Alkyl Groups or Alkyl Radicals

 

Alkyl Groups or Alkyl Radicals

 

Definition of Alkyl Radicals                                                                     

Alkyl groups are the basic structural unit of all aliphatic organic compounds.

 

The residual hydrocarbon group or radical left after the removal of a hydrogen atom from a saturated hydrocarbon alkane is called an alkyl group or radical. Stated differently, the radicals obtained from alkanes by the removal of one hydrogen atom are called alkyl group or radical.

 

Alkyl radicals are derivatives of alkanes. They are generally represented by ‘R–'. 

General Formula of Alkyl Radicals

All alkyl radicals are expressed by a general formula C_nH_2n+1.

 

Type Formula of Alkyl Radicals

All alkyl radicals are expressed by a type formula R.

 

Nature of Alkyl Radicals

Alkyl groups are monovalent radical.

 

Suffix for Alkyl Radical

The name of an alkyl radical is derived from the parent alkane by substituting ane of alkane by yl.

 

Isomerism in Alkyl Groups

Propyl and higher alkyl groups exist in two or more isomeric forms.

 

Italicized Prefixes

Note the usage of the prefixes iso-, neo-, sec-, and tert-, and note when italics are employed. Italicized prefixes are never involved in alphabetization, except among themselves; thus sec-butyl would precede isobutyl, isohexyl would precede isopropyl, and sec-butyl would precede tert-butyl.

 

Names and molecular formulae of alkanes and their alkyl radicals

 

Common alkyl groups

 

(i)       Methyl Group;

H₃C–    OR       CH₃–                           

 

(ii)      Ethyl Group;  

H₃C–CH₂–  or  C₂H₅–

 

Propyl Radical; C₃H₇– (It may exist in two isomeric forms)

Propyl radical is obtained from propane either by the removal of its terminal H or central (second C) H.  Propane has a straight chain structure. When terminal H is removed, it is called n-propyl. When hydrogen from central carbon is removed, it is called isopropyl, as explained below:

 

Butyl Group; (C₄H₉–); It may exist in 4 isomeric forms:

It is derived from butane (C₄H₁₀) having formula C₄H₉–.It may exist in 4 isomeric forms.

Butane has two isomers namely strength chain n-butane and branched chain iso-butane. When terminal H is removed from n-butane, it is called n-butyl. When hydrogen from central carbon is removed from n-butane, it is called sec-butyl. When terminal H is removed from iso-butane, it is called iso-butyl. When hydrogen from central carbon is removed from n-butane, it is called ter-butyl. 

(v)      Pentyl Group or Amyl group (C₅H₁₁–); It may exist in 8 isomeric forms:     

In organic chemistry, pentyl is a five-carbon alkyl group (substituent) with chemical formula –C₅H₁₁. It is the substituent form of the alkane pentane. In older literature, the common non-systematic name "amyl" was often used for the pentyl group. Conversely, the name pentyl was used for several five-carbon branched alkyl groups, distinguished by various prefixes. (A cyclopentyl group is a ring with the formula -C₅H₉). 

Pentyl group (C₅H₁₁–) has the following 8 isomers out of them 6 have special trivial names. Out of 8 isomers, 4 are primary, 3 are secondary and 1 is tertiary: