8.4 Electrophilic Substitution in Substituted Arenes | Hydrocarbons | Chemistry 12

When a benzene ring already carries a substituent, that substituent influences both the rate and the position of any further electrophilic aromatic substitution (EAS) reaction.

The Sigma Complex (Arenium Ion)

In every EAS reaction the electrophile ( $E^{+}$ ) attacks the $π$ -electron cloud of the ring to form a sigma complex (also called an arenium ion or Wheland intermediate). This is a carbocation in which one carbon is $s p^{3}$ -hybridised and the positive charge is delocalised over the remaining five carbons. A base then removes a proton to restore aromaticity.

$ArH + E^{+} ⟶ [Ar (H) (E)]^{+} - H^{+} ArE$

Directing Effects of Substituents

Electron-Donating Groups (EDGs) — Activating, Ortho/Para Directors

Groups such as $- O H$ , $- O R$ , $- N H_{2}$ , $- N H R$ , $- C H_{3}$ , and $- C_{2} H_{5}$ donate electron density into the ring by resonance or hyperconjugation. This:

Activates the ring (faster than benzene)
Directs the electrophile to the ortho and para positions

The sigma complex formed at ortho/para positions is more stable because the positive charge can be delocalised onto the carbon bearing the EDG, which then stabilises it further.

Electron-Withdrawing Groups (EWGs) — Deactivating, Meta Directors

Groups such as $- N O_{2}$ , $- C H O$ , $- C O R$ , $- C O O H$ , $- C N$ , and $- S O_{3} H$ withdraw electron density from the ring by inductive and/or resonance effects. This:

Deactivates the ring (slower than benzene)
Directs the electrophile to the meta position

At the meta position the positive charge in the sigma complex is never placed on the carbon bearing the EWG, making it the least destabilised option.

Halogens — Deactivating but Ortho/Para Directors

Halogens ( $- F$ , $- C l$ , $- B r$ , $- I$ ) show a dual character:

Effect	Direction	Result
Inductive (electronegativity)	Withdraws $σ$ electrons	Deactivates ring
Resonance (lone pairs)	Donates into ring at o/p	Directs to ortho/para

Overall the ring is less reactive than benzene, but substitution still occurs predominantly at the ortho and para positions.

Example: Bromination of chlorobenzene gives mainly o-bromochlorobenzene and p-bromochlorobenzene.

Predicting Major Products

Substituent	Type	Position
$- O H$ , $- N H_{2}$ , $- O R$	Activating	ortho/para
$- C H_{3}$ , alkyl	Activating	ortho/para
$- C l$ , $- B r$	Deactivating	ortho/para
$- N O_{2}$ , $- C H O$ , $- C O O H$	Deactivating	meta

Example: Nitration of nitrobenzene gives 1,3-dinitrobenzene because $- N O_{2}$ is a meta-director.

Electrophilic Substitution Reactions of Phenol (SLO C-12-E-27)

Phenol ( $C_{6} H_{5} O H$ ) is highly activated toward EAS because the $- O H$ group donates electron density into the ring by resonance, making the ortho and para positions electron-rich.

a. Reaction with NaOH(aq)

Phenol reacts with aqueous sodium hydroxide as a weak acid:

$C_{6} H_{5} O H + N a O H (a q) ⟶ C_{6} H_{5} O^{-} N a^{+} (a q) + H_{2} O$

Product: sodium phenoxide (sodium phenate)

b. Reaction with Na(s)

Phenol reacts with sodium metal more vigorously than water:

$2 C_{6} H_{5} O H + 2 N a (s) ⟶ 2 C_{6} H_{5} O^{-} N a^{+} + H_{2} (g)$

Products: sodium phenoxide and hydrogen gas

c. Reaction with Diazonium Salts in NaOH(aq) — Azo Coupling

In alkaline solution, phenol exists as the phenoxide ion ( $C_{6} H_{5} O^{-}$ ), which is an even stronger activator. It couples with a diazonium salt ( $A r N_{2}^{+}$ ) at the para position to give a brightly coloured azo compound:

$C_{6} H_{5} O^{-} + A r N_{2}^{+} ⟶ A r - N = N - C_{6} H_{4} - O H + (coloured azo dye)$

Azo compounds contain the $- N = N -$ chromophore and are used as dyes.

d. Nitration with Dilute HNO₃ at Room Temperature

Unlike benzene (which requires concentrated $H N O_{3}$ / concentrated $H_{2} S O_{4}$ at 55 °C), phenol reacts with dilute $H N O_{3}$ at room temperature to give a mixture of:

2-nitrophenol (ortho product)
4-nitrophenol (para product)

$C_{6} H_{5} O H + H N O_{3} (dil) r.t. 2-nitrophenol + 4-nitrophenol$

The milder conditions are needed because the $- O H$ group makes the ring far more reactive than benzene.

e. Bromination with Br₂(aq) — No Catalyst Required

Phenol reacts instantly with bromine water at room temperature without any catalyst (unlike benzene which needs $F e B r_{3}$ ):

$C_{6} H_{5} O H + 3 B r_{2} (a q) ⟶ 2, 4, 6 -tribromophenol ↓ + 3 H B r$

All three ortho and para positions are brominated. 2,4,6-tribromophenol forms as a white precipitate. This reaction is used as a test for phenol.

When a benzene ring already carries a substituent, that substituent influences both the rate and the position of any further electrophilic aromatic substitution (EAS) reaction.

The Sigma Complex (Arenium Ion)

$ArH + E^{+} ⟶ [Ar (H) (E)]^{+} - H^{+} ArE$

Directing Effects of Substituents

Electron-Donating Groups (EDGs) — Activating, Ortho/Para Directors

Groups such as $- O H$ , $- O R$ , $- N H_{2}$ , $- N H R$ , $- C H_{3}$ , and $- C_{2} H_{5}$ donate electron density into the ring by resonance or hyperconjugation. This:

Activates the ring (faster than benzene)
Directs the electrophile to the ortho and para positions

The sigma complex formed at ortho/para positions is more stable because the positive charge can be delocalised onto the carbon bearing the EDG, which then stabilises it further.

Electron-Withdrawing Groups (EWGs) — Deactivating, Meta Directors

Groups such as $- N O_{2}$ , $- C H O$ , $- C O R$ , $- C O O H$ , $- C N$ , and $- S O_{3} H$ withdraw electron density from the ring by inductive and/or resonance effects. This:

Deactivates the ring (slower than benzene)
Directs the electrophile to the meta position

At the meta position the positive charge in the sigma complex is never placed on the carbon bearing the EWG, making it the least destabilised option.

Halogens — Deactivating but Ortho/Para Directors

Halogens ( $- F$ , $- C l$ , $- B r$ , $- I$ ) show a dual character:

Effect	Direction	Result
Inductive (electronegativity)	Withdraws $σ$ electrons	Deactivates ring
Resonance (lone pairs)	Donates into ring at o/p	Directs to ortho/para

Overall the ring is less reactive than benzene, but substitution still occurs predominantly at the ortho and para positions.

Example: Bromination of chlorobenzene gives mainly o-bromochlorobenzene and p-bromochlorobenzene.

Predicting Major Products

Substituent	Type	Position
$- O H$ , $- N H_{2}$ , $- O R$	Activating	ortho/para
$- C H_{3}$ , alkyl	Activating	ortho/para
$- C l$ , $- B r$	Deactivating	ortho/para
$- N O_{2}$ , $- C H O$ , $- C O O H$	Deactivating	meta

Example: Nitration of nitrobenzene gives 1,3-dinitrobenzene because $- N O_{2}$ is a meta-director.

Electrophilic Substitution Reactions of Phenol (SLO C-12-E-27)

Phenol ( $C_{6} H_{5} O H$ ) is highly activated toward EAS because the $- O H$ group donates electron density into the ring by resonance, making the ortho and para positions electron-rich.

a. Reaction with NaOH(aq)

Phenol reacts with aqueous sodium hydroxide as a weak acid:

$C_{6} H_{5} O H + N a O H (a q) ⟶ C_{6} H_{5} O^{-} N a^{+} (a q) + H_{2} O$

Product: sodium phenoxide (sodium phenate)

b. Reaction with Na(s)

Phenol reacts with sodium metal more vigorously than water:

$2 C_{6} H_{5} O H + 2 N a (s) ⟶ 2 C_{6} H_{5} O^{-} N a^{+} + H_{2} (g)$

Products: sodium phenoxide and hydrogen gas

c. Reaction with Diazonium Salts in NaOH(aq) — Azo Coupling

$C_{6} H_{5} O^{-} + A r N_{2}^{+} ⟶ A r - N = N - C_{6} H_{4} - O H + (coloured azo dye)$

Azo compounds contain the $- N = N -$ chromophore and are used as dyes.

d. Nitration with Dilute HNO₃ at Room Temperature

Unlike benzene (which requires concentrated $H N O_{3}$ / concentrated $H_{2} S O_{4}$ at 55 °C), phenol reacts with dilute $H N O_{3}$ at room temperature to give a mixture of:

2-nitrophenol (ortho product)
4-nitrophenol (para product)

$C_{6} H_{5} O H + H N O_{3} (dil) r.t. 2-nitrophenol + 4-nitrophenol$

The milder conditions are needed because the $- O H$ group makes the ring far more reactive than benzene.

e. Bromination with Br₂(aq) — No Catalyst Required

Phenol reacts instantly with bromine water at room temperature without any catalyst (unlike benzene which needs $F e B r_{3}$ ):

$C_{6} H_{5} O H + 3 B r_{2} (a q) ⟶ 2, 4, 6 -tribromophenol ↓ + 3 H B r$

All three ortho and para positions are brominated. 2,4,6-tribromophenol forms as a white precipitate. This reaction is used as a test for phenol.