Alkyl Group

Alkanes from which one atom of hydrogen has been removed become monovalent radicals. These radicals, which are molecular fragments having an unpaired electron, are known as alkyl groups. The names of the alkyl groups are formed by substituting the suffix -yl for -ane in the names of the alkanes from which they are derived.

The methyl group (CH 3 ) is formed from methane, CH₄. The ethyl group, C₂H 5 , is formed from ethane, C₂H₆. Two different alkyl groups can be formed from propane, CH₃CH₂CH₃. Removal of a hydrogen atom from one of the carbon atoms at the end of the chain forms C₃H 7 . This CH₃CH₂CH 2 group is called a normal propyl group (n-propyl group). Removing a hydrogen from the second carbon produces an isopropyl group (i-propyl group).

The next member of the alkanes has the formula C₄H₁₀. There are four isomers of this molecular formula. Removal of a hydrogen from one of the end carbons in n-butane, CH₃CH₂CH₂CH₃, produces the n-butyl group (CH₃CH₂CH₂CH 2 ). Removing a hydrogen atom from carbon 2 or 3 produces the secondary butyl group (sec-butyl group; CH CH 3 2CH(CH₃) ). Removal of a hydrogen from carbon atoms 1, 3, or 4 forms the isobutyl group (i-butyl group; (CH₃)₂CHCH 2 ). Finally, removing a hydrogen from carbon 2 gives the tertiary butyl group (t-butyl group; (CH ) 3 3C ).

Frequently the terms primary, secondary, and tertiary are used to describe carbon atoms in a molecule. A primary carbon atom is one that is attached to only one other carbon atom; a secondary carbon atom is attached to two other carbon atoms; and a tertiary carbon atom is attached to three other carbon atoms in the molecule. Thus the ethyl group defined above is a primary group, since the carbon atom that has lost the hydrogen is attached to only one other carbon atom in the molecule. The n-propyl group is also a primary group for the same reason. But the i-propyl group is a secondary group, because the central carbon atom from which the hydrogen has been removed is attached to two other carbon atoms. The n-butyl and i-butyl groups are both primary groups; the secondary butyl group is a secondary group; and the tertiary butyl group is tertiary, because the central atom is attached to the three other carbon atoms in the molecule. The methyl group, however, cannot be defined using this classification scheme.

In general, the letter R is used to designate any alkyl group (R = CH₃, C₂H₅, etc.). With this convention, the alkanes are represented by the general formula R-H, and the alkyl halides by R-X, where X is a halogen.