Nucleophilic Aromatic Substitution

Nucleophilic aromatic substitution is a classical reaction in which a nucleophile displaces a leaving group on an aromatic ring. The presence of the electron-withdrawing group increases the rate of nucleophilic aromatic substitution. The nucleophilic compound doesn’t give substitution reaction easily, but some aromatic compounds were found to undergo nucleophilic aromatic substitution successfully.

This section will discuss the nucleophilic aromatic substitution reaction in detail.

Table of Content

• What is a Nucleophilic Aromatic Substitution?
• Conditions of Nucleophilic Aromatic Substitution
• Example of Nucleophilic Aromatic Substitution
• Mechanism of Nucleophilic Aromatic Substitution
• Frequently Asked Questions – FAQs

What is a Nucleophilic Aromatic Substitution?

Nucleophilic aromatic substitution is a classical reaction in which a nucleophile displaces a leaving group on an aromatic ring. The presence of the electron-withdrawing group increases the rate of nucleophilic aromatic substitution. The nucleophilic compound doesn’t give substitution reaction easily, but some aromatic compounds were found to undergo nucleophilic aromatic substitution successfully.

• Nucleophilic aromatic substitution does not follow the S_N2 reaction mechanism, as the substitution takes place at the trigonal carbon atom having sp³ hybridisation.
• Nucleophilic aromatic substitution does not follow the S_N2 reaction mechanism because of the steric hindrance of the benzene ring.
• Nucleophilic aromatic substitution may follow the S_N1 reaction mechanism in the presence of an excellent leaving group.
• S_N1 reaction would involve elimination of the leaving group and formation of aryl carbocation.
• Generally, Nucleophilic aromatic substitution follows the elimination addition or addition elimination pathway.

Conditions of Nucleophilic Aromatic Substitution

• The nitro group is generally used as an electron-withdrawing group to activate the aromatic ring for nucleophilic aromatic substitution.
• Ortho or Para position of the electron-withdrawing group to the leaving group supports the resonance stabilisation of the negative charge in the transition state.
• –OH, –OR, –NH₂, –SR, NH₃, and other amines are nucleophiles that generally show nucleophilic aromatic substitution.
• Alike SN1 or SN2 greater the halogen atoms electronegativity, the more readily it will leave.

Examples of Nucleophilic Aromatic Substitution

• The para nitro fluoro benzene reacts with sodium methoxide yields a nucleophilic aromatic substitution product, i.e. para nitro methoxy benzene and sodium fluoride.
• The ortho bromo acetophenone reacts with ethanethiol yields a nucleophilic aromatic substitution product.

Mechanism of Nucleophilic Aromatic Substitution

SNAr Mechanism

• It is also known as the elimination addition or addition elimination mechanism.
• The presence of an electron-withdrawing group makes an aromatic ring prone to a nucleophilic attack.
• It is similar to electrophilic substitution except that an anionic intermediate is formed rather than a cationic intermediate.
• Substitution between the leaving group and a nucleophile takes place.
• The electron-withdrawing group’s ortho or Para position to the leaving group is preferred as resonance stabilised.
• In contrast, meta is stabilised by the inductive, but no resonance.
• Thus ortho or para position concerning electron-withdrawing group is preferred.

SN1 Mechanism

This mechanism is encountered with diazonium salts and nucleophiles. The rate depends on the strength of nitrogen molecule bonding.

Benzyne Mechanism

Aryl halides with no activating group proceed through the benzyne mechanism.

Aryl halide reacts with a strong base to yield benzyne intermediate, which reacts with a nucleophile to give regioisomers.