Electrophilic Substitution Mechanism of Benzene - Organic Chem

In this video we want to discuss the electrophilic substitution mechanism of benzene using 2 reactions as example.

Nitration of Benzene

The first reaction is nitration of benzene to nitrobenzene.

Nitration of benzene to nitrobenzene

This reaction is done using concentrated nitric acid in concentrated sulphuric acid catalyst at 50 to 60 degree celsius.

The first step involves the generation of electrophile NO2+ which is formed from the reaction between conc HNO3 and conc H2SO4.

ES nitration step 1 - generate electrophile

In the second step, which is the rate determining step or slow step, benzene will react with the electrophile NO2+.

ES nitration step 2 - electrophilic attack

For arrow pushing we have to draw the full arrow, which represents the movement of 2 electrons, from benzene ring (electron rich region) to nitrogen in NO2+ (electron poor region).

The intermediate formed is highly unstable as the benzene is positively charged and it loses the resonance stability due to the opening of delocalised ring.

In the third step, the intermediate will deprotonate or remove a H+ to form product nitrobenzene. The H+ will combine with HSO4- intermediate to regenerate H2SO4 catalyst.

ES nitration step 3 - deprotonation and regenerate H2SO4 catalyst

Bromination of Benzene

The second reaction is bromination of benzene to bromobenzene.

bromination of benzene to form bromobenzene

This reaction is done via Br2 in FeBr3 catalyst.

The mechanism is effectively the same, namely:

Step 1 - generate electrophile Br+ from Br2 and FeBr3

ES mechanism bromination step 1 - generate electrophile Br+

Step 2 - carbon in benzene uses 2 electrons from delocalised pi system to form a bond with Br+, generating an unstable intermediate

ES mechanism bromination step 2 - electrophilic attack

Step 3 - deprotonation of intermediate to form bromobenzene and regeneration of FeBr3 catalyst.

ES mechanism bromination step 3 - deprotonation and regenerate FeBr3 catalyst

For the detailed step-by-step discussion on how to draw the mechanism, check out this video!

Topic: Benzene (Hydrocarbon), Organic Chemistry, A Level Chemistry, Singapore

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