Mechanism Of Organic Reaction

IUPAC & GOC of Class 12

A chemical equation is only a symbolic representation of chemical reaction which indicates the initial reactants and final products involved in a chemical change. Reactants generally consist of two species.

(a) Substrate: One which is being attacked in a chemical reaction

(b) Reagents: The species which attack the substrate molecule

Substrate + Reagent  → Products

Mechanism Of Organic Reaction

It is important to know not only what happens in a chemical reaction but also how it happens. Most of the reactions are complex and take place via reactive intermediates which may be or may not be isolated. The reaction intermediates are generally very reactive which readily react with other species present in the environment to form the products. The detailed step by step description of chemical reaction is called its mechanism. Mechanism is only a hypothesis to explain various facts regarding a chemical reaction.

Substrate → Reactive intermediates –— Products

Mechanism Of Organic Reaction

By knowing the mechanism we can predict the product of a chemical reaction, adjust the experimental conditions to improve the yield of the products or even alter the course of reaction to get the different products.

Most of the attacking reagents carry either positive charge (an electron deficient species) or a negative charge (electron rich species). The positively charged reagents attack the substrate at points of high electron density while (-vely) charged reagents attack the point of low electron density. The organic reactions essentially involve changes in the existing covalent bonds present in the molecules. These changes may involve electronic displacements in covalent bonds breaking of some of the existing bonds (bond fission), formation of new bonds as well as energy change accompanying the bond fission and bond cleavage.

We can understand the mechanism of various organic reactions in terms of following well established basic concepts.

(i) Electronic displacement in covalent bond

(ii) Fission (cleavage) of covalent bonds

(iii) Nature of attacking reagents

Types of Organic Reactions:

All organic reactions can be broadly classified into four catagories.

(a) Substitution reactions

(b) Addition reactions

(c) Elimination reactions and

(d) Rearrangement reactions


In these an atom or a group of atoms in an organic molecule is replaced by another atom or group of atoms without any change in the remaining part of the molecule. These reactions may be initiated by free radical, electrophile or nucleophile.

(i) Free radical substitution reaction:

This substitution reaction is brought about by free radicals. For example chlorination of methane in presence of diffused sunlight. The mechanism of the reaction is as follows.

Cl : Cl → 2Cl∙ Chain initiation

Mechanism Of Organic Reaction

Mechanism Of Organic Reaction

(ii) Nucleophilic substitution reoactions:

These reactions are brought about by nucleophile. The reaction can proceed either via SN1 or SN2 mechanism.

SN1 mechanism:

Rate determining step involves only the species. For example the reaction.

Mechanism Of Organic Reaction

takes place as follows

1st step:  Mechanism Of Organic Reaction

2nd step: Attack by nucleophile

Mechanism Of Organic Reaction

The stability of carbocation is the controlling factor for this mechanism the formation of 3° carbocation as an intermediate proceeds via this mechanism. In an optically active compound substitution at chiral centre through SN1 mechanism produces recemic mixture (No 100% recemization is observed?).

SN2 mechanism: Rate determining step involves two species and reaction proceeds through transition state.

Mechanism Of Organic Reaction

Mechanism Of Organic Reaction

Since 1° carbocation is less stable than the transition state formed above, the reaction involving 1° alkyl halides proceed via SN2 mechanism. During reaction configuration of carbon is inverted which is referred to as Walden inversion.

Mechanism Of Organic Reaction

Points to Remember

  • The higher the polarity of solvent greater the tendency for SN1 reaction.
  • High concentration of the nucleophile favours SN2 reaction while low concentration favours SN1 reaction.
  • Rearrangement of the carbocation (formed in SN1 reaction) leading to more stable carbocation is observed in SN1 reaction (discussed latter).
  • In general SN2 mechanism is strongly inhibited by increasing steric bulk of the reagents. In such case SN1 mechanism is favoured.

(iii) Electrophilic substitution reactions:

The reaction initiated by an electrophile is known as electrophilic substitution reaction. Aromatic substitution reactions are the examples of this type of reaction.

C6H6 + Cl2 Mechanism Of Organic Reaction C6H5Cl + HCl

The mechanism of this reaction as follows.

Formation of electrophile: Cl : Cl + AlCl3 ⎯→ Cl+  Mechanism Of Organic Reaction

Electrophile attack:

Mechanism Of Organic Reaction

Elimination of proton:

Mechanism Of Organic Reaction


Reactions which involve combination between two molecules to give a single molecule of the product are called addition reactions. Such reactions are typical of compounds containing multiple (double or tripe) bonds. Depending upon the nature of the attacking species (electrophiles, nucleophiles or free radicals) addition reactions are of the following types.

(i) Electrophilic addition reaction:

These reactions are brought about by electrophiles and are typical reactions of alkenes and alkynes.

Mechanism Of Organic Reaction

Mechanism Of Organic Reaction

(ii) Nucleophilic addition reactions:

These reactions are brought about by nucleophiles. The characteristics reaction of aldehyde and ketone are nucleophilic addition reaction i.e., base catalysed addition of HCN to aldehydes or ketones.

HO– + HCN ⎯⎯→ H2O + CN–

Mechanism Of Organic Reaction

(iii) Free radical addition reactions:

Addition reactions brought about by free radicals are called free radical addition reactions for example addition of HBr to alkenes in presence of peroxides.

CH3—CH=CH2+HBr Mechanism Of Organic Reaction Mechanism Of Organic Reaction

The reaction proceeds through following mechanism.

R - O - O -R Mechanism Of Organic Reaction 2RO°

RO∙ + HBr ⎯⎯→ ROH + Br∙

CH3—CH=CH2 + Br Mechanism Of Organic ReactionCH3—∙CH —CH2—Br

CH3—∙CH—CH2—BrMechanism Of Organic ReactionCH3—CH2CH2—Br + Br

Br∙ + Br∙ ⎯⎯→ Br2


An elimination reaction is one which involves the loss of two atoms or groups of atoms from the same or adjacent atoms of a substrate molecule leading to formation of multiple (double or triple) bond. These are of two types.

(i) β - elimination reactions. In these reactions, loss of two atoms or groups occurs from the adjacent atoms of the substrate molecule e.g., acid catalysed dehydration of alcohol and base catalysed dehydrogenation of alkyl halides.

Mechanism Of Organic Reaction


Mechanism Of Organic Reaction

Mechanism Of Organic Reaction

E1 mechanism

Mechanism Of Organic Reaction

E2 Mechanism

Mechanism Of Organic Reaction

E1 — CB mechanism

Mechanism Of Organic Reaction

(ii) α - Elimination – In these reactions loss of two atoms or groups occurs from the same atom of the substrate molecule. E.g., base catalysed dehydrohalogenation of chloroform to form dichlorocarbene.

Mechanism Of Organic Reaction

Dichlorocarbene is the reactive intermediate involved in carbylamine reaction and
Reimer – Tieman reaction.


These reactions involve the migration of an atom or a group of atoms from one atom to another within the same molecule.

Some reactions involving rearrangement.

Mechanism Of Organic ReactionMechanism Of Organic Reaction

Mechanism Of Organic Reaction

Mechanism Of Organic Reaction

Mechanism Of Organic Reaction

Mechanism Of Organic Reaction

1, 2 Hydride shift

Mechanism Of Organic Reaction

1, 2 Methyl shift

Mechanism Of Organic Reaction

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