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Chapter 16: Problem 21

Write equations showing how 2-phenylethanol could be prepared from each of the following starting materials: (a) Bromobenzene (c) 2 -Phenylethanal \(\left(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{CH}_{2} \mathrm{CHO}\right)\) (b) Styrene (d) 2-Phenylethanoic acid \(\left(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{CH}_{2} \mathrm{CO}_{2} \mathrm{H}\right)\)

Short Answer

Expert verified
2-Phenylethanol can be prepared using Grignard reaction, oxymercuration, reduction with NaBH鈧�, or reduction with LiAlH鈧� depending on the starting material.

Step by step solution

01

Synthesis from Bromobenzene

To convert bromobenzene (C鈧咹鈧匓r) to 2-phenylethanol, we begin by performing a Grignard reaction. Treat bromobenzene with magnesium to form phenylmagnesium bromide (C鈧咹鈧匨gBr). Then, react this Grignard reagent with ethylene oxide. The ethylene oxide ring opens to attach the phenylmagnesium group, yielding 2-phenylethanol after protonation with acid. The reaction sequence is: 1. Formation of Grignard reagent: C鈧咹鈧匓r + Mg 鈫� C鈧咹鈧匨gBr 2. Reaction with ethylene oxide: C鈧咹鈧匨gBr + (CH鈧�)鈧侽 + H鈦� 鈫� C鈧咹鈧匔H鈧侰H鈧侽H.
02

Synthesis from Styrene

To k茅sz铆theti from styrene, use an oxymercuration-demercuration reaction. First, react styrene with mercuric acetate in the presence of water to add a hydroxyl group across the double bond. Finally, reduce with sodium borohydride to obtain the alcohol. The reactions are: 1. Oxymercuration: C鈧咹鈧匔H=CH鈧� + Hg(OAc)鈧� + H鈧侽 鈫� C鈧咹鈧匔H(OH)CH鈧侶gOAc 2. Demercuration: C鈧咹鈧匔H(OH)CH鈧侶gOAc + NaBH鈧� 鈫� C鈧咹鈧匔H鈧侰H鈧侽H + Hg + B(OH)鈧勨伝.
03

Synthesis from 2-Phenylethanal

By reducing 2-phenylethanal, you can obtain 2-phenylethanol. Use a mild reducing agent like sodium borohydride (NaBH鈧�). The carbonyl group in the aldehyde is reduced to an alcohol (hydroxyl group). The reaction is simply: C鈧咹鈧匔H鈧侰HO + NaBH鈧� 鈫� C鈧咹鈧匔H鈧侰H鈧侽H.
04

Synthesis from 2-Phenylethanoic Acid

To synthesize 2-phenylethanol from 2-phenylethanoic acid, perform a reduction using lithium aluminium hydride (LiAlH鈧�). The carboxylic acid group is reduced to a primary alcohol during this transformation: C鈧咹鈧匔H鈧侰O鈧侶 + LiAlH鈧� 鈫� C鈧咹鈧匔H鈧侰H鈧侽H.

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Key Concepts

These are the key concepts you need to understand to accurately answer the question.

Grignard Reaction
The Grignard Reaction is a powerful tool in organic chemistry for forming carbon-carbon bonds. This reaction involves the addition of a Grignard reagent, which is an organomagnesium compound, to a carbonyl group. For example, when starting with bromobenzene, the introduction of magnesium creates phenylmagnesium bromide, a typical Grignard reagent.
  • First, create the Grignard reagent: \( C_6H_5Br + Mg \rightarrow C_6H_5MgBr \).
  • Next, react it with ethylene oxide: \( C_6H_5MgBr + (CH_2)_2O \rightarrow C_6H_5CH_2CH_2OMgBr \).
  • Finally, the reaction is completed by adding an acid (like HCl), which protonates the alkoxide, forming 2-phenylethanol: \( C_6H_5CH_2CH_2OMgBr + H^+ \rightarrow C_6H_5CH_2CH_2OH \).
This reaction showcases how Grignard reagents facilitate the transformation of bromobenzene to a useful alcohol, underscoring its significance in synthetic chemistry.
Reduction Reactions
Reduction reactions are useful for converting carbonyl compounds to alcohols. These reactions involve adding hydrogen (or removing oxygen) and are typically carried out with reducing agents. For the reduction of 2-phenylethanal ( \( C_6H_5CH_2CHO \)), sodium borohydride (NaBH鈧�) is often used.
  • The reaction proceeds as: \( C_6H_5CH_2CHO + NaBH_4 \rightarrow C_6H_5CH_2CH_2OH \).
Here, the carbonyl group (C=O) in the aldehyde is reduced to a hydroxyl group (OH), turning an aldehyde into a primary alcohol.Reduction reactions are vital in organic synthesis because they allow chemists to selectively reduce carbonyl compounds, transforming them into valuable alcohols.
Oxymercuration-Demercuration
Oxymercuration-demercuration is a two-step process for converting alkenes into alcohols without rearrangement. This method is convenient for fixing the hydroxyl group on a specific position across the double bond, generating alcohols from alkenes such as styrene.
  • First, the oxymercuration step: \( C_6H_5CH=CH_2 + Hg(OAc)_2 + H_2O \rightarrow C_6H_5CH(OH)CH_2HgOAc \).
  • Next, the demercuration step with sodium borohydride (NaBH鈧�): \( C_6H_5CH(OH)CH_2HgOAc + NaBH_4 \rightarrow C_6H_5CH_2CH_2OH + Hg + B(OH)_4^- \).
This reaction sequence efficiently adds a hydroxyl group to the less substituted carbon, conforming to Markovnikov's rule.In summary, oxymercuration-demercuration offers a valuable pathway for styrene conversion to 2-phenylethanol due to its high precision in adding water across double bonds.
Preparation of Alcohols
Preparation of alcohols is a fundamental process in organic chemistry, often achieved through various methods such as Grignard reactions, reduction reactions, and oxymercuration-demercuration. These transformations of organic compounds open numerous pathways to synthesize alcohols like 2-phenylethanol.
  • From carboxylic acids, such as 2-phenylethanoic acid, alcohols are prepared by reduction. Using lithium aluminium hydride (LiAlH鈧�), the carboxylic acid are reduced to aldehydes and then to primary alcohols: \( C_6H_5CH_2CO_2H + LiAlH_4 \rightarrow C_6H_5CH_2CH_2OH \).
  • Each method provides distinct advantages, like the ability to selectively reduce functional groups without affecting others, or to manipulate which carbon atoms in a compound receive specific modifications.
The preparation of alcohols is essential not only for creating building blocks in pharmaceutical synthesis but also for producing intermediates in complex organic synthesis.

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Most popular questions from this chapter

(a) The cis isomer of 3 -hexen-1-ol \(\left(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH} \square \mathrm{CHCH}_{2} \mathrm{CH}_{2} \mathrm{OH}\right)\) has the characteristic odor of green leaves and grass. Suggest a synthesis for this compound from acetylene and any necessary organic or inorganic reagents. (b) One of the compounds responsible for the characteristic odor of ripe tomatoes is the cis isomer of \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH} \square \mathrm{CHCH}_{2} \mathrm{CH} \square \mathrm{O}\). How could you prepare this compound?

On heating \(1,2,4\) -butanetriol in the presence of an acid catalyst, a cyclic ether of molecular formula \(\mathrm{C}_{4} \mathrm{H}_{8} \mathrm{O}_{2}\) was obtained in \(81-88 \%\) yield. Suggest a reasonable structure for this product.

When trans-2-butene was subjected to enantioselective dihydroxylation, the 2,3-butanediol that was formed had the (R)-configuration at one carbon. What was the confiquration at the other?

Write chemical equations, showing all necessary reagents, for the preparation of 2 -butanol by each of the following methods: (a) Hydroboration-oxidation of an alkene (b) Use of a Grignard reagent (c) Use of a Grignard reagent different from that used in part (b) (d-f) Three different methods for reducing a ketone

Predict the products formed on oxidation of each of the following with periodic acid: Sample Solution (a) The carbon鈥揷arbon bond of 1,2-ethanediol is cleaved by periodic acid to give two molecules of formaldehyde:
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