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Chapter 12: Problem 257

How might you prepare ethylbenzene from: (a) benzene and ethyl alcohol; (b) acetophenone, \(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{COCH}_{3}\); (c) styrene, \(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{CH}=\mathrm{CH}_{2}\); (d) \(\alpha\) -phenylethyl alcohol, \(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{CHOHCH}_{3}\); and (e) \(\beta\) -phenylethyl chloride, \(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{Cl}\) ?

Short Answer

Expert verified
(a) Convert ethyl alcohol to ethyl chloride, then use Friedel-Crafts alkylation with anhydrous aluminium chloride. (b) Reduce acetophenone using Clemmensen's reagent or Wolff-Kishner reagent. (c) Perform catalytic hydrogenation of styrene using palladium on carbon. (d) Dehydrate α-phenylethyl alcohol to form styrene, then catalytically hydrogenate. (e) Substitute β-phenylethyl chloride with hydroxide ions to form ethylbenzene.

Step by step solution

01

(a) Preparation of Ethylbenzene from Benzene and Ethyl Alcohol

This reaction involves electrophilic substitution of ethyl group in the presence of a Lewis acid. However, directly using ethyl alcohol is not effective, instead ethyl chloride, which can be derived from ethyl alcohol, is used. So, the procedure involves two steps. In the first step, ethyl alcohol is protonated and loses a molecule of water to form ethyl chloride. In the second step, ethyl chloride reacts with benzene in presence of anhydrous aluminium chloride (\(\mathrm{AlCl}_{3}\)) to form ethylbenzene. This is known as Friedel-Crafts alkylation.
02

(b) Preparation of Ethylbenzene from Acetophenone

It involves reduction of the carbonyl group (\(=\mathrm{O}\)) in acetophenone (\(\mathrm{C}_{6}\mathrm{H}_{5}\mathrm{COCH}_{3}\)) to form an ethyl group (\(-\mathrm{CH}_{2}\mathrm{CH}_{3}\)). The reduction can be achieved by using reducing agents like Clemmensen's reagent (Zinc amalgam, \(\mathrm{Hg}(Zn)-\mathrm{HCl}\)), or Wolff-Kishner reagent (Hydrazine, \(N_{2}H_{4}\) in a basic medium). This carbonyl reduction step transforms an acetophenone to ethylbenzene.
03

(c) Preparation of Ethylbenzene from Styrene

Styrene contains a vinyl group (--\(\mathrm{CH}=\mathrm{CH}_{2}\)), which can be hydrogenated to form an ethyl group (\(-\mathrm{CH}_{2}\mathrm{CH}_{3}\)). This involves the addition of molecular hydrogen (\(H_{2}\)) across the double bond of styrene in the presence of a suitable catalyst like palladium on carbon (\(\mathrm{Pd/C}\)). This process, known as catalytic hydrogenation, results in the formation of ethylbenzene.
04

(d) Preparation of Ethylbenzene from α-Phenylethyl Alcohol

This involves dehydration, a process to remove a molecule of water. The group of two carbon atoms can have one of its hydrogen atoms removed along with the \(-\mathrm{OH}\) group from the carbon atom next to it. This loses a molecule of water to form a double bond results in styrene. The styrene can then be converted to ethylbenzene by catalytic hydrogenation as described in (c).
05

(e) Preparation of Ethylbenzene from β-Phenylethyl Chloride

The \(β\)-phenylethyl chloride, essentially a benzene ring with an ethyl chloride group attached, already has the essential composition for ethylbenzene. The only difference is the \(-Cl\) substitution at the terminal carbon atom instead of a hydrogen atom. A substitution reaction using a strong base, such as hydroxide ions (\(\mathrm{OH}^{-}\)), can remove the \(-\mathrm{Cl}\) by a nucleophilic substitution reaction to form the final ethylbenzene structure.

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