substitution reaction and some tips, Exercises of Chemistry

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Substitution Reactions In

Aliphatic and Aromatic

Compounds

V. Sri Krishna

A. Chakravarthy

Substitution Reactions-Definition

• Reactions which involve the replacement

or substitution of one or more atoms or

groups of a compound by other atoms or

groups are known as SUBSTITUTION

REACTIONS.

Free Radical Substitution

• Radical substitution reactions are initiated by radicals in the gas phase or in non-polar solvents.
• For example, methane and chlorine react in presence of sunlight or heat to give methylchloride

Mechanism of free radical substitution

• Light energy or heat causes homolytic fission of chlorine producing chlorine radicals which attack methane to form methylchloride.

Electrophilic Substitution

• When the substitution involves attack by an electrophile , it is electrophilic substitution.
• This occurs in both aliphatic and aromatic compounds and hence classified as: - Electrophilic Aliphatic Substitution
• Electrophilic Aromatic Substitution

Electrophilic Aliphatic Substitution

• This reaction is similar to Nucleophilic Substitution reaction and are differentiated based on their mechanisms as SE 1 and SE 2
• Examples: Nitrosation Ketone halogenation Keto-enol tautomerism Aliphatic diazonium coupling Carbene insertion into C-H bonds

Examples

• Nitration:
• Sulphonation:
• Halogenation:

Examples

• Friedel-Craft’s alkylation:
• Friedel-Craft’s acylation:

Activating

Substituents:

• They stabilize the cationic intermediate formed during the substitution by donating electrons into the ring system, by either inductive effect or resonance effects. - These destabilize the intermediate cation and thus decrease the reaction rate by withdrawing electron density from the aromatic ring.

Deactivating

Substituents:

Examples are toluene, aniline and phenol

Examples are nitrobenzene, benzaldehyde and trifluoromethylbenzene

Ortho/para directors

Ortho substitution

Ortho/para directors

Meta substitution

• Non-halogen groups with atoms that are more electronegative than carbon, such as a carboxylic acid group (CO 2 H) draw substantial electron density from the pi system.
• These groups are strongly deactivating groups.
• Additionally, since the substituted carbon is already electron-poor, the resonance contributor with a positive charge on this carbon (produced by ortho/para attack) is less stable than the others.
• Therefore, these electron-withdrawing groups are meta directing

Meta directors

Aliphatic Nucleophilic substitution

• In 1935, Edward D. Hughes and Sir Christopher Ingold studied nucleophilic substitution reactions of alkyl halides and related compounds.
• They proposed two main mechanisms—

the SN1 reaction and the SN2 reaction.

• S stands for chemical substitution, N stands for nucleophilic, and the number represents the kinetic order of the reaction.

SN1 Reaction Mechanism

• L = Leaving group
• Nu-^ = Attacking Nucleophile