Electrophilic substitution

In a nutshell

Benzene can undergo electrophilic substitution in the presence of a catalyst. Some common reactions of benzenes are halogenation and nitration.

Equations

These are general chemical equations. You will need to be able to use these when you write equations for specific substances.

$R-X:\rightarrow AlX_3 \xrightarrow{polarisation} R^+ AlX_4^- \\ \ \\C_6H_6 \ + \ X_2 \xrightarrow{halogen \ carrier \ catalyst} C_6H_5X \ + \ HX$

Equation for specific chemical reactions.

$C_6H_6 \ + \ HNO_3 \xrightarrow{c.H_2SO_4, \ <50 \degree C} C_6H_5NO_2 \ + \ H_2O$

Arenes

Benzenes do not undergo electrophilic addition reactions due to the stable delocalised ring of $\pi$ -electrons. However they do undergo electrophilic substitution. The hydrogen atom(s) in benzene is replaced by an electrophile, via a two step mechanism.

General Mechanism

Chemistry; Organic chemistry III; KS5 Year 12; Electrophilic substitution

Good electrophiles

As the electron density in the benzene ring is spread out, in order for a reaction to occur there must be a strong electrophile.

Halogen carriers, which are a type of catalyst, can produce strong electrophiles via polarisation. Common halogen carriers are aluminium halides and iron halides. The halogen carrier accepts a pair of electrons from the halogen on an electrophile. The difference in electron density in the carbon-halogen bond is increased whereby at times ionic species are produced.

$R-X:\rightarrow AlX_3 \xrightarrow{polarisation} R^+ AlX_4^-$

Example

$R-Cl:\rightarrow AlCl_3 \xrightarrow{polarisation} R^+ AlCl_4^-$

Halogenation

Benzene can react with a halogen in the presence of a catalyst. The catalyst polarises the electrophile so it is strong enough to react with the benzene.

$C_6H_6 \ + \ X_2 \xrightarrow{halogen \ carrier \ catalyst} C_6H_5X \ + \ HX$

Example

$C_6H_6 \ + \ Br_2 \xrightarrow{FeBr_3} C_6H_5Br \ + \ HBr$

Mechanism

$Br-Br \ + \ FeBr_3 \rightarrow Br^+ \ + \ FeBr_4^-$

Chemistry; Organic chemistry III; KS5 Year 12; Electrophilic substitution

Nitration

Benzene can be warmed with concentrated nitric acid and concentrated sulfuric acid to form a nitrobenzene. Nitronium ion is produced in situ.

In order for mononitration to occur the temperature must be below $50 \degree C$ . A temperature above this will lead to multiple substitutions. The reaction is as follows:

$C_6H_6 \ + \ HNO_3 \xrightarrow{c.H_2SO_4, \ <50 \degree C} C_6H_5NO_2 \ + \ H_2O$

Mechanism

Generation of nitronium ion

$HNO_3 \ + \ H_2SO_4 \rightarrow H_2NO_3^+ \ + \ HSO_4^- \\ \ \\H_2NO_3^+ \rightarrow NO_2^+ \ + \ H_2O$

Electrophilic Substitution

Chemistry; Organic chemistry III; KS5 Year 12; Electrophilic substitution

Regeneration of catalyst

$H^+ \ + \ HSO_4^- \rightarrow H_2SO_4$