5 Steps to a 5 AP Chemistry

Organic Chemistry  269

alkanes have the general formula of CnH 2 n+ 2 , whereas the cyclic alkanes have the general

formula of CnH 2 n. The nstands for the number of carbon atoms in the compound. The

first 10 straight-chained alkanes are shown in Table 18.1.

There can be many more carbon units in a chain than are shown in Table 18.1, but

these are enough to allow us to study alkane nomenclature: the naming of alkanes.

Alkane Nomenclature

The naming of alkanes is based on choosing the longest carbon chain in the structural for-

mula, then naming the hydrocarbon branches while indicating onto which carbon that

branch is attached. Here are the specific rules for naming simple alkanes:

1. Find the continuous carbon chain in the compound that contains the most carbon
   atoms. This will provide the base name of the alkane.


2. This base name will be modified by adding the names of the branches (substituent
   groups) in front of the base name. Alkane branches are named by taking the name of
   the alkane that contains the same number of carbon atoms, dropping the –aneending
   and adding –yl. Methane becomes methyl, propane becomes propyl, etc. If there is
   more than one branch, list them alphabetically.

Table 18.1 The First Ten Straight-Chained Alkanes

NAME MOLECULAR FORMULA STRUCTURAL FORMULA

methane CH 4 CH 4

ethane C 2 H 6 CH 3 -CH 3

propane C 3 H 8 CH 3 -CH 2 -CH 3

butane C 4 H 10 CH 3 -CH 2 -CH 2 -CH 3

pentane C 5 H 12 CH 3 -CH 2 -CH 2 -CH 2 -CH 3

hexane C 6 H 14 CH 3 -CH 2 -CH 2 -CH 2 -CH 2 -CH 3

heptane C 7 H 16 CH 3 -CH 2 -CH 2 -CH 2 -CH 2 -CH 2 -CH 3

octane C 8 H 18 CH 3 -CH 2 -CH 2 -CH 2 -CH 2 -CH 2 -CH 2 -CH 3

nonane C 9 H 20 CH 3 -CH 2 -CH 2 -CH 2 -CH 2 -CH 2 -CH 2 -CH 2 -CH 3

decane C 10 H 22 CH 3 -CH 2 -CH 2 -CH 2 -CH 2 -CH 2 -CH 2 -CH 2 -CH 2 -CH 3

3. The position where a particular substituent is attached to the chain is indicated by a
   location number. These numbers are assigned by consecutively numbering the carbons
   of the base hydrocarbon, starting at one end of the hydrocarbon chain. Choose the end
   that will result in the lowest sum of location numbers for the substituent groups. Place
   this location number in front of the substituent name and separate it from the name by
   a hyphen (for example, 2-methyl).


4. Place the substituent names with their location numbers in front of the base name of
   the alkane in alphabetical order. If there are identical substituents (two methyl groups,
   for example), give the location numbers of each, separated by commas using the
   common Greek prefixes (di-, tri-, tetra-, etc.) to indicate the number of identical sub-
   stituent groups (i.e. 2,3-dimethyl). These Greek prefixes are not considered in the
   alphabetical arrangement.