7.4: Alkanes and Alkane Isomers
After completing this section, you should be able to
- draw the Kekulé structure, condensed structure and shorthand structure of each of the first ten straight-chain alkanes.
- name each of the first ten straight-chain alkanes, given its molecular formula, Kekulé structure, condensed structure or shorthand structure.
- explain the difference in structure between a straight- and a branched-chain alkane, and illustrate the difference using a suitable example.
- explain why the number of possible isomers for a given molecular formula increases as the number of carbon atoms increases.
- draw all the possible isomers that correspond to a given molecular formula of the type C n H 2n +2 , where n is ≤ 7.
Make certain that you can define, and use in context, the key terms below.
- branched-chain alkane
- constitutional or structural isomer
- homologous series
- isomer
- saturated hydrocarbon
- straight-chain alkane (or normal alkane)
A series of compounds in which successive members differ from one another by a CH 2 unit is called a homologous series. Thus, the series CH 4 , C 2 H 6 , C 3 H 8 . . . C n H 2n +2 , is an example of a homologous series.
It is important that you commit to memory the names of the first 10 straight-chain alkanes (i.e., from CH 4 to C 10 H 22 ). You will use these names repeatedly when you begin to learn how to derive the systematic names of a large variety of organic compounds. You need not remember the number of isomers possible for alkanes containing more than seven carbon atoms. Such information is available in reference books when it is needed. When drawing isomers, be careful not to deceive yourself into thinking that you can draw more isomers than you are supposed to be able to. Remember that it is possible to draw each isomer in several different ways and you may inadvertently count the same isomer more than once.
Alkanes are organic compounds that consist entirely of single-bonded carbon and hydrogen atoms and lack any other functional groups. Alkanes are often called saturated hydrocarbons because they have the maximum possible number of hydrogens per carbon. In Section 1.7, thealkane molecule, ethane, was shown to contain a C-C sigma bond. By adding more C-C sigma bond larger and more complexed alkanes can be formed. Methane (CH 4 ), ethane (C 2 H 6 ), and propane (C 3 H 8 ) are the beginning of a series of compounds in which any two members in a sequence differ by one carbon atom and two hydrogen atoms—namely, a CH 2 unit. Any family of compounds in which adjacent members differ from each other by a definite factor (here a CH 2 group) is called a homologous series. The members of such a series, called homologs , have properties that vary in a regular and predictable manner.
Methane (CH 4 ), ethane (C 2 H 6 ), and propane (C 3 H 8 ) are the beginning of a series of compounds in which any two members in a sequence differ by one carbon atom and two hydrogen atoms—namely, a CH 2 unit. Consider the series in Figure 25.3.3. The sequence starts with C 3 H 8 , and a CH 2 unit is added in each step moving up the series. Any family of compounds in which adjacent members differ from each other by a definite factor (here a CH 2 group) is called a homologous series. The members of such a series, called homologs , have properties that vary in a regular and predictable manner.
Figure 25.3.2: Members of a Homologous Series. Each succeeding formula incorporates one carbon atom and two hydrogen atoms more than the previous formula.
The homologous series allows us to write a general formula for alkanes: C n H 2 n + 2 . Using this formula, we can write a molecular formula for any alkane with a given number of carbon atoms. For example, an alkane with eight carbon atoms has the molecular formula C 8 H (2 × 8) + 2 = C 8 H 18 .
Molecular Formulas
Alkanes are the simplest family of hydrocarbons - compounds containing carbon and hydrogen only. Alkanes only contain carbon-hydrogen bonds and carbon-carbon single bonds. The first six alkanes are as follows:
Table \(\PageIndex{1}\): Molecular formulas for small alkanes
| methane | CH 4 |
| ethane | C 2 H 6 |
| propane | C 3 H 8 |
| butane | C 4 H 10 |
| pentane | C 5 H 12 |
| hexane | C 6 H 14 |
You can work out the formula of any of the alkanes using the general formula C n H 2n +2
Isomerism
All of the alkanes containing 4 or more carbon atoms show structural isomerism, meaning that there are two or more different structural formulas that you can draw for each molecular formula. Isomers (from the Greek isos + meros , meaning "made of the same parts") are molecules that have the same molecular formula, but have a different arrangement of the atoms in space. Alkanes with 1-3 carbons, methane (CH 4 ), ethane (C 2 H 6 ), and propane (C 3 H 8 ,) do not exist in isomeric forms because there is only one way to arrange the atoms in each formula so that each carbon atom has four bonds. However, C 4 H 10 , has more than possible structure. The four carbons can be drawn in a row to form butane or the can branch to form isobutane. The two compounds have different properties—for example, butane boils at −0.5°C, while isobutane boils at −11.7°C.
Likewise the molecular formula: C 5 H 12 has three possible isomer. The compound at the far left is pentane because it has all five carbon atoms in a continuous chain. The compound in the middle is isopentane; like isobutane, it has a one CH 3 branch off the second carbon atom of the continuous chain. The compound at the far right, discovered after the other two, was named neopentane (from the Greek neos , meaning “new”). Although all three have the same molecular formula, they have different properties, including boiling points: pentane, 36.1°C; isopentane, 27.7°C; and neopentane, 9.5°C.
Of the structures show above, butane and pentane are called normal alkanes or straight-chain alkanes , indicating that all contain a single continuous chain of carbon atoms and can be represented by a projection formula whose carbon atoms are in a straight line. The other structures, isobutane, isopentane, and neopentane are called called branched-chain alkanes. As the number of carbons in an akane increases the number of possible isomers also increases as shown in the table below.
Table \(\PageIndex{2}\): Number of isomers for hydrocarbons
| Molecular Formula | Number of Structural Isomers |
|---|---|
| CH 4 | 1 |
| C 2 H 6 | 1 |
| C 3 H 8 | 1 |
| C 4 H 10 | 2 |
| C 5 H 12 | 3 |
| C 6 H 14 | 5 |
| C 7 H 16 | 9 |
| C 8 H 18 | 18 |
| C 9 H 20 | 35 |
| C 10 H 22 | 75 |
| C 14 H 30 | 1858 |
| C 18 H 38 | 60,523 |
| C 30 H 62 | 4,111,846,763 |
Akanes can be represented in many different ways. The figure below shows some of the different ways straight-chain butane can be represented. Most often chemists refer to butane by the condensed structure CH 3 CH 2 CH 2 CH 3 or n -C 4 H 10 where n denotes a normal straight alkane.
Note that many of these structures only imply bonding connections and do not indicate any particular geometry. The bottom two structures, referred to as "ball and stick" and "space filling" do show 3D geometry for butane. Because the four-carbon chain in butane may be bent in various ways the groups can rotate freely about the C–C bonds. However, this rotation does not change the identity of the compound. It is important to realize that bending a chain does not change the identity of the compound; all of the following represent the same compound, butane:
The nomenclature of straight alkanes is based on the number of carbon atoms they contain. The number of carbons are indicated by a prefix and the suffix -ane is added to indicate the molecules is an alkane. The prefix for three carbons is prop so adding -ane, the IUPAC name for C 3 H 8 is propane. Likewise, the prefix for six is hex so the name for the straight chain isomer of C 6 H 14 is called hexane. The first ten prefixes should be memorized, because these alkane names from the basis for naming many other organic compounds.
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| Molecular Formula | Prefix | Condensed Structural Formula | Name |
|---|---|---|---|
| CH 4 | Meth | CH 4 | methane |
| C 2 H 6 | Eth | CH 3 CH 3 | ethane |
| C 3 H 8 | Prop | CH 3 CH 2 CH 3 | propane |
| C 4 H 10 | But | CH 3 CH 2 CH 2 CH 3 | butane |
| C 5 H 12 | Pent | CH 3 CH 2 CH 2 CH 2 CH 3 | pentane |
| C 6 H 14 | Hex | CH 3 (CH 2 ) 4 CH 3 | hexane |
| C 7 H 16 | Hept | CH 3 (CH 2 ) 5 CH 3 | heptane |
| C 8 H 18 | Oct | CH 3 (CH 2 ) 6 CH 3 | octane |
| C 9 H 20 | Non | CH 3 (CH 2 ) 7 CH 3 | nonane |
| C 10 H 22 | Dec | CH 3 (CH 2 ) 8 CH 3 | decane |
Pentane, C 5 H 12 , has three chain isomers. If you think you can find any others, they are simply twisted versions of the ones below. If in doubt make some models.
Exercises
Draw all of the isomers for C 6 H 14 O that contain a 6 carbon chain and an alcohol (-OH) functional group.
- Answer
Draw all possible isomers for C 6 H 14 (There are five total).
- Answer
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The top structure is when it is a 6 carbon chain. The middle row contains the 5 carbon chained isomers with branching at the 2 nd and 3 rd carbon. The bottom row contains the two 4 carbon chain isomers that can be drawn.
Draw all possible isomers for C 3 H 8 O .
- Answer
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The first structure is when an alcohol comes off the first carbon. The second structure is when the alcohol is coming off the central carbon. The third structure is the only possible ether form of C 3 H 8 O .
Draw all possible isomers for C 4 H 8 O 2 that contain a carboxylic acid .
- Answer
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There are only 2 possibilities.
Draw all possible isomers for C 3 H 9 N and indicate whether each amine is primary, secondary, or tertiary.
- Answer
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The first and second structures are primary amines. The third structure is a secondary amine. The last structure is a tertiary amine.
Indicate whether each of the following sets are constitutional isomers, the same compound, or different compounds.
- Answer
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a) Both structures have formulas of C 10 H 22 and have different connectivity which makes these constitutional isomers .
b) Both structures have formulas of C 7 H 16 and have the same connectivity which makes these the same compound .
c) Both structure have formulas of C 7 H 16 and have different connectivity which makes these constitutional isomers .
d) The structure on the left has a formula of C 9 H 20 and the structure on the right has a formula of C 10 H 22 so these are different compounds .
Draw the 5 constitutional isomers of C 7 H 16 (of the 9 total isomers possible) that have 5 carbons as the longest carbon chain length.
- Answer
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The 5 constitutional isomers with a 5 carbon chain length are shown above. Since there needs to be 7 carbons total, the 2 extra carbons are added as substituents. From left to right, the methyl group substitution pattern is 2,2, 2,3, 2,4, and 3,3, and the last one (on right) has a 3-ethyl substituent.
The other 4 possible constitutional isomers (with different length carbon chains) are shown below.