7.5.1: Naming Alkanes
After completing this section, you should be able to
- provide the correct IUPAC name for any given alkane structure (Kekulé, condensed or shorthand).
- draw the Kekulé, condensed or shorthand structure of an alkane, given its IUPAC name.
Make certain that you can define, and use in context, the key term below.
- IUPAC system
The IUPAC system of nomenclature aims to ensure
- that every organic compound has a unique, unambiguous name.
- that the IUPAC name of any compound conveys the structure of that compound to a person familiar with the system.
One way of checking whether the name you have given to an alkane is reasonable is to count the number of carbon atoms implied by the chosen name. For example, if you named a compound 3‑ethyl-4‑methylheptane, you have indicated that the compound contains a total of 10 carbon atoms—seven carbon atoms in the main chain, two carbon atoms in an ethyl group, and one carbon atom in a methyl group. If you were to check the given structure and find 11 carbon atoms, you would know that you had made a mistake. Perhaps the name you should have written was 3‑ethyl-4,4‑dimethylheptane!
When naming alkanes, a common error of beginning students is a failure to pick out the longest carbon chain. For example, the correct name for the compound shown below is 3‑methylheptane, not 2‑ethylhexane.
Remember that every substituent must have a number, and do not forget the prefixes: di, tri, tetra, etc.
You must use commas to separate numbers, and hyphens to separate numbers and substituents. Notice that 3‑methylhexane is one word.
Hydrocarbons having no double or triple bond functional groups are classified as alkanes or cycloalkanes, depending on whether the carbon atoms of the molecule are arranged only in chains or also in rings. Although these hydrocarbons have no functional groups, they constitute the framework on which functional groups are located in other classes of compounds, and provide an ideal starting point for studying and naming organic compounds. The alkanes and cycloalkanes are also members of a larger class of compounds referred to as aliphatic. Simply put, aliphatic compounds are compounds that do not incorporate any aromatic rings in their molecular structure.
The following table lists the IUPAC names assigned to simple continuous-chain alkanes from C-1 to C-10. A common “ane” suffix identifies these compounds as alkanes. Longer chain alkanes are well known, and their names may be found in many reference and text books. The names methane through decane should be memorized, since they constitute the root of many IUPAC names. Fortunately, common numerical prefixes are used in naming chains of five or more carbon atoms.
| Name | Molecular Formula |
Structural Formula |
Isomers | Name | Molecular Formula |
Structural Formula |
Isomers | |
|---|---|---|---|---|---|---|---|---|
| methane | CH4 | CH4 | 1 | hexane | C6H14 | CH3(CH2)4CH3 | 5 | |
| ethane | C2H6 | CH3CH3 | 1 | heptane | C7H16 | CH3(CH2)5CH3 | 9 | |
| propane | C3H8 | CH3CH2CH3 | 1 | octane | C8H18 | CH3(CH2)6CH3 | 18 | |
| butane | C4H10 | CH3CH2CH2CH3 | 2 | nonane | C9H20 | CH3(CH2)7CH3 | 35 | |
| pentane | C5H12 | CH3(CH2)3CH3 | 3 | decane | C10H22 | CH3(CH2)8CH3 | 75 |
- The formulas and structures of these alkanes increase uniformly by a CH2 increment.
- A uniform variation of this kind in a series of compounds is called homologous.
- These formulas all fit the CnH2n+2 rule. This is also the highest possible H/C ratio for a stable hydrocarbon.
- Since the H/C ratio in these compounds is at a maximum, we call them saturated (with hydrogen).
Beginning with butane (C4H10), and becoming more numerous with larger alkanes, we note the existence of alkane isomers. For example, there are five C6H14 isomers, shown below as abbreviated line formulas (A through E):
Although these distinct compounds all have the same molecular formula, only one (A) can be called hexane. How then are we to name the others?
The IUPAC system requires first that we have names for simple unbranched chains, as noted above, and second that we have names for simple alkyl groups that may be attached to the chains. Examples of some common alkyl groups are given in the following table. Note that the “ane” suffix is replaced by “yl” in naming groups. The symbol R is used to designate a generic (unspecified) alkyl group.
| Group | CH3– | C2H5– | CH3CH2CH2– | (CH3)2CH– | CH3CH2CH2CH2– | (CH3)2CHCH2– | CH3CH2CH(CH3)– | (CH3)3C– | R– |
|---|---|---|---|---|---|---|---|---|---|
| Name | Methyl | Ethyl | Propyl | Isopropyl | Butyl | Isobutyl | sec-Butyl | tert-Butyl | Alkyl |
IUPAC Rules for Alkane Nomenclature
- Find and name the longest continuous carbon chain.
- Identify and name groups attached to this chain.
- Number the chain consecutively, starting at the end nearest a substituent group.
- Designate the location of each substituent group by an appropriate number and name.
- Assemble the name, listing groups in alphabetical order.
- The prefixes di, tri, tetra etc., used to designate several groups of the same kind, are not considered when alphabetizing
The IUPAC names of the isomers of hexane are: A hexane B 2-methylpentane C 3-methylpentane D 2,2-dimethylbutane E 2,3-dimethylbutane
Halogen Groups
Halogen substituents are easily accommodated, using the names: fluoro (F-), chloro (Cl-), bromo (Br-) and iodo (I-).
For example, (CH3)2CHCH2CH2Br would be named 1-bromo-3-methylbutane. If the halogen is bonded to a simple alkyl group an alternative “alkyl halide” name may be used. Thus, C2H5Cl may be named chloroethane (no locator number is needed for a two carbon chain) or ethyl chloride.
Alkyl Groups
Alkanes can be described by the general formula CnH2n+2. An alkyl group is formed by removing one hydrogen from the alkane chain and is described by the formula CnH2n+1. The removal of this hydrogen results in a stem change from -ane to -yl. Take a look at the following examples.
The same concept can be applied to any of the straight chain alkane names provided in the table below.
| Name | Molecular Formula | Condensed Structural Formula |
|---|---|---|
| Methane | CH4 | CH4 |
| Ethane | C2H6 | CH3CH3 |
| Propane | C3H8 | CH3CH2CH3 |
| Butane | C4H10 | CH3(CH2)2CH3 |
| Pentane | C5H12 | CH3(CH2)3CH3 |
| Hexane | C6H14 | CH3(CH2)4CH3 |
| Heptane | C7H16 | CH3(CH2)5CH3 |
| Octane | C8H18 | CH3(CH2)6CH3 |
| Nonane | C9H20 | CH3(CH2)7CH3 |
| Decane | C10H22 | CH3(CH2)8CH3 |
| Undecane | C11H24 | CH3(CH2)9CH3 |
| Dodecane | C12H26 | CH3(CH2)10CH3 |
| Tridecane | C13H28 | CH3(CH2)11CH3 |
| Tetradecane | C14H30 | CH3(CH2)12CH3 |
| Pentadecane | C15H32 | CH3(CH2)13CH3 |
| Hexadecane | C16H34 | CH3(CH2)14CH3 |
| Heptadecane | C17H36 | CH3(CH2)15CH3 |
| Octadecane | C18H38 | CH3(CH2)16CH3 |
| Nonadecane | C19H40 | CH3(CH2)17CH3 |
| Eicosane | C20H42 | CH3(CH2)18CH3 |
Three Rules for Naming Alkanes
- Choose the longest, most substituted carbon chain containing a functional group.
- A carbon bonded to a functional group must have the lowest possible carbon number. If there are no functional groups, then any substituent present must have the lowest possible number.
- Take the alphabetical order into consideration; that is, after applying the first two rules given above, make sure that your substituents and/or functional groups are written in alphabetical order.
What is the name of the follow molecule?
Solution
Rule #1: Choose the longest, most substituted carbon chain containing a functional group. This example does not contain any functional groups, so we only need to be concerned with choosing the longest, most substituted carbon chain. The longest carbon chain has been highlighted in blue and consists of eight carbons.
Rule #2: Carbons bonded to a functional group must have the lowest possible carbon number. If there are no functional groups, then any substitute present must have the lowest possible number. Because this example does not contain any functional groups, we only need to be concerned with the two substitutes present, that is, the two methyl groups. If we begin numbering the chain from the left, the methyls would be assigned the numbers 4 and 7, respectively. If we begin numbering the chain from the right, the methyls would be assigned the numbers 2 and 5. Therefore, to satisfy the second rule, numbering begins on the right side of the carbon chain as shown below. This gives the methyl groups the lowest possible numbering.
Rule 3: In this example, there is no need to utilize the third rule. Because the two substitutes are identical, neither takes alphabetical precedence with respect to numbering the carbons. This concept will become clearer in the following examples.
The name of this molecule is 2,5-dimethyloctane
What is the name of the follow molecule?
Solution
Rule #1: Choose the longest, most substituted carbon chain containing a functional group. This example contains two functional groups, bromine and chlorine. The longest carbon chain has been highlighted in blue and consists of seven carbons.
Rule #2: Carbons bonded to a functional group must have the lowest possible carbon number. If there are no functional groups, then any substituent present must have the lowest possible number. In this example, numbering the chain from either the left or the right would satisfy this rule. If we number the chain from the left, bromine and chlorine would be assigned the second and sixth carbon positions, respectively. If we number the chain from the right, chlorine would be assigned the second position and bromine would be assigned the sixth position. In other words, whether we choose to number from the left or right, the functional groups occupy the second and sixth positions in the chain. To select the correct numbering scheme, we need to utilize the third rule.
Rule #3: After applying the first two rules, take the alphabetical order into consideration. Alphabetically, bromine comes before chlorine. Therefore, bromine is assigned the second carbon position, and chlorine is assigned the sixth carbon position.
The name of this molecule is: 2-bromo-6-chloroheptane
What is the name of the follow molecule?
Solution
Rule #1: Choose the longest, most substituted carbon chain containing a functional group. This example contains two functional groups, bromine and chlorine, and one substitute, the methyl group. The longest carbon chain has been highlighted in blue and consists of seven carbons.
Rule #2: Carbons bonded to a functional group must have the lowest possible carbon number. After taking functional groups into consideration, any substitutes present must have the lowest possible carbon number. This particular example illustrates the point of difference principle. If we number the chain from the left, bromine, the methyl group and chlorine would occupy the second, fifth and sixth positions, respectively. This concept is illustrated in the second drawing below. If we number the chain from the right, chlorine, the methyl group and bromine would occupy the second, third and sixth positions, respectively, which is illustrated in the first drawing below. The position of the methyl, therefore, becomes a point of difference. In the first drawing, the methyl occupies the third position. In the second drawing, the methyl occupies the fifth position. To satisfy the second rule, we want to choose the numbering scheme that provides the lowest possible numbering of this substitute. Therefore, the first of the two carbon chains shown below is correct.
Therefore, the first numbering scheme is the appropriate one to use.
Once you have determined the correct numbering of the carbons, it is often useful to make a list, including the functional groups, substitutes, and the name of the parent chain.
Rule #3: After applying the first two rules, take the alphabetical order into consideration. Alphabetically, bromine comes before chlorine. Therefore, bromine is assigned the second carbon position, and chlorine is assigned the sixth carbon position.
Parent chain: heptane Substitutents: 2-chloro 3-methyl 6-bromo
The name of this molecule is: 6-bromo-2-chloro-3-methylheptane
Exercises
Give the proper IUPAC names of the following compounds.
- Answer
-
a) Since this structure is an unbranched alkane (all single bonds) with a 5 carbon chain length, its name would be pentane.
b) This alkane has a 7 carbon longest continuous chain length that we number from right to left to get the first methyl substituent we encounter to have the lowest possible number (3 versus being 4 if numbering from left to right). This causes it to have 2 methyl substituents at positions 3 & 4 so we would name it indicating those numbers and the prefix dimethyl which gives a proper IUPAC name of 3,4-dimethylheptane.
c) This alkane has a 5 carbon longest continuous chain length (which could be numbered from left to right or right to left due to the symmetry at C-3). It has two methyl substituents off of C-3 so the proper IUPAC name is 3,3-dimethylpentane.
Give the proper IUPAC names of the following compounds.
- Answer
-
a) This alkane has a 9 carbon longest continuous chain length that we number from left to right (structure on left numbered in blue) to make the ethyl substituent be number 4. For the structure on the right (numbered in red) going from right to left, the methyl substituent is number 4. Since ethyl is higher in the alphabetical order, you want to make it have the lower number so the structure on the left (blue numbering) takes priority and the name is 4-ethyl-6-methylnonane.
b) This alkane has a 6 carbon longest continuous chain length that we number from right to left to make the first methyl be C-2 (versus the opposite direction which would make the first methyl C-3). Since there are 3 methyl substituents at positions 2,3, & 4, this compound would have the name 2,3,4-trimethylhexane.
c) This 6 carbon alkane can be numbered along different chains (see below) as well as in the opposite directions. This shows the two different chains that can be drawn (making the first substituent in that chain the lowest number). The structure on the left (numbered in blue) is the correct choice since it causes more substituents to be on the longest continuous chain (3 vs 2 in the structure on the right). This would make the IUPAC name of the structure 3-ethyl-2,4-dimethylhexane. (Notice how ethyl takes priority over methyl and the di- is not considered for alphabetizing.)
All of the following names represent a compound that has been named improperly. Draw out the structure from the name and give the proper IUPAC name for the compounds.
- 1,3-dimethylbutane
- 4-ethylpentane
- 2-ethyl-3-methylpentane
- Answer
-
a) The structure that can be drawn for the improper name is shown below on the left. When you renumber it properly (structure on the right), the correct name should be 2-methylpentane.
b) The structure that can be drawn for the improper name is shown below on the left. When you renumber it properly (structure on the right), notice that the longest chain is 6 C’s and we start the numbering on the end to the right to make the methyl substituent come off at C-3 (instead of beong at C-4 if we numbered it the opposite direction) the correct name should be 3-methyl hexane.
c) The structure that can be drawn for the improper name is shown below on the left. When you renumber it properly (structure on the right), notice that the longest chain is 6 C’s and since this molecule is symmetrical (between carbon 3 & 4), you can start the numbers from either end. In this case, we have methyl substituents coming off of carbons 3 & 4 so the proper name is 3,4-dimethyl hexane.
All of the following names represent a compound that has been named improperly. Draw out the structure from the name and give the proper IUPAC name for the compounds.
- 2,2-diethylheptane
- 2-propylpentane
- 4,4-diethylbutane
- Answer
-
a) The structure that can be drawn for the improper name is shown below on the left. When you renumber it properly (structure on the right), notice that the longest chain is now 8 C’s and you have an ethyl substituent at C-3 and a methyl substituent also at C-3 so the proper name is 3- ethyl-3-methyloctane.
b) The structure that can be drawn for the improper name is shown below on the left. When you renumber it properly (structure on the right), notice that the longest chain is now 7 C’s and since this molecule is symmetrical (at carbon 4), you can start the numbers from either end. There is a methyl substituent at C-4 so the proper name is 4-methylheptane.
c) The structure that can be drawn for the improper name is shown below on the left. When you renumber it properly (structure on the right) going from right to left (to make the ethyl substituent have the lowest number possible), the correct name is 3-ethylhexane
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