The chemistry of carbon-containing compounds is broadly known as organic chemistry. This science is a subdiscipline of chemistry and it focuses on the chemistry within medicine, natural products, and the materials we come into contact with daily. In this chapter we will focus on hydrocarbons, nomenclature of hydrocarbons, oxygen-containing organic compounds, and nitrogen containing organic compounds. Additionally, we will focus on some basic properties that each class of organic compounds possess and look at their applications.
OBJECTIVES
After completing this chapter, you will be able to
name alkanes and alkene hydrocarbons;
identify and label constitutional isomers of hydrocarbons;
identify and describe the structures and properties for alcohols, ethers, ketones, aldehydes, esters, and carboxylic acids;
identify and describe the structures and properties for amines, imines, nitriles, and amides.
The simplest organic compounds are known as hydrocarbons. These compounds are composed of only two types of elements: carbon and hydrogen. Even when a compound is mostly carbon and hydrogen and has only one other element within its structure, then it is not a hydrocarbon. Consider the following molecular formulas for these common organic compounds. A molecular formula provides the number of atoms of each type of element that makes up a molecule.
Propane is used as a fuel. It has the molecular formula C3H8.
Acetone is an organic compound that is found in fingernail polish remover. It has the molecular formula C3H6O.
Ethanol is known as drinking alcohol and it also is an organic compound. It has the molecular formula C2H6O.
Octane is the main component found in gasoline and has the molecular formula C8H18.
Ethylamine is a common organic solvent used and has the molecular formula C2H7N.
Which of the previous organic compounds are hydrocarbons? _______________
Answer: propane and octane
Molecular formulas provide some information about organic compounds but they don't help in terms of the actual structure of the molecule. Structural information can help us understand the physical and chemical properties of a molecule. As such we will look at structural formulas. Structure formulas help us see how the atoms of a molecule are connected together.
Before we begin making hydrocarbon structures we must remember that the carbon atom has four valence electrons, since it is a Group IVA element. This means carbon will require four bonds in its structure and it will obtain this by the sharing of four additional electrons. By sharing the electrons the carbon atom will satisfy its octet (Chapter 3).
Let's draw the structure for the simplest hydrocarbon, CH4, using a type of structural formula known as the dash formula. The dash formula is a Lewis structure that shows bonds as dashes and lone pairs of electrons where they exist on a molecule. The dash formula is not often accurate to the actual three-dimensional structure but it does serve well as a two-dimensional representation of a molecule.
The simplest hydrocarbon has one carbon atom and it is known as methane, CH4.
There are four hydrogen atoms connected to the carbon atom. The dash formula for methane is
The structure shows us how the atoms are connected to one another.
How many valence electrons does the carbon atom possess? ____________
Answer: Four
The next simplest hydrocarbon is known as ethane and has the molecular formula C2H6.
Notice that there are now two carbons with four single bonds each. Connecting the two carbon atoms together with three additional bonds each will give
What is the molecular formula for ethane? ____________
Answer: C2H6
Before we go any further let's discuss the classifications for hydrocarbons. Hydrocarbons can be separated into three main groups:
Saturated hydrocarbons: this comprises the alkanes and the cycloalkanes.
Unsaturated hydrocarbons: this comprises the alkenes and the alkynes.
Aromatic hydrocarbons: this comprises the benzene rings.
The following structures illustrate these groups.
What common feature can you find in unsaturated hydrocarbons? _____________________
Answer: double or triple carbon bonds
BOND GEOMETRY
A hydrocarbon that is saturated contains only carbon and hydrogen single bonds. These are known as alkanes. Alkanes can be cyclic or acyclic. A cyclic compound does not have an end to it but resembles a ring. An acyclic is a molecule that has at least two ends to it. We also say that these carbons are sp3 hybridized. In other words, the carbon atom contains four “bonding directions.” You can think of this when considering the sp3 hybridization (1s + 3p = 4 total orbitals). The four orbitals contribute to four bonding directions. The carbon in methane has four bonding directions as seen in its dash structure. Therefore, the carbon atom in methane is sp3 hybridized. The language of hybridization will be seen as we cover various organic compounds in this chapter.
An unsaturated hydrocarbon is a compound that possesses a carbon–carbon double or triple bond. An alkene is an unsaturated hydrocarbon that has at least one carbon–carbon double bond. An alkyne is also an unsaturated hydrocarbon that has at least one carbon–carbon triple bond. An alkene carbon has a total of three bonding directions (two single bonds and one double bond). Since the alkene carbon has three bonding directions it is sp2 hybridized (1s + 2p = 3 total orbitals). The alkyne has two bonding directions (one single bond and one triple bond) and is sp hybridized (1s + 1p = 2 total orbitals). Each hybridization has its own geometry and set of bond angles. See the following table.
Hybridization
Geometry
Bond angle (°)
sp
Linear
180
sp2
Trigonal planar
120
sp3
Tetrahedral
109.5
An aromatic hydrocarbon possesses a benzene ring or some similar structure. The benzene ring is a six-membered ring with three alternating carbon–carbon double bonds.
Below are a variety of hydrocarbons represented by dash formula structures. Dash formulas show all atoms and bonds within a structure.
Which hydrocarbons are saturated and why? _______________
Answer: Compounds A and D. Each comprises only carbon and hydrogen single bonds.
Which hydrocarbons are unsaturated and why? _______________
Answer: Compound B, C, and E. These have either carbon–carbon double or triple bonds.
Which hydrocarbon is an alkyne? _______________
Answer: E
Which hydrocarbon is an alkene? _______________
Answer: B
Which hydrocarbon is an aromatic? _______________
Answer: C
Which hydrocarbons are alkanes? _______________
Answer: A and D
Which hydrocarbon is cyclic? _______________
Answer: D
Which hydrocarbons are acyclic? _______________
Answer: A, B, and E
Remember that methane is the simplest alkane. The number of hydrogens can be obtained quickly by knowing the number of carbons in an alkane. Alkanes have the general formula CnH2n + 2. Methane has one carbon atom n = 1 and the number of hydrogen atoms is 2n + 2 = 2(1) + 2 = 4. The molecular formula is therefore CH4.
What is the molecular formula for butane, a fuel found in lighters? Butane has a total of four carbons. _______________
Answer:C4H10
Following ethane, the name of the next molecule is propane. This gas is used as a fuel for heating and cooking, and it has a total of three carbons in its molecular formula. Using the formula CnH2n + 2 a three-carbon alkane will have eight hydrogen atoms.
What is the molecular formula of this alkane? __________________
Answer: C3H8
What are the names of the first four hydrocarbons (containing 1, 2, 3, and 4 saturated carbon atoms)? _______________
Answer: Methane, ethane, propane, and butane
After butane, the next alkanes incorporate Greek prefixes within their names. This makes it easier to know the number of carbons. The following table lists the Greek prefixes and their corresponding alkane names.
Greek prefix
Number of carbon atoms
Alkane name
Penta–
Five
Pentane
Hexa–
Six
Hexane
Hepta–
Seven
Heptane
Octa–
Eight
Octane
Nona–
Nine
Nonane
Deca–
Ten
Decane
What is the molecular formula and dash formula for heptane?
Answer:C7H16
Another formula representation is known as the condensed structural formula or condensed formula. In this representation the bonds are not explicitly drawn but there is more information to their structure than the simpler molecular formula. For instance, the molecular formula for hexane is C6H14. Additionally, its condensed formula is CH3CH2CH2CH2CH2CH3. With the condensed formula we see how the atoms are grouped but we do not see how each is bonded, as in the case with the dashed formula structure.
Write the condensed formula for nonane. _______________
Answer:CH3CH2CH2CH2CH2CH2CH2CH2CH3
As you can imagine there is a lot of drawing to do as the molecular formula increases in the number of carbon and hydrogen atoms. The most common type of structural formula that is fastest to draw is the bond-line formula. There are a few rules to know when drawing molecules using the bond-line formula.
Bonds are represented by lines. Each bend in a line represents a carbon atom, unless another atom is explicitly shown.
Elemental symbols for carbon atoms are not drawn. Elemental symbols for hydrogen atoms are not drawn on carbon atoms unless there is a need to represent a three-dimensional perspective.
The number of hydrogen atoms are inferred by assuming carbon must complete its octet.
Hydrogen atoms bonded to heteroatoms (atoms other than C and H) are drawn as well as the elemental symbol for the heteroatom.
Below is a bond-line formula. Notice how the carbons have been highlighted with an asterisk (*).
Identify the alkane represented by this bond-line formula. _________________
Answer: pentane
Although not drawn, hydrogen atoms are inferred on the above structure for pentane. The end carbons only have one bond drawn. Therefore the end carbons must have three hydrogen atoms in order for them to have four bonds in total. Each of the three middle carbons has two bonds drawn. Therefore, each middle carbon must have two hydrogens, providing a total of four bonds. The following highlights the number of hydrogen atoms bonded to each carbon atom in pentane. Again, these hydrogen atoms are inferred, not drawn onto the bond-line structure.
How many carbon atoms and hydrogen atoms are there for the following structure?
Answer: This structure has nine carbon atoms and 20 hydrogen atoms.
So far you have only been shown the first 10 alkanes, beginning with methane and ending in decane. These alkanes belong to a homologous series, which is a series of compounds differing from each other by a fixed group of atoms. In the case with alkanes the fixed group is an additional –CH2–.
Are ethane, propane, butane, and pentane part of a homologous series? ______________
What is the difference in the structure between ethane and propane? Butane and pentane? ______________
Answer: yes; The difference between ethane and propane is a –CH2– group. The difference between butane and pentane is a –CH2– group.
You have also seen these alkanes as straight-chain alkanes. In other words, the carbon atoms are bonded to one another in a linear fashion. However, there are also branched-chain alkanes too. In a branched-chain alkane a hydrogen along the longest chain (known as the parent chain) is replaced with a CH group known as an alkyl group. The name of the branched portion is similar to one of the 10 straight-chain alkanes but the “–ane” ending is replaced with “–yl.” Below is a table that shows the first four alkyl groups and the parent alkane each alkyl is derived from.
Branched group
Alkyl name
Derived from the alkane
CH3–
Methyl
Methane
CH3CH2–
Ethyl
Ethane
CH3CH2CH2–
Propyl
Propane
CH3CH2CH2CH2–
Butyl
Butane
What is the difference between propane and a propyl group? ______________
Answer: one less hydrogen atom, thus leaving a bond for attachment to another atom or group
The simplest branched-chain alkane has the following structure.
You'll notice the numbers are shown underneath the molecule. There is no standard placement for the numbers to go so long as these are in order. For instance it would be just as accurate to label the molecule as follows. Again, the placement of the numbers does not matter in a molecule's numbering.
What is the parent chain and the branched alkyl group name for the following compound?
Answer:
In naming this branched alkane, we need to know to which carbon of the octane parent chain the ethyl group is attached. The ethyl group is located on which carbon? ____________ (caution, tricky question)
Answer: Counting from the right, the fourth carbon; counting from the left, the fifth carbon. (We need some counting rules here.)
Although the parent chain is octane this is not its name. The actual name must include both its branched alkyl group and its parent name. This complicates naming so as a result we need to have rules for naming branched alkanes. Fortunately, the International Union for Pure and Applied Chemistry (IUPAC) has provided those rules.
Rule 1 Count the longest carbon chain. This chain is not always straight. In fact it could be bent. However, before we go any further, you must count from the direction in which your substituent appears sooner. Remember, the substituent is the alkyl group(s) that is connected to the parent chain.
Rule 2 Identify the substituent by name and find its location along the parent chain.
Rule 3 Name the hydrocarbon by writing the substituent location first, then the substituent name followed by the parent name. There are no spaces in a hydrocarbon's name. However, numbers are written first followed by dashes with the alkyl name proceeding the parent name. If multiple substituents are present dashes are found between numbers.
Example 1: Number-alkyl group/parent name such as in 3-methyloctane
Example 2: Number-alkyl group–number–alkyl group by parent name such as in 4-ethyl-5-methyl-decane
Based on these rules, which is the correct name for the structure in frame 16?
5-methyloctane
4-octanemethyl
5-octanemethyl or
4-methyloctane
Answer: (d) (4-methyl octane follows the naming rules. The longest chain is octane, the smallest number for the carbon to which the substituent is attached is 4 on the parent chain from either side, and the substituent is named first.)
Identify this molecule's parent chain and substituent.
Answer:
Count the number of carbons in the parent chain. Make sure to number correctly.
Answer:
The parent chain is five carbons long so its parent name is pentane.
In the structure in frame 21, what is the substituent name and where is its location along the parent chain?
Answer: The substituent is a CH3– group which is known as the methyl group. It is located at carbon-2.
What is the name for the organic compound in frame 21?
Answer: 2-Methylpentane
Simpler alkanes can be named quickly, but alkanes can have multiple substituents. When you have an alkane with multiple substituents you will need to use Greek prefixes prior to the substituent name. Additionally, for every substituent you must have a number written with it, even if it is on the same carbon!
Below are the first five Greek prefixes used in naming alkanes. Rarely does one exceed tetra or penta in a name.
Number of identical substituents
Greek prefix
Two
Di
Three
Tri
Four
Tetra
Five
Penta
What is the name for this alkane and why?
Answer: The name for this compound is 2,3,4-trimethylpentane. After counting the longest carbon chain you have identified the parent chain as pentane. The three substituents are all the same, methyl. They are located on carbons-2,3,4 along the chain. Since there are three identical substituents the Greek prefix tri is placed between the last number and the alkyl name.
Remember the rule that if two substituents are on the same carbon, both numbers must be given. Try naming this alkane.
Answer: The name for this alkane is 2,2-dimethylpropane. The longest carbon chain has three carbons so its parent name is propane. It has two identical substituents, methyl. Both are located on carbon-2. Since there are two substituents the name must also have two numbers, even if they are the same number.
For your previous two alkanes, 2,3,4-trimethylpentane and 2,2-dimethylpropane, did the direction for numbering matter when identifying the parent chain?
Answer: No. In both cases the substituents appeared at the same distance from either end so the direction for numbering didn't matter in naming this compound.
If a structure has two or more different alkyl groups, they must be named alphabetically. Let's look at a compound that offers a bit more challenge. What is the parent name, the substituent names (and their locations) for the following alkane? Finally, what is the overall name for this molecule?
Answer: Since there are 10 carbons within the longest carbon chain the parent name is decane. Numbering from the bottom carbon will provide the first substituent at carbon-3 whereas numbering from the top carbon will provide the first substituent at carbon-4. Therefore, the numbering begins from the bottom carbon since it provides a substituent sooner.
The substituents and their locations are 3-methyl and 7-ethyl. Combining these substituents in alphabetical order with the parent name at the end provides 7-ethyl-3-methyldecane. The name 3-methyl-7-ethyldecane would be incorrect as the alkyl groups are not listed alphabetically. There is one more bit of information when it comes to listing alkyl groups alphabetically, which we will see in the following section.
There is a rule for the order of naming multiple different substituents. The alkyl name is still listed by alphabetical order. For instance, butyl is written before ethyl. Ethyl will come before methyl, and methyl is listed before propyl. However, Greek prefixes do not play a role in the order. Consider the compound below. Its longest carbon chain is nine. Can you identify the longest carbon chain?
Answer:
For instance, it would be incorrect to name this 3-methyl-5,5,6-triethylnonane thinking that the “t” for triethyl is part of the alphabetical order. Instead, triethyl's “e” for ethyl is used for alphabetizing. Therefore, the name of this compound is 5,5,6-triethyl-3-methylnonane.
Here is a list of six alkane names. Which alkane name is written incorrectly? What is its correct name? Assume the carbon locations are correct (i.e., the numbering scheme) for each.
3-Ethyl-4-methylheptane
4-Methyl-4-propyloctane
2,2-Dimethyl-4-ethylhexane
3-Methyl-5,6-dipropyldecane
2,2-Dimethylpropane
Answer: 2,2-Dimethyl-4-ethylhexane (the “m” for methyl was not alphabetized correctly). Its correct name (assuming the numbering is correct) is 4-ethyl-2,2-dimethylhexane.
The variety of structures that arise from branched-chain alkanes allow for constitutional isomers (or structural isomers). These are compounds with the same molecular formula but different structural formulas.
Let's examine the two simplest alkane structural isomers, 2-methylpropane and butane.
They both have the same molecular structure but their connectivity is different. In other words they have the same molecular formula but a different structure. As such, structural isomers will have different physical properties, such as different melting points and boiling points. When heteroatoms are involved (compounds other than hydrocarbons), chemical properties can drastically be different between structural isomers.
How many structural isomers exist for an alkane having the molecular formula C4H10?
Answer: Two
How many structural isomers exist for an alkane having the molecular formula C5H12? Make sure to draw and name all of these structural isomers.
Answer: Three
Circle the alkane that is not a structural isomer for C6H14. Explain your choice.
Answer: The circled alkane has the formula C5H12 while the other alkanes are isomers of C6H14.
Cyclics are another group of alkanes but their structure does not have ends to them. Instead they are connected into a ring structure. The simplest cyclic is cyclopropane. Cyclopropane is a three-carbon ring shaped like a triangle. Notice that when naming cyclics the prefix cyclo is included. The following shows the first four cyclics' dash formula, bond-line formula, and name.
Cyclics have the formula CnH2n. Notice that it's different than the acyclic alkane formula CnH2n + 2. Since the two ends connect to make a loop (aka, a ring) there are two fewer hydrogens. These compounds are still saturated since they are only composed of carbon and hydrogen single bonds.
Let's consider the four-carbon hydrocarbon chain butane, CH3CH2CH2CH3. Butane is fully saturated and as a result it cannot connect the two end carbons together in a loop. Remember, the maximum number of bonds to a carbon atom is four! The question is, how could we have a four-carbon chain loop up at the two end carbons?
Would the following structure work for looping the two end carbons? CH3CH2CH2CH2–
Answer: No. Notice that only one of the end carbons (the carbon on the far right) has a single hydrogen atom removed. The dash (–) reflects an available bond. However, if we were to remove two hydrogen atoms (one from each end carbon) we get –CH2CH2CH2CH2–. The two end carbons have only three bonds, leaving enough room to form a final bond to each other to give the cyclic known as cyclobutane.
Cyclobutane (C4H8)
What are the molecular formulas for cycloheptane and cyclooctane?
Answer: cycloheptane: C7H14; cyclooctane: C8H16
Alkenes are unsaturated hydrocarbons that possess at least one carbon–carbon double bond. They have the same general formula as cyclic hydrocarbons, CnH2n. The simplest alkene is ethene. Note the ending is no longer “–ane” but rather “–ene” describing an alkene. Below are the various ways to represent ethene.
An alkene requires at least two carbon atoms. This is why the simplest alkene is ethene. The next larger alkene is three carbons long and is called propene. A four-carbon alkene is either 1-butene or 2-butene. The “1” in front of the name provides us the information as to where the C=C bond starts. If the double bond begins on the first carbon then it is 1-butene. If it begins at carbon-2, then it is 2-butene.
You can draw these compounds more quickly by using the condensed formula or bond-line formula.
CH2=CH – CH3 CH2=CH – CH2 – CH3 CH3 – CH=CH – CH3
What are the condensed and bond line formulas for 1-heptene, 2-heptene, and 3-heptene?
A fact about alkenes is that the C=C bond is not allowed to rotate, unlike the carbon–carbon single bonds found within alkanes. Because of that rotational restriction alkenes can give rise to geometric isomers. These geometric isomers result when there are two different groups located on the alkene's double-bonded carbons. Geometric isomers are a specific type of isomer where the atoms are joined to one another in the same manner but differ in their orientation in space.
2-Butene has two geometric isomers. Notice that butene's C=C carbon atoms each have a (1) hydrogen atom and (2) a methyl group (CH3–). When the larger groups are on the same side of the double bond, then the geometric isomer begins with the prefix “cis.” If the two larger groups are on opposite sides of the double bond then the geometric isomer begins with the prefix “trans.”
The double bond in each 2-Butene isomer is highlighted below with the larger groups (the methyl groups) circled. The first isomer shows the two methyl groups on opposite sides of the double bond while the second drawing shows the two methyl groups located on the same side of the double bond.
Based on what you have just learned, name each of the following structures as ______-2-butene.
Given the information you have just learned about cis and trans isomers, circle the alkene that represents cis-2-pentene.
Answer:
Can cis–trans isomers be possible if one of the carbons of a double bond has two identical groups?
Answer: No
Consider the following alkene to help illustrate this point. Notice the C=C bond and the carbon on the left. It is bonded to two other hydrogen atoms. Because of this we cannot say one hydrogen takes priority over the other. In other words, one group (in this case a hydrogen) does not have greater priority than the other group (also a hydrogen atom) since both are identical. Because of this we cannot label this as cis or trans.
Name and draw the two geometric isomers for 3-hexene.
Answer:
Although cis and trans isomers have the same molecular formula they have different physical (melting points and boiling points) and chemical properties.
What is the name for the following alkene?
Answer:
First, let's identify if this alkene is cis or trans? Notice that the two circled groups are on opposite sides of the double bond. This is therefore a trans alkene.
Next, count the parent chain from the end that produces the alkene double bond sooner and then identify the substituent groups. You'll notice on carbon-2 and carbon-5 methyl groups.
Now let us put all of this information together. The alkene's name is trans-2,5-dimethyl-3-octene.
Alkynes are unsaturated hydrocarbons that have at least one carbon–carbon triple bond. Alkynes have the formula CnH2n−2. The simplest alkyne is ethyne, which has two carbons, and according to the equation it will also have two hydrogen atoms. Below are the structures for ethyne, propyne, 1-butyne, and 2-butyne.
What are the molecular formulas for an alkyne that has (a) five carbons, (b) six carbons, and (c) seven carbons in its structure?
Answer: Using the formula CnH2n−2 and plugging in the number of carbon atoms for n we get (a) C5H8 (b) C6H10 and (c) C7H12.
Naming alkynes is similar to alkenes. The triple bond is counted from the acyclic's closest end. Propyne's triple bond is not numbered since it gets a carbon-1 placement from either end of the molecule. However, butyne can have two possible locations for its triple bond. As such, both must be numbered to identify the two structural isomers.
Using bond-line formulas, draw all of the straight chain isomers for hexyne, C6H10.
Answer:
There is a great deal of stored chemical energy found within the double bond of the alkene and even more energy found within the triple bond of the alkyne. Acetylene (C2H2) is an alkyne that burns so hot that it is used in welding.
Which hydrocarbon should have the most stored chemical energy in it and why? (octane, 1-octene, or 1-octyne)
Answer: 1-Octyne should have the most stored chemical energy in it because it has a carbon–carbon triple bond.
Alkynes are either internal (found within the carbon chain of the molecule) or terminal (found at the end of the carbon chain). 3-Heptyne and 1-heptyne are alkyne isomers of one another. Which alkyne is the terminal alkyne and why?
Answer: 1-Heptyne is the terminal alkyne since the alkyne triple bond begins at the end carbon (or rather carbon-1) of the chain.
Draw the bond-line structure for 2-hexyne.
Answer:
Identify the structural isomers by circling them. Also, what are the isomers' molecular formula?
Answer:
Aromatic hydrocarbons are compounds that have a six-membered ring with three alternating carbon–carbon single and double bonds. The simplest aromatic hydrocarbon is benzene, C6H6.
This is the structure for benzene.
Other aromatic hydrocarbons are found in fused-ring structures. A fused-ring structure has two or more aromatic rings fused together. Naphthalene, anthracene, phenanthrene, and pentacene are all aromatic hydrocarbons.
All of these compounds are specifically known as polycyclic aromatic hydrocarbons. These are named polycyclic as they are made up of more than one aromatic ring. When looking at these compounds it is important to identify the aromatic ring structures found in each. Naphthalene is the main ingredient used in moth balls. This compound is used to make a variety of other compounds.
Anthracene is a compound found in coal tar and is used to make dyes.
Phenanthrene is found in cigarette smoke and also a known skin irritant.
Pentacene is a compound used in semiconductive materials.
Aromatic rings are also found in a variety of medications. Below is a list of compounds that possess the aromatic ring. The following are not considered aromatic hydrocarbons since there are atoms other than carbon and hydrogen within their structure. However, these compounds contain the aromatic ring structure.
Circle the aromatic ring found in the structure of ibuprofen.
Answer:
Aromatic rings are also found in a variety of natural products.
Benzaldehyde (shown below) contributes to the smell of oyster mushrooms. Circle the aromatic ring found in benzaldehyde.
Answer:
OXYGEN-CONTAINING ORGANIC COMPOUNDS
The next class of organic compounds are the ones that contain the oxygen atom as part of their functional group. A functional group is a particular connection of atoms that have unique chemical and physical properties. Functional groups are the reactive portion of a molecule. Alcohols, ethers, ketones, aldehydes, carboxylic acids, and esters all contain oxygen as part of their functional group.
Alcohols
Alcohols are an important class of organic compounds that are used as solvents, cleaners, and chemicals to make more complicated organic compounds. Common alcohols include isopropyl alcohol and ethanol. Isopropyl alcohol is used in antiseptics and disinfectants. Ethanol is also known as drinking alcohol.
When an sp3 hybridized carbon atom is single-bonded to an –OH group that functional group is known as an alcohol group (aka, a hydroxyl group).
The basic structure of the alcohol functional group is C–O–H. However, the carbon atom requires its other connected atoms to fulfill its octet. The simplest alcohol is methanol, CH3–OH.
Methanol can be represented the following ways.
Naming alcohols is rather straightforward and is also similar to alkane nomenclature. The number of carbons is drawn from the parent alkane names. The suffix “–ane” is changed to “–ol” to reflect the hydroxyl functional group. Ethanol contains two carbons and the –OH functional group. Below are the different ways to represent ethanol.
What is the molecular formula and condensed formula for propanol? Propanol is an alcohol with three carbons.
Answer:C3H8O,CH3CH2CH2OH
As the number of carbon atoms in alcohols increase so does their complexity. The three carbon-containing alcohols have the molecular formula C3H8O and can come in the form of two structural isomers.
The numbering system shows where the –OH group is attached.
Alcohols are usually classified by the number of carbon atoms attached to the C–OH carbon. A primary (1°) alcohol has one carbon bonded to it, a secondary (2°) alcohol has two, and a tertiary (3°) alcohol has three.
For each of the following alcohols circle the carbon atom(s) directly attached to the C–OH carbon atom. Then label the C–OH carbon as primary (1°), secondary (2°), or tertiary (3°).
Answer:
Draw the line structure for 3-hexanol and state whether it is a primary, secondary, or tertiary alcohol.
Answer:
, this is a secondary alcohol.
Ethers
Our next oxygen-containing functional group has the C–O–C connection. These are known as ethers. Ethers have been used in chemical reactions and in medicine. The simplest ether has two methyl groups connected by an oxygen atom. This compound is known as dimethyl ether. Below are the molecular formula, the condensed formula, the dashed formula, and the bond-line structure for dimethyl ether.
Are ethanol and dimethyl ether structural isomers? Why or why not?
Answer: Yes, because ethanol and dimethyl ether have the molecular formula C2H6O but different molecular structures.
CARBONYL GROUPS
Some carbon atoms are connected to oxygen atoms through double bonds. The C=O group is known as the carbonyl group and is common to several larger functional groups. In particular, the carbonyl group is part of the aldehyde and ketone functional groups. The simplest functional groups that contain these are known as the aldehyde and the ketone (shown below). The aldehyde and ketone are similar but there is a slight structural difference between the two. Both possess the C=O bond (the carbonyl group) but the aldehyde has a hydrogen atom bonded to the C=O carbon. The ketone has the basic structure C–C(=O)–C. The (=O) implies this bond is connected to the carbon to its left. A ketone is a C=O carbon connected to two other carbons through the C=O carbon.
Ethanal is the simplest aldehyde having the basic structure –C(=O)–H while propanone, the simplest ketone, has its C=O flanked by two alkyl groups. In this case, propanone's C=O carbon is bonded to two methyl groups.
So far you have seen the alcohol, ether, ketone, and aldehyde functional groups. Circle and label the functional groups found in the following organic compounds.
Answer:
The next two oxygen-containing functional groups have the carbonyl group with its carbon also connected to one other oxygen atom through a single bond. This oxygen atom is an sp3 hybridized oxygen atom. The R in the formulas below represent any alkyl group. The oxygen atom in –OH or –OR has four total bonding directions (two bonds and two lone pairs). The two lone pairs on the oxygen atom count as two bonding directions. However, what is connected to the other end of the single-bonded oxygen atom determines the functional group's identity. Consider the basic structure of the carboxylic acid and ester, shown below. Notice that the carboxylic acid single-bonded oxygen has a bond to hydrogen. This is unique to the carboxylic acid, and this group is often shortened to –COOH or –CO2H. As for the ester its sp3 hybridized oxygen is instead connected to an alkyl group.
Circle and label the following oxygen-containing functional groups in vitamin C and aspirin.
Answer:
ORGANIC COMPOUNDS CONTAINING NITROGEN
Another type of organic compound has the nitrogen atom incorporated into its structure. As in the case with oxygen-containing organic compounds there are several types of nitrogen-containing functional groups. These include amines, imines, nitriles, and amides. These nitrogen-containing compounds have different chemical and physical properties than the oxygen-containing functional groups.
Most organic bases are amines, which are compounds derived from ammonia, NH3. These derivatives are made by replacing one or more hydrogen atoms of ammonia with alkyl groups (CH groups generically written as R groups). The number of N–alkyl groups determines the substitution on the amine (1°, 2°, or 3°).
How many alkyl groups are directly attached to a secondary amine?
Answer: 2
How many lone pairs exist on the nitrogen atom for any nitrogen atom with three single bonds? Also, what is the hybridization on the amine's N atom?
Answer: 1, sp3
As mentioned, amines are bases, since they have a lone pair available to accept a proton (H+). After accepting a proton the amine becomes a substituted ammonium ion. In the example below methylamine (CH3NH2) deprotonates the water molecule to become the methylammonium ion and the hydroxide ion.
In this reaction, identify the acid, base, conjugate acid, and conjugate base.
Answer:acid = CH3NH2, base = H2O, , conjugate base = OH−
Identify the acid, base, conjugate acid, and conjugate base for the reaction of ethylamine (CH3CH2NH2) with water.
Answer:acid = CH3CH2NH2, base = H2O, , conjugate base = OH−
Diethylamine, (CH3CH2)2NH, has the dash line formula
What is the structure of diethylamine after it deprotonates a water molecules?
Answer:
Amines are one type of nitrogen-containing organic compounds. Remember, amines are nitrogen atoms with a lone pair on the central atom with three single bonds connected to alkyl groups. Just as hydrocarbons can have carbon–carbon single bonds, carbon–carbon double bonds, and carbon–carbon triple bonds so they can have multiple carbon–nitrogen bonds.
An imine functional group is composed of a nitrogen atom with a lone pair on the central atom having a single bond to one alkyl group and a double bond to a second alkyl group. The simplest amine (CH2NH) is shown below.
Consider the following nitrogen-containing compounds. Circle the one that is an imine.
Answer:
Is an imine basic? Why or why not? Also, what is the hybridization of the imine's nitrogen atoms?
Answer: The imine is basic since its nitrogen atom possesses a lone pair available to accept a proton. The N atom is sp2 hybridized.
Imines can be made from reacting an aldehyde or ketone with a 1° amine. For instance, methanal, an aldehyde, reacts with methyl amine to produce an imine and water.
In the reaction with methanal and methyl amine circle the portion of the imine that came from the aldehyde.
Answer:
Can you make an imine from diethylamine, (CH3CH2)2NH? Why or why not?
Answer: No, because diethylamine is a 2° amine.
Think about the imine produced from this reaction. Why must the amine be a primary amine? Hint: What other product must be made?
Answer: The amine must be a primary amine. Its two hydrogen atoms will combine with the ketone or aldehyde's oxygen atom to make water as a by-product.
The nitrile functional group contains a carbon–nitrogen triple bond with a lone pair on the nitrogen atom. The carbon atom of the nitrile connects to the alkyl group. The simplest nitrile is CH3CN, known as acetonitrile, which is a common organic solvent. The hybridization of the nitrile's N atom is sp.
Nitriles are often abbreviated in their structures with –CN. Acetonitrile can also be written as . The condensed formula provides CH3CN and the bond-line formula is where only the nitrogen atom is written. All of these provide insight into the arrangement of atoms with the nitrile.
Nitriles undergo reactions that can make our final nitrogen-containing organic compound, the amide. The amide is a C=O directly connected to an sp3-hybridized nitrogen atom. Amide bonds are the backbone that makes up proteins. The simplest amide is acetamide, CH3CONH2. Amides can also be made through a variety of other organic reactions.
Identify the nitrogen-containing functional groups for the following compounds.
Answer:
Amides can be made by reacting nitriles with acid. The simplest nitrile, acetonitrile, can be made by reacting acetonitrile with acid. Treating acetonitrile with acid can generate acetamide.
Draw the product that is made from the reaction.
Answer:
In the box provided draw the structure of the nitrile that makes the following amide.
Answer:
SUMMARY
You should now be able to name and draw the structures for a variety of alkanes and alkenes with a reasonable degree of proficiency. Also, you should be able to draw constitutional and geometric isomers for a variety of hydrocarbons. Finally, you should be able to identify several of the common organic chemistry functional groups.
SELF-TEST
Which of the following hydrocarbons is saturated: alkanes, cycloalkanes, alkenes, alkynes, or aromatics?
Fill in the following table for the labeled carbon atoms.
Carbon atom
Hybridization
Geometry
Bond angle (°)
A
B
C
D
E
Fill in the missing content in the following table concerning straight-chain alkane molecular formulas, condensed formulas, bond-line formulas, and their names.
Molecular formula
Condensed formula
Bond-line formula
Name
CH3CH2CH2CH3
C6H14
Heptane
Provide the names for alkanes #1 and #2.
Draw the bond-line formula for 4,5-diethyl-2,9-dimethyldecane.
Provide the molecular formula for each of the structures below and circle the compounds that are isomers.
Draw the following compounds using bond-line formulas and name them.
A seven-membered cycloalkane.
An alkene that has eight carbons with the double bond beginning on carbon-1.
Consider an alkene that has six carbon atoms. Using bond-line formulas draw (a) all of the straight chain isomers and (b) at least four branched chain isomers where the parent chain is pentene.
Draw and name the two geometric isomers for 3-methyl-3-hexene.
What are the formulas for alkanes, alkenes, cycloalkanes, and alkynes?
Name the following alkynes.
Identify the following organic compounds by their functional group.
Identify the following organic compounds by their functional group.
Label the following alcohols and amines as primary (1°), secondary (2°), or tertiary (3°).
Draw the product of the following reaction.
ANSWERS
Alkanes and cycloalkanes [frame 4]
Carbon atom
Hybridization
Geometry
Bond angle (°)
A
sp3
Tetrahedral
109.5
B
sp2
Trigonal planar
120
C
sp2
Trigonal planar
120
D
sp3
Tetrahedral
109.5
E
sp
Linear
180
[frame 4]
Molecular formula
Condensed formula
Bond-line formula
Name
C4H10
CH3CH2CH2CH3
Butane
C6H14
CH3CH2CH2CH2CH2CH3
Hexane
C5H12
CH3CH2CH2CH2CH3
Pentane
C7H16
CH3CH2CH2CH2CH2CH2CH3
Heptane
[frames 6–11]
Alkane #1 is 2,2-dimethylbutane; alkane #2 is 2,3,5-trimethylhexane. [frames 12–29]
[frames 12–28]
[frames 1, 29–34]
[frames 34–38]
Although you are asked for four, all are provided. [frames 37–41]
The simplest organic compounds are known as hydrocarbons. These compounds are composed of only two types of elements: carbon and hydrogen. Even when a compound is mostly carbon and hydrogen and has only one other element within its structure, then it is not a hydrocarbon. Consider the following molecular formulas for these common organic compounds. A molecular formula provides the number of atoms of each type of element that makes up a molecule.
Molecular formulas provide some information about organic compounds but they don't help in terms of the actual structure of the molecule. Structural information can help us understand the physical and chemical properties of a molecule. As such we will look at structural formulas. Structure formulas help us see how the atoms of a molecule are connected together.
The next simplest hydrocarbon is known as ethane and has the molecular formula C2H6.
Before we go any further let's discuss the classifications for hydrocarbons. Hydrocarbons can be separated into three main groups:
Below are a variety of hydrocarbons represented by dash formula structures. Dash formulas show all atoms and bonds within a structure.
Remember that methane is the simplest alkane. The number of hydrogens can be obtained quickly by knowing the number of carbons in an alkane. Alkanes have the general formula CnH2n + 2. Methane has one carbon atom n = 1 and the number of hydrogen atoms is 2n + 2 = 2(1) + 2 = 4. The molecular formula is therefore CH4.
Following ethane, the name of the next molecule is propane. This gas is used as a fuel for heating and cooking, and it has a total of three carbons in its molecular formula. Using the formula CnH2n + 2 a three-carbon alkane will have eight hydrogen atoms.
What are the names of the first four hydrocarbons (containing 1, 2, 3, and 4 saturated carbon atoms)? _______________
After butane, the next alkanes incorporate Greek prefixes within their names. This makes it easier to know the number of carbons. The following table lists the Greek prefixes and their corresponding alkane names.
Another formula representation is known as the condensed structural formula or condensed formula. In this representation the bonds are not explicitly drawn but there is more information to their structure than the simpler molecular formula. For instance, the molecular formula for hexane is C6H14. Additionally, its condensed formula is CH3CH2CH2CH2CH2CH3. With the condensed formula we see how the atoms are grouped but we do not see how each is bonded, as in the case with the dashed formula structure.
As you can imagine there is a lot of drawing to do as the molecular formula increases in the number of carbon and hydrogen atoms. The most common type of structural formula that is fastest to draw is the bond-line formula. There are a few rules to know when drawing molecules using the bond-line formula.
Below is a bond-line formula. Notice how the carbons have been highlighted with an asterisk (*).
Although not drawn, hydrogen atoms are inferred on the above structure for pentane. The end carbons only have one bond drawn. Therefore the end carbons must have three hydrogen atoms in order for them to have four bonds in total. Each of the three middle carbons has two bonds drawn. Therefore, each middle carbon must have two hydrogens, providing a total of four bonds. The following highlights the number of hydrogen atoms bonded to each carbon atom in pentane. Again, these hydrogen atoms are inferred, not drawn onto the bond-line structure.
So far you have only been shown the first 10 alkanes, beginning with methane and ending in decane. These alkanes belong to a homologous series, which is a series of compounds differing from each other by a fixed group of atoms. In the case with alkanes the fixed group is an additional –CH2–.
You have also seen these alkanes as straight-chain alkanes. In other words, the carbon atoms are bonded to one another in a linear fashion. However, there are also branched-chain alkanes too. In a branched-chain alkane a hydrogen along the longest chain (known as the parent chain) is replaced with a CH group known as an alkyl group. The name of the branched portion is similar to one of the 10 straight-chain alkanes but the “–ane” ending is replaced with “–yl.” Below is a table that shows the first four alkyl groups and the parent alkane each alkyl is derived from.
The simplest branched-chain alkane has the following structure.
In naming this branched alkane, we need to know to which carbon of the octane parent chain the ethyl group is attached. The ethyl group is located on which carbon? ____________ (caution, tricky question)
Although the parent chain is octane this is not its name. The actual name must include both its branched alkyl group and its parent name. This complicates naming so as a result we need to have rules for naming branched alkanes. Fortunately, the International Union for Pure and Applied Chemistry (IUPAC) has provided those rules.
Based on these rules, which is the correct name for the structure in frame 16?
Identify this molecule's parent chain and substituent.
Count the number of carbons in the parent chain. Make sure to number correctly.
In the structure in frame 21, what is the substituent name and where is its location along the parent chain?
What is the name for the organic compound in frame 21?
Simpler alkanes can be named quickly, but alkanes can have multiple substituents. When you have an alkane with multiple substituents you will need to use Greek prefixes prior to the substituent name. Additionally, for every substituent you must have a number written with it, even if it is on the same carbon!
Remember the rule that if two substituents are on the same carbon, both numbers must be given. Try naming this alkane.
For your previous two alkanes, 2,3,4-trimethylpentane and 2,2-dimethylpropane, did the direction for numbering matter when identifying the parent chain?
If a structure has two or more different alkyl groups, they must be named alphabetically. Let's look at a compound that offers a bit more challenge. What is the parent name, the substituent names (and their locations) for the following alkane? Finally, what is the overall name for this molecule?
There is a rule for the order of naming multiple different substituents. The alkyl name is still listed by alphabetical order. For instance, butyl is written before ethyl. Ethyl will come before methyl, and methyl is listed before propyl. However, Greek prefixes do not play a role in the order. Consider the compound below. Its longest carbon chain is nine. Can you identify the longest carbon chain?
Here is a list of six alkane names. Which alkane name is written incorrectly? What is its correct name? Assume the carbon locations are correct (i.e., the numbering scheme) for each.
The variety of structures that arise from branched-chain alkanes allow for constitutional isomers (or structural isomers). These are compounds with the same molecular formula but different structural formulas.
Let's examine the two simplest alkane structural isomers, 2-methylpropane and butane.
How many structural isomers exist for an alkane having the molecular formula C4H10?
How many structural isomers exist for an alkane having the molecular formula C5H12? Make sure to draw and name all of these structural isomers.
Circle the alkane that is not a structural isomer for C6H14. Explain your choice.
Cyclics are another group of alkanes but their structure does not have ends to them. Instead they are connected into a ring structure. The simplest cyclic is cyclopropane. Cyclopropane is a three-carbon ring shaped like a triangle. Notice that when naming cyclics the prefix cyclo is included. The following shows the first four cyclics' dash formula, bond-line formula, and name.
What are the molecular formulas for cycloheptane and cyclooctane?
Alkenes are unsaturated hydrocarbons that possess at least one carbon–carbon double bond. They have the same general formula as cyclic hydrocarbons, CnH2n. The simplest alkene is ethene. Note the ending is no longer “–ane” but rather “–ene” describing an alkene. Below are the various ways to represent ethene.
What are the condensed and bond line formulas for 1-heptene, 2-heptene, and 3-heptene?
A fact about alkenes is that the C=C bond is not allowed to rotate, unlike the carbon–carbon single bonds found within alkanes. Because of that rotational restriction alkenes can give rise to geometric isomers. These geometric isomers result when there are two different groups located on the alkene's double-bonded carbons. Geometric isomers are a specific type of isomer where the atoms are joined to one another in the same manner but differ in their orientation in space.
Given the information you have just learned about cis and trans isomers, circle the alkene that represents cis-2-pentene.
Can cis–trans isomers be possible if one of the carbons of a double bond has two identical groups?
Name and draw the two geometric isomers for 3-hexene.
Although cis and trans isomers have the same molecular formula they have different physical (melting points and boiling points) and chemical properties.
Alkynes are unsaturated hydrocarbons that have at least one carbon–carbon triple bond. Alkynes have the formula CnH2n−2. The simplest alkyne is ethyne, which has two carbons, and according to the equation it will also have two hydrogen atoms. Below are the structures for ethyne, propyne, 1-butyne, and 2-butyne.
Naming alkynes is similar to alkenes. The triple bond is counted from the acyclic's closest end. Propyne's triple bond is not numbered since it gets a carbon-1 placement from either end of the molecule. However, butyne can have two possible locations for its triple bond. As such, both must be numbered to identify the two structural isomers.
There is a great deal of stored chemical energy found within the double bond of the alkene and even more energy found within the triple bond of the alkyne. Acetylene (C2H2) is an alkyne that burns so hot that it is used in welding.
Alkynes are either internal (found within the carbon chain of the molecule) or terminal (found at the end of the carbon chain). 3-Heptyne and 1-heptyne are alkyne isomers of one another. Which alkyne is the terminal alkyne and why?
Draw the bond-line structure for 2-hexyne.
Identify the structural isomers by circling them. Also, what are the isomers' molecular formula?
Aromatic hydrocarbons are compounds that have a six-membered ring with three alternating carbon–carbon single and double bonds. The simplest aromatic hydrocarbon is benzene, C6H6.
Aromatic rings are also found in a variety of natural products.
What is the molecular formula and condensed formula for propanol? Propanol is an alcohol with three carbons.
As the number of carbon atoms in alcohols increase so does their complexity. The three carbon-containing alcohols have the molecular formula C3H8O and can come in the form of two structural isomers.
Draw the line structure for 3-hexanol and state whether it is a primary, secondary, or tertiary alcohol.
Our next oxygen-containing functional group has the C–O–C connection. These are known as ethers. Ethers have been used in chemical reactions and in medicine. The simplest ether has two methyl groups connected by an oxygen atom. This compound is known as dimethyl ether. Below are the molecular formula, the condensed formula, the dashed formula, and the bond-line structure for dimethyl ether.
So far you have seen the alcohol, ether, ketone, and aldehyde functional groups. Circle and label the functional groups found in the following organic compounds.
The next two oxygen-containing functional groups have the carbonyl group with its carbon also connected to one other oxygen atom through a single bond. This oxygen atom is an sp3 hybridized oxygen atom. The R in the formulas below represent any alkyl group. The oxygen atom in –OH or –OR has four total bonding directions (two bonds and two lone pairs). The two lone pairs on the oxygen atom count as two bonding directions. However, what is connected to the other end of the single-bonded oxygen atom determines the functional group's identity. Consider the basic structure of the carboxylic acid and ester, shown below. Notice that the carboxylic acid single-bonded oxygen has a bond to hydrogen. This is unique to the carboxylic acid, and this group is often shortened to –COOH or –CO2H. As for the ester its sp3 hybridized oxygen is instead connected to an alkyl group.
Another type of organic compound has the nitrogen atom incorporated into its structure. As in the case with oxygen-containing organic compounds there are several types of nitrogen-containing functional groups. These include amines, imines, nitriles, and amides. These nitrogen-containing compounds have different chemical and physical properties than the oxygen-containing functional groups.
How many lone pairs exist on the nitrogen atom for any nitrogen atom with three single bonds? Also, what is the hybridization on the amine's N atom?
As mentioned, amines are bases, since they have a lone pair available to accept a proton (H+). After accepting a proton the amine becomes a substituted ammonium ion. In the example below methylamine (CH3NH2) deprotonates the water molecule to become the methylammonium ion 
and the hydroxide ion.
, conjugate base = OH−
, conjugate base = OH−
Diethylamine, (CH3CH2)2NH, has the dash line formula
Amines are one type of nitrogen-containing organic compounds. Remember, amines are nitrogen atoms with a lone pair on the central atom with three single bonds connected to alkyl groups. Just as hydrocarbons can have carbon–carbon single bonds, carbon–carbon double bonds, and carbon–carbon triple bonds so they can have multiple carbon–nitrogen bonds.
Is an imine basic? Why or why not? Also, what is the hybridization of the imine's nitrogen atoms?
Imines can be made from reacting an aldehyde or ketone with a 1° amine. For instance, methanal, an aldehyde, reacts with methyl amine to produce an imine and water.
Can you make an imine from diethylamine, (CH3CH2)2NH? Why or why not?
Think about the imine produced from this reaction. Why must the amine be a primary amine? Hint: What other product must be made?
The nitrile functional group contains a carbon–nitrogen triple bond with a lone pair on the nitrogen atom. The carbon atom of the nitrile connects to the alkyl group. The simplest nitrile is CH3CN, known as acetonitrile, which is a common organic solvent. The hybridization of the nitrile's N atom is sp.
where only the nitrogen atom is written. All of these provide insight into the arrangement of atoms with the nitrile.
Amides can be made by reacting nitriles with acid. The simplest nitrile, acetonitrile, can be made by reacting acetonitrile with acid. Treating acetonitrile with acid can generate acetamide.
