5 Steps to a 5 AP Chemistry 2019

Organic Chemistry ❮ 271

Alkanes

Alkanes are members of a family of organic compounds called hydrocarbons, compounds

of carbon and hydrogen. These hydrocarbons are the simplest of organic compounds, but

are extremely important to our society as fuels and raw materials for chemical industries.

We heat our homes and run our automobiles through the combustion (burning) of these

hydrocarbons. Paints, plastics, and pharmaceuticals are often made from hydrocarbons.

Alkanes are hydrocarbons that contain only single covalent bonds within their molecules.

They are called saturated hydrocarbons because they are bonded to the maximum number

of other atoms. These alkanes may be straight-chained hydrocarbons, in which the carbons

are sequentially bonded; branched hydrocarbons, in which another hydrocarbon group is

bonded to the hydrocarbon “backbone”; or they may be cyclic, in which the hydrocarbon

is composed entirely or partially of a ring system. The straight-chained and branched

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

formula of CnH 2 n. The n stands 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

formula, 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 -ane ending
   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 10 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