Electrophilic substitution

Electrophilic Substitution

Electrophilic substitution is a fundamental class of reactions in organic chemistry, particularly important in the chemistry of aromatic compounds. This type of reaction involves the replacement of an atom, typically hydrogen, in an organic molecule by an electrophile.

Mechanism

The general mechanism of electrophilic substitution can be divided into two main steps:

1. Formation of the Electrophile

The first step involves the generation of a strong electrophile. This can occur through various means, such as the protonation of a molecule or the formation of a positively charged species. For example, in the nitration of benzene, the electrophile is the nitronium ion (NO₂⁺), which is generated from nitric acid and sulfuric acid.

2. Attack on the Aromatic Ring

The electrophile then attacks the π-electron-rich aromatic ring, forming a sigma complex (also known as an arenium ion or Wheland intermediate). This step is the rate-determining step of the reaction.

3. Deprotonation

Finally, the sigma complex loses a proton to regenerate the aromaticity of the ring, resulting in the substitution of a hydrogen atom by the electrophile.

Types of Electrophilic Substitution Reactions

Electrophilic substitution reactions are classified based on the type of electrophile involved. Some common types include:

Aromatic Nitration

In aromatic nitration, a nitro group (NO₂) is introduced into an aromatic ring. This is typically achieved using a mixture of concentrated nitric acid and sulfuric acid.

Aromatic Halogenation

Aromatic halogenation involves the substitution of a hydrogen atom by a halogen (such as chlorine or bromine). This reaction requires a halogen carrier, such as iron(III) chloride (FeCl₃) or iron(III) bromide (FeBr₃), to generate the electrophilic halogen species.

Friedel-Crafts Alkylation

In Friedel-Crafts alkylation, an alkyl group is introduced into an aromatic ring. This reaction uses an alkyl halide and a Lewis acid catalyst, such as aluminum chloride (AlCl₃).

Friedel-Crafts Acylation

Friedel-Crafts acylation involves the introduction of an acyl group into an aromatic ring. This reaction uses an acyl chloride and a Lewis acid catalyst.

Factors Affecting Electrophilic Substitution

Several factors influence the rate and outcome of electrophilic substitution reactions:

Nature of the Electrophile

Stronger electrophiles tend to react more readily with aromatic rings.

Substituents on the Aromatic Ring

Substituents can either activate or deactivate the ring towards electrophilic substitution. Activating groups, such as hydroxyl (OH) and methoxy (OCH₃), increase the electron density of the ring, making it more reactive. Deactivating groups, such as nitro (NO₂) and carbonyl (C=O), decrease the electron density, making the ring less reactive.

Solvent Effects

The choice of solvent can also affect the reaction rate and selectivity.

Applications

Electrophilic substitution reactions are widely used in the synthesis of various aromatic compounds, including dyes, pharmaceuticals, and agrochemicals. They are also crucial in the production of intermediates for further chemical transformations.

See Also

• Nucleophilic substitution
• Aromaticity
• Lewis acid

References

• Smith, M. B., & March, J. (2007). March's Advanced Organic Chemistry: Reactions, Mechanisms, and Structure. Wiley.
• Carey, F. A., & Sundberg, R. J. (2007). Advanced Organic Chemistry: Part A: Structure and Mechanisms. Springer.