Question

Which one of the following compounds undergoes substitution at a slower rate than benzene and yet yields predominantly ortho and para products? (a) phenol (b) chlorobenzene (c) nitrobenzene (d) benzene sulphonic acid

Short answer

Benzene sulphonic acid undergoes substitution slower than benzene, yielding ortho and para products.

Step-by-step solution

Identify the Substituents

First, identify the substituents on the benzene ring for each compound listed: (a) phenol has an -OH group, (b) chlorobenzene has a -Cl group, (c) nitrobenzene has a -NO2 group, and (d) benzene sulphonic acid has a -SO3H group.

Analyze Electron Donating/Withdrawing Characteristics

Determine if these groups are electron donating or withdrawing. The hydroxyl group (-OH) in phenol is electron-donating. The chloro group (-Cl) in chlorobenzene is slightly electron-withdrawing but can donate electrons through resonance. The nitro group (-NO2) and the sulfonic acid group (-SO3H) are strong electron-withdrawing groups.

Determine Reactivity in Substitution

Analyze how these substituents affect the rate of substitution. Nitrobenzene, with the -NO2 group, significantly deactivates the ring, slowing substitution. The -SO3H group also deactivates the ring but less so compared to -NO2. Phenol and chlorobenzene, being less deactivating or even mildly activating, react faster than benzene under substitution reactions.

Determine Position of Predominant Products

For ortho and para directing influences, consider resonance and inductive effects. Both phenol and chlorobenzene predominantly direct substitution ortho and para due to resonance effects. Nitrobenzene directs meta due to strong electron withdrawal, while benzene sulphonic acid, although deactivating, allows ortho/para substitution due to the way sulfonic acid influences electronic density distribution through resonance.

Evaluate Each Compound's Characteristics

Combine the findings: phenol undergoes fast ortho/para substitution, chlorobenzene is slower than phenol but faster than benzene, and it gives ortho/para products, nitrobenzene is slower and gives meta products, while benzene sulphonic acid is slower than benzene but can yield predominantly ortho/para despite the slow rate.

Key concepts

Reaction Mechanisms

Electrophilic Aromatic Substitution (EAS) is a fundamental reaction mechanism in organic chemistry used to introduce substituents onto an aromatic ring, such as benzene. During EAS, an electrophile replaces a hydrogen atom on the benzene. Studying this mechanism helps us appreciate how substituents influence reactivity and product orientation. Generally, the process involves the formation of an arenium ion intermediate following the attack of the electrophile on the aromatic ring. Subsequent loss of a proton from the arenium ion restores aromaticity. It's important to note how the nature of the substituent affects the stability of intermediates, which plays a crucial role in determining the rate and orientation of substitution reactions.

Substituent Effects

Substituents on the benzene ring can either donate or withdraw electrons, influencing the reactivity of the aromatic system.

• Electron Donating Groups (EDGs): These groups stabilize the positive charge of the intermediate by donating electron density, usually through resonance or inductive effects. As a result, they generally activate the benzene ring, making it more reactive than benzene itself.
• Electron Withdrawing Groups (EWGs): In contrast, these groups pull electron density away from the aromatic ring, thus deactivating the ring towards EAS. This usually results in a slower reaction compared to benzene.

In our exercise, chlorobenzene and benzene sulphonic acid are key examples of EWGs that slow down the reaction rate, with benzene sulphonic acid more strongly deactivating the ring than chlorobenzene. Meanwhile, phenol illustrates how an EDG can significantly speed up electrophilic substitution.

Ortho-Para and Meta Directing Groups

Another fascinating aspect of electrophilic aromatic substitution is the directionality imparted by different substituents. This refers to where the new substituent is most likely to be added to the ring.

• Ortho-Para Directors: These are typically EDGs. They encourage substitution at the positions adjacent to and opposite the existing substituent. Phenol, with its -OH group, and chlorobenzene, with its -Cl group, exemplify ortho-para direction due to the partial electron donation via resonance.
• Meta Directors: Generally, strong EWGs are meta directors since they stabilize the positive charge via resonance or inductive effects at these positions. Nitrobenzene is an example where the strong electron-withdrawing -NO2 group directs new substitution predominantly to the meta position.

Understanding substituent direction helps predict the major products in electrophilic aromatic substitution, aligning with the nature and effects of each functional group.