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Alkyl Groups

Whenever there is a combination of two or more different elements to form a molecule which has its own characteristic individual properties we term it as a compound. A compound can be divided in to its individual components to show the nature of bonding.

In inorganic compounds there are mostly cations and anions due to their electrovalent or ionic bondage. The organic compounds bonded in covalent bond always split in to units with an unshared electron. Such a part of the  molecule is called free radical. When the bond is polar covalent the split depends on the polarity of the shared pair of electrons. In such cases the cation and anions are formed.
When the split is covalent and free radical is the product then the free radical with the carbon or carbon chain is called an alkyl free radical and the group is called the alkyl group. If the split is ionic then the carbon containing ion is either carbocation or carbanion. In this topic we will be discussing the alkyl free radicals and alkyl groups in some detail.

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Alkyl Group Definition

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The alkyl group definition can be given as the unstable unit or reactive unit of a hydrogen containing carbon atom or chain which is possessing an unpaired electron from a broken bond between another similar unit or different unit as a result of homolytic fission.

The name is in general alkyl but depending on the number of carbon atoms and their position the particular name is given to the radical. For example 3-methyl heptane will be called 3-methyl heptyl radical.

Alkyl Chain

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When the carbon atom which has an unshared electron is connected with other carbon atom or carbon atoms a carbon chain compound is formed. If the unshared electron is attached to the terminal carbon atom and the chain is straight it is termed as a normal alkyl chain.

For example, n-butyl radical is CH3-CH2-CH2-CH2 and CH3-(CH2)7-CH2 is n-nonyl radical. If the chain is branched then the alkyl chain gets the name according to the chain. Such branched alkyl chains can be iso alkyl and neo alkyl chains. When a hydrogen atom from any carbon other than the terminal carbon of a carbon chain is removed, branched chain alkyl groups are obtained.

For example,
Iso Alkyl Group

When two -CH3 branches are present at the end of the chain the alkyl group in designated as neo alkyl group. This is the alkyl group in which the second last carbon atom in the chain is branched to two -CH3 groups.

Neo Alkyl Group


If the unshared electron is there not on the terminal carbon atom but on the middle carbon atoms which has already an alkyl group, the carbon atom is called secondary carbon atom (CH) and the radical is termed as sec-alkyl radical.
If the middle carbon atom is already with two alkyl groups, the carbon atom is called tertiary carbon atom (C) and the radical is called Ter-alkyl radical. When the name of the alkyl group has the prefix sec- before it indicates that the H-atom was removed from a secondary carbon atom.
Sec Alkyl Group


The prefix 'tert'- before the name of the alkyl group indicates that the H-atom has been removed from a tertiary carbon atom.

Tert Alkyl Group

Formation of alkyl groups is shown below.

Alkyl Group Formation



Alkyl Groups Electron Donating

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Alkyl groups that are electron donating are formed by the heterolytic fission of the molecule. They are called carbanions. In these species the bonded pair of electrons stays with it leaving a proton.

An ion in which the carbon bears the negative charge is called carbon anion or carbanion. The carbanion is formed by the removal of acidic hydrogen ion(H+).

R-CH2-H R-CH2- + H+
(or)
R-COOH
R-COO- + H+ ; R-COO- on heating R- + CO2

or when a negative ion attacks a double bonded compound.

R-CH=CH2 + X- R-CH¯- CH2X

Carbanions can be primary, secondary and tertiary depending on the carbon atom on which the negative charge is present.

Alkyl Groups Electron Withdrawing

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Alkyl groups that are electron withdrawing are formed when a positive charged ion adds to the double bond. These are called carbocations.
  1. Carbonyl carbon - C=O + H+ CH+-OH
  2. Alkene - C = C + H+ C+—CH
  3. Alcohol - R—OH + H+ ⇔ R— OH2+; R— OH2+ ⇔ R+ + H2O
  4. Halides - R—X + H+ ⇔ R— X-H+; R— X-H+ ⇔ R+ + HX
Carbocations are also primary, secondary and tertiary depending upon the carbon atom on which the positive charge is present.

More topics in Alkyl Groups
Naming Alkyl Halides Alkyl Halides
Alkyl Chloride Carbonyl Group
Alkyl Iodide
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