Question

Which of the following reactions will give acetophenone as a product? (a) Acid catalysed hydration of styrene followed by oxidation with hot alkaline \(\mathrm{KMnO}_{4}\). (b) Phenyl acetylene \(\stackrel{\mathrm{d} \mathrm{H}, \mathrm{SO}_{4}^{\prime} \mathrm{H}_{\mathrm{E}}^{2+}}{\longrightarrow}\) Aldehydes, Ketones and Carboxylic Acids (c) Reaction of benzoyl chloride with dimethyl cadmium. (d) Dry distillation of a mixture of calcium acetate and calcium benzoate.

Short answer

a) Acid-catalyzed hydration of styrene, followed by oxidation with hot alkaline \(\mathrm{KMnO}_{4}\) b) Reaction of phenyl acetylene with sulfuric acid and mercury sulfate (\(\mathrm{H}_{2}\mathrm{SO}_{4}, \mathrm{HgSO}_{4}\)) c) Reaction of benzoyl chloride with dimethyl cadmium d) Dry distillation of a mixture of calcium acetate and calcium benzoate Answer: c) Reaction of benzoyl chloride with dimethyl cadmium.

Step-by-step solution

Analyzing Option (a)

Acid catalyzed hydration of styrene involves the addition of water across the double bond in styrene in the presence of an acid catalyst, followed by oxidation with hot alkaline \(\mathrm{KMnO}_{4}\). The hydration of styrene can be shown as: \(\mathrm{C}_{6}\mathrm{H}_{5}\mathrm{CH}= \mathrm{CH}_{2} + \mathrm{H}_{2}\mathrm{O} \stackrel{\mathrm{H}^{+}}{\longrightarrow} \mathrm{C}_{6}\mathrm{H}_{5}\mathrm{CH}\mathrm{(OH)CH}_{3}\) (Phenylethanol) Oxidation with hot alkaline \(\mathrm{KMnO}_{4}\) of phenylethanol will lead to the formation of a carboxylic acid, not a ketone. So, option (a) does not produce acetophenone.

Analyzing Option (b)

Phenyl acetylene reacts with sulfuric acid and mercury sulfate as a catalyst, \(\mathrm{d} \mathrm{H}, \mathrm{SO}_{4}^{\prime} \mathrm{H}_{\mathrm{E}}^{2+}\) to form aldehydes, not ketones. This reaction is the hydrolysis of alkynes and can be shown as: \(\mathrm{C}_{6}\mathrm{H}_{5}-\mathrm{C}\equiv \mathrm{CH} \stackrel{\mathrm{H}_{2}\mathrm{SO}_{4}, \mathrm{HgSO}_{4}}{\longrightarrow} \mathrm{C}_{6}\mathrm{H}_{5}-\mathrm{CHO}\) (Benzaldehyde) Hence, option (b) does not produce acetophenone.

Analyzing Option (c)

The reaction of benzoyl chloride with dimethyl cadmium is given by: \(\mathrm{C}_{6}\mathrm{H}_{5}\mathrm{C(O)}\mathrm{Cl} + \mathrm{(CH}_{3})_{2}\mathrm{Cd} \rightarrow \mathrm{C}_{6}\mathrm{H}_{5}\mathrm{C(O)}\mathrm{CH}_{3} + \mathrm{CH}_{3}\mathrm{CdCl}\) Here, the product formed is acetophenone. So, option (c) gives acetophenone as a product.

Analyzing Option (d)

Dry distillation of a mixture of calcium acetate and calcium benzoate creates a carboxylic acid, not a ketone. \(\mathrm{Ca(OOCCH}_{3})_{2} + \mathrm{Ca(OOC-C}_{6}\mathrm{H}_{5})_{2} \rightarrow \mathrm{C}_{6}\mathrm{H}_{5}\mathrm{-COOCH}_{3} + 2\mathrm{CaCO}_{3}\) Here, the product formed is methyl benzoate, not acetophenone. So, option (d) does not produce acetophenone. After analyzing all the options, we can conclude that the reaction in option (c) will give acetophenone as a product.

Key concepts

Styrene Hydration Oxidation

Understanding the hydration and subsequent oxidation of styrene is key to grasping why it does not yield acetophenone. Initially, water is added across the double bond of styrene in the presence of an acid catalyst, creating phenylethanol.

When styrene, which has the structural formula \(\mathrm{C}_{6}\mathrm{H}_{5}\mathrm{CH}= \mathrm{CH}_{2}\), undergoes hydration, an \(\mathrm{OH}\) group is introduced at the site of the unsaturated carbon-carbon bond. As a result, you get the intermediate:\[\mathrm{C}_{6}\mathrm{H}_{5}\mathrm{CH}(\mathrm{OH})\mathrm{CH}_{3}\] (phenylethanol).

However, when this intermediate is subjected to hot alkaline \(\mathrm{KMnO}_{4}\), it is oxidized further to a carboxylic acid rather than a ketone. Therefore, despite involving a crucial oxidative step, this particular pathway does not produce the desired product, acetophenone, due to excessive oxidation.

Phenyl Acetylene Hydrolysis

The process of hydrolyzing phenyl acetylene can sometimes be confusing; it's essential to note the products that result from this reaction. With phenyl acetylene as the starting material and sulfuric acid and mercury sulfate as catalysts, the reaction proceeds through the addition of water to the triple bond of the alkyne.

This addition forms an enol, which tautomerizes into the more stable aldehyde, benzaldehyde: \[\mathrm{C}_{6}\mathrm{H}_{5}-\mathrm{C}\equiv \mathrm{CH} \stackrel{\mathrm{H}_{2}\mathrm{SO}_{4}, \mathrm{HgSO}_{4}}{\longrightarrow} \mathrm{C}_{6}\mathrm{H}_{5}-\mathrm{CHO}\].

However, this reaction stops at the aldehyde stage and does not proceed to form a ketone, let alone the specific ketone acetophenone. Thus, understanding the limitation of the reaction's product is crucial.

Benzoyl Chloride Dimethyl Cadmium Reaction

Among the reactions considered for the synthesis of acetophenone, the reaction involving benzoyl chloride and dimethyl cadmium is the only one that hits the target.

In this reaction, dimethyl cadmium (\((\mathrm{CH}_{3})_{2}\mathrm{Cd}\)) is utilized to transfer a methyl group to the suitable precursor, benzoyl chloride (\(\mathrm{C}_{6}\mathrm{H}_{5}\mathrm{C(O)}\mathrm{Cl}\)), through nucleophilic acyl substitution. The mechanism facilitates the replacement of the chloride ion with a methyl group, forming the desired ketone, acetophenone: \[\mathrm{C}_{6}\mathrm{H}_{5}\mathrm{C(O)}\mathrm{CH}_{3}\].

This is the only pathway that appropriately creates the ketone structure associated with acetophenone without further oxidation or reduction steps. A clear understanding of the nucleophilic acyl substitution mechanisms can illuminate why this reaction is successful.

Calcium Acetate Benzoyate Dry Distillation

The dry distillation of calcium acetate with calcium benzoate might seem like a probable method for synthesizing acetophenone, but the chemistry does not support this outcome.

Instead, this process involves decomposing the salts, primarily leading to the formation of ketene, which subsequently reacts with the benzoate to form methyl benzoate: \[\mathrm{Ca(OOCCH}_{3})_{2} + \mathrm{Ca(OOC-C}_{6}\mathrm{H}_{5})_{2} \rightarrow \mathrm{C}_{6}\mathrm{H}_{5}\mathrm{-COOCH}_{3} + 2\mathrm{CaCO}_{3}\].

The reaction fundamentally yields an ester rather than a ketone due to the esterification that takes place as a part of the dry distillation process. Understanding the differences between esterification and ketone formation is necessary to discern why this method does not yield acetophenone.