CK-12-Chemistry Intermediate

http://www.ck12.org Chapter 25. Organic Chemistry

TABLE25.1:(continued)

Name Molecular Formula Condensed Structural

Formula

Boiling Point (°C)

Heptane C 7 H 16 CH 3 CH 2 CH 2 CH 2 CH 2 CH 2 CH98.5 3

Octane C 8 H 18 CH 3 CH 2 CH 2 CH 2 CH 2 CH 2 CH125.6 2 CH 3

Nonane C 9 H 20 CH 3 CH 2 CH 2 CH 2 CH 2 CH 2 CH150.7 2 CH 2 CH 3

Decane C 10 H 22 CH 3 CH 2 CH 2 CH 2 CH 2 CH 2 CH174.1 2 CH 2 CH 2 CH 3

Note that the table shows a variation of a structural formula called a condensed structural formula. In this formula,
the covalent bonds are understood to exist between each carbon and the hydrogens associated with it as well as
between carbons. The table above (Table25.1) also shows that the boiling points of the alkanes steadily increase as
the length of the carbon chain increases. This is due to an increase in the strength of the attractive London dispersion
forces between molecules, which are the only relevant intermolecular forces for nonpolar hydrocarbons.

Branched Alkanes

Beginning with butane, there is an alternate structure possible that is not a straight chain. The structural formula
below shows a four-carbon structure consisting of a three-carbon chain with a –CH 3 group attached to the middle
carbon.

The official name of this molecule is 2-methylpropane. Note that the molecular formula is still C 4 H 10 , which is the
same as that of butane. Astructural isomeris one of multiple molecules that have the same molecular formula but
different structural formulas. Butane and 2-methylpropane are structural isomers. Except for the simplest molecules,
nearly all organic molecules have multiple potential isomers, which means that a chemical formula is not sufficient
to uniquely identify most organic compounds.

2-methylpropane is an example of a branched alkane. The official IUPAC system of nomenclature for branched
alkanes follows a set of steps that will be outlined in the following example.

1. Find the longest continuous chain of carbon atoms in the molecule. This is called the parent chain. In the
   example above, the longest chain is eight carbon atoms, so the parent hydrocarbon is octane.


2. Number the carbon atoms in the parent chain. To do this, start at the end that will give the smallest numbers
   possible to the carbon atoms where the branches originate. In the example above, the branches are on carbons