Naming the Alkanes 2-5 Many common or trivial names are still used widely used to name certain alkanes. Systematic IUPAC names are more precise. The first 20 straight chain alkanes are:

An alkyl group is formed by removing a hydrogen from an alkane An alkyl group is formed by removing a hydrogen from an alkane. It is named by removing the –ane suffix and replacing it by –yl. CH3- methyl; CH3CH2- ethyl; CH3CH2CH2- propyl Additional prefixes are also used: sec- (or s-) for secondary, and tert- (or t-) for tertiary. A secondary carbon is directly attached to two other carbons. A tertiary carbon is directly attached to three other carbons.

Some common branched alkyl groups are:

For systematically naming branched alkanes, four IUPAC rules are used: IUPAC RULE 1: Find the longest chain (stem) in the molecule and name it. Groups other than hydrogen attached to this chain are called substituents. If the molecule has two or more stems of equal length, the one with the most substituents is the base stem chain.

IUPAC RULE 2: Name all groups attached to the longest chain as alkyl substituents. If a substituents chain is branched, find the longest chain in the substituents and then name all of its substituents.

IUPAC RULE 3: Number the carbons of the longest chain beginning with the end closest to a substituents. If there are two substituents at equal distance from the ends of the chain, assign the lower number to the substituents coming first in alphabetical order.

If there are three or more substituents, the chain is numbered to give the lower number at the first difference between the two possible numbering schemes (first point of difference principle). Substituents are numbered outward from the main chain. C1 will be the carbon attached to the main stem.

IUPAC RULE 4: Write the name of the alkane by first arranging all the substituents in alphabetical order. Precede each with the carbon number to which it is attached (to the stem) and a hyphen. When a molecule contains more than one instance of a particular substituents, precede the name by the attachment positions separated by commas and a prefix: di, tri, tetra, etc. In general, the Greek prefixes, sec- and tert- are not considered in the alphabetical ordering process.

If a particular complex substituents is present more than once, the prefixes bis, trix, tetrakis, pentakis, etc. are used. Remember the substituents carbon number 1 is the one attached to the stem chain.

Haloalkanes are named treating the halogen as a substituents to the longest stem, as for other substituents: