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Alkyl

What is an alkyl group?

Alkyl group is formed by removing a hydrogen atom from the molecule of alkane. Alkanes are quite often represented as R-H and here R stands for alkyl group.

The general formula of the alkyl group is CnH2n+1. The smallest alkyl group is CH3 called methyl.

A cycloalkyl is derived from a cycloalkane by removal of a hydrogen atom from a ring. The general formula of cycloalkyl is CnH2n-1.

Nomenclature of alkyl groups

An alkyl group is named by replacing the suffix “ane” of the alkane with the suffix – “yl”.

Example:

CH4 – H → CH3

(Methane) (Methyl)

Number of carbon atom
Word Root
Name of alkyl group
Structure of alkyl group

1
C1
Meth
Methyl
CH3–

2
C2
Eth
Ethyl
CH3-CH2–

3
C3
Prop
Propyl
CH3-CH2-CH2–

4
C4
But
Butyl
CH3-CH2-CH2-CH2–

5
C5
Pent
Pentyl
CH3-CH2-CH2-CH2-CH2–

6
C6
Hex
Hexyl
CH3-CH2-CH2-CH2-CH2-CH2–

7
C7
Hept
Heptyl
CH3-CH2-CH2-CH2-CH2-CH2-CH2–

8
C8
Oct
Octyl
CH3-CH2-CH2-CH2-CH2-CH2-CH2-CH2–

9
C9
Non
Nonyl
CH3-CH2-CH2-CH2-CH2-CH2-CH2-CH2-CH2–

10
C10
Dec
Decyl
CH3-CH2-CH2-CH2-CH2-CH2-CH2-CH2-CH2-CH2–

Formation of alkyl groups

Alkyl group is formed by removing a hydrogen atom from the molecules of alkane. It may be noted that an alkane molecule can lead to more than one alkyl group depending upon the nature of the carbon atoms from which hydrogen is to be removed. For example,

(1) In ethane (C2H6), both the carbon atoms are primary so only one alkyl group can be formed.

(2) In Propane (C3H8), there are two types of carbon atoms present therefore, two different alkyl groups can be formed.

The secondary propyl group is called the isopropyl group.

(3) n-Butane has also two types of carbon atoms that give two alkyl groups.

(4) Iso-Butane has primary and tertiary carbon so it gives the following alkyl groups.

The tertiary butyl group is also called the neo butyl group.

Alkyl groups are attached to other atoms or groups of atoms. Free alkyls occur as neutral compounds, as anions, or as cations. Such species are usually encountered only as transient intermediates, but some are quite stable.

Alkyl cations

In the alkyl carbocation the carbon atom contains six electrons in the valence shell to form a positively charged species. Depending upon the nature of carbon atoms these are classified into three types primary, secondary and tertiary carbocation.

Alkyl anions

In the alkyl anion or carbanion, the carbon atom containing eight electrons in the valence shell forms a negatively charged species. These are

Alkyl radicals

In the alkyl free radical carbon contains seven electrons in the valence shell. Depending upon the nature of carbon atoms bearing unpaired electrons, alkyl free radical is classified as primary, secondary or tertiary in nature. Examples are: