Question

Name and draw the major product(s) of electrophilic chlorination of the following compounds: (a) \(m\) -Nitrophenol (b) \(o\) -Xylene (dimethylbenzene) (c) \(p\) -Nitrobenzoic acid (d) \(p\) -Bromobenzenesulfonic acid

Short answer

Major products are 4-chloro-3-nitrophenol, 3-chloro-o-xylene, 3-chloro-p-nitrobenzoic acid, and 2-chloro-p-bromobenzenesulfonic acid.

Step-by-step solution

Understand Electrophilic Chlorination

Electrophilic chlorination involves an electrophile (Cl⁺) attacking aromatic rings. The position of the chlorine atom in the final product is influenced by the substituents already present on the benzene ring.

Analyze Substituent Effects for m-Nitrophenol

In m-nitrophenol, the nitro group is a deactivating meta-director, while the hydroxyl group is an activating ortho/para-director. However, due to steric and electronic factors, chlorination will most likely occur at the position that's ortho to the hydroxyl group and meta to the nitro group.

Determine Product for m-Nitrophenol

The major product will be chlorination at the 4-position relative to the -OH group, resulting in 4-chloro-3-nitrophenol.

Analyze Substituent Effects for o-Xylene

In o-xylene, both methyl groups are ortho/para-directors and activating. Chlorination will occur at positions ortho or para to one of the methyl groups, which are the least hindered.

Determine Product for o-Xylene

The major product will be 3-chloro-o-xylene, with chlorine added to the 3-position relative to one methyl group, as it is ortho to both methyl groups.

Analyze Substituent Effects for p-Nitrobenzoic Acid

p-Nitrobenzoic acid has a nitro group, a meta-director, deactivating the ring, and a carboxylic acid, which is also a meta-director. Thus, chlorination will most likely occur at the position meta to both groups.

Determine Product for p-Nitrobenzoic Acid

The major product will be 3-chloro-p-nitrobenzoic acid, with the chlorine added to the position meta to both substituents.

Analyze Substituent Effects for p-Bromobenzenesulfonic Acid

In p-bromobenzenesulfonic acid, bromine is a weak ortho/para-director while sulfonic acid is meta-director and deactivating. Chlorination prefers the less hindered position ortho to bromine.

Determine Product for p-Bromobenzenesulfonic Acid

Chlorination will occur at the position ortho to bromine and meta to sulfonic acid, resulting in 2-chloro-p-bromobenzenesulfonic acid.

Key concepts

Aromatic Substitution

Aromatic substitution is a reaction where an atom in an aromatic ring is replaced with another atom or group of atoms. In the context of electrophilic chlorination, it involves the addition of a chlorine atom to an aromatic ring, typically benzene. This type of reaction is common in organic chemistry and is crucial for modifying the chemical structure of aromatic compounds. During this process, the aromatic ring acts as a nucleophile, providing electron density to accommodate the incoming electrophile, which in this case is the chlorine cation (Cl⁺). This reaction retains the aromaticity of the ring but requires careful consideration of the effects of any pre-existing substituents on the ring. Such substituents influence reactivity and determine the chlorination position, making each scenario unique.

Substituent Effects

Substituents attached to an aromatic ring greatly influence the behavior and outcome of electrophilic aromatic substitution reactions. They affect two key aspects: the reactivity of the ring and the position where substitution occurs.

• Activating Substituents: These increase the reactivity of the benzene ring towards electrophilic attack. Examples include -OH and -CH₃ groups. They often direct incoming electrophiles to ortho/para positions relative to themselves.
• Deactivating Substituents: These decrease the ring’s reactivity, making substitution more challenging. Groups like -NO₂ and -COOH are typical examples. They usually direct substitution to meta positions.

Understanding these effects is crucial when predicting which positions will be favored in the substitution reaction. It is these traits of the substituents that guide the major product outcome in each reaction, dictating both speed and pathway.

Meta-Director

Meta-directors are specific types of substituents that guide electrophilic aromatic substitution to the meta position on the ring. These are generally electron-withdrawing groups. When present, they withdraw electron density through resonance or inductive effects. This decreases the electron density at the ortho and para positions, culminating in electrophiles favoring the meta position instead.

• Common Meta-Directors: Nitro groups (-NO₂), carboxylic acids (-COOH), and sulfonic acid groups (-SO₃H) serve as typical examples of meta-directors.
• Impact on Reactivity: These groups also tend to deactivate the ring by making it less reactive to electrophilic attack. This is why certain reactions with deactivating meta-directors can be slower compared to those involving activating groups.

Meta-directors are essential considerations for accurately anticipating the substitution pattern on aromatic rings, especially when predicting major chlorination products.

Ortho/Para-Director

Ortho/para-directors are substituents that influence the position of a new substituent during an electrophilic aromatic substitution. These groups facilitate substitution at the ortho or para positions compared to others on the aromatic ring. This directing effect is due to their electron-donating capability, which increases electron density at these positions, making them more attractive to electrophiles.

• Examples of Ortho/Para-Directors: Hydroxyl (-OH), amino (-NH₂), and methyl (-CH₃) groups are well-known ortho/para-directors.
• Influence on Electrophilic Reactions: They enhance the reactivity of the benzene ring by activating it towards electrophilic attack. This is because their presence increases electron density through resonance or inductive effects.

Understanding ortho/para directors helps predict where electrophilic substitution will occur, providing critical insights into reaction sites and final product structures during processes like chlorination.