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Magazine Chapter 17: Problem 47
How would you prepare the following compounds from 2-phenylethanol? More than one step may be required. (a) Styrene \(\left(\mathrm{PhCH}=\mathrm{CH}_{2}\right.\) ) (b) Phenylacetaldehyde (PhCH \(_{2}\) CHO) (c) Phenylacetic acid (PhCH \(_{2} \mathrm{CO}_{2} \mathrm{H}\) ) (d) Benzoic acid (e) Ethylbenzene (f) Benzaldehyde (g) 1 -Phenylethanol (h) 1 -Bromo- 2 -phenylethane
Short Answer
Expert verified
Convert 2-phenylethanol through dehydration, oxidation, reduction, or substitution, depending on the target compound.
Step by step solution
01
Convert 2-phenylethanol to styrene
To prepare styrene from 2-phenylethanol, first dehydrate the alcohol to form phenylethene:
1. Use concentrated sulfuric acid (H鈧係O鈧�) or phosphoric acid (H鈧働O鈧�) to protonate the alcohol and promote the formation of a carbocation.
2. The carbocation then loses a molecule of water, leading to the formation of styrene (PhCH=CH鈧�).
02
Convert 2-phenylethanol to phenylacetaldehyde
To synthesize phenylacetaldehyde from 2-phenylethanol, perform a mild oxidation reaction:
1. Use pyridinium chlorochromate (PCC) or Dess-Martin periodinane (DMP) to oxidize the primary alcohol group in 2-phenylethanol.
2. This will yield the corresponding aldehyde, phenylacetaldehyde, PhCH鈧侰HO.
03
Convert 2-phenylethanol to phenylacetic acid
To make phenylacetic acid from 2-phenylethanol, oxidize the alcohol to a carboxylic acid:
1. First, oxidize 2-phenylethanol to phenylacetaldehyde using PCC or DMP.
2. Then, further oxidize phenylacetaldehyde to phenylacetic acid using a stronger oxidizing agent like potassium permanganate (KMnO鈧�) or chromium trioxide (CrO鈧�).
04
Convert 2-phenylethanol to benzoic acid
To transform 2-phenylethanol into benzoic acid, you must oxidize the side chain entirely:
1. Use a strong oxidizing agent such as potassium permanganate (KMnO鈧�) or sodium dichromate (Na鈧侰r鈧侽鈧�) in acidic conditions.
2. This will oxidize the entire side chain, cleaving it to form benzoic acid (C鈧咹鈧匔OOH).
05
Convert 2-phenylethanol to ethylbenzene
Preparing ethylbenzene from 2-phenylethanol involves reduction followed by alkylation:
1. First, reduce the hydroxyl group to a hydrogen to form ethylbenzene by treating 2-phenylethanol with hydrogen in the presence of a palladium catalyst (Pd/C) or using lithium aluminum hydride (LiAlH鈧�).
2. The hydrogenation reaction should yield ethylbenzene (C鈧咹鈧匔H鈧侰H鈧�).
06
Convert 2-phenylethanol to benzaldehyde
To prepare benzaldehyde, oxidize the primary carbon in 2-phenylethanol selectively:
1. Use manganese dioxide (MnO鈧�) to oxidize the primary alcohol to an aldehyde without further oxidation.
2. This should convert 2-phenylethanol to benzaldehyde (C鈧咹鈧匔HO).
07
Convert 2-phenylethanol to 1-phenylethanol
To prepare 1-phenylethanol, you must rearrange the alcohol position:
1. Convert 2-phenylethanol to the corresponding ketone, acetophenone (C鈧咹鈧匔(O)CH鈧�), by using an oxidizing agent like chromic acid.
2. Then, reduce the ketone to an alcohol with lithium aluminum hydride (LiAlH鈧�) to yield 1-phenylethanol.
08
Convert 2-phenylethanol to 1-bromo-2-phenylethane
To synthesize 1-bromo-2-phenylethane from 2-phenylethanol, perform a substitution reaction:
1. Convert 2-phenylethanol to its corresponding tosylate by reacting with p-toluenesulfonyl chloride (TsCl) in the presence of a base like pyridine.
2. Replace the tosyl group with a bromide ion by reacting it with sodium bromide (NaBr) in acetone to yield 1-bromo-2-phenylethane.
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Key Concepts
These are the key concepts you need to understand to accurately answer the question.
Phenylacetaldehyde Preparation
To prepare phenylacetaldehyde from 2-phenylethanol, you'll need to gently oxidize the alcohol. This process involves converting the primary alcohol group into an aldehyde. A suitable reagent for this transformation is pyridinium chlorochromate (PCC) or Dess-Martin periodinane (DMP). These reagents selectively oxidize primary alcohols to aldehydes, without further oxidation to carboxylic acids.
Here is the basic idea:
Here is the basic idea:
- The 2-phenylethanol is treated with PCC or DMP.
- The reagent oxidizes the hydroxyl group (OH) to form phenylacetaldehyde (PhCH2CHO).
Styrene Synthesis
To convert 2-phenylethanol to styrene, a dehydration process is essential. Dehydration involves the removal of a water molecule from alcohol, forming an alkene such as styrene. This reaction requires an acid catalyst to facilitate the elimination of water. Concentrated sulfuric acid (H鈧係O鈧�) or phosphoric acid (H鈧働O鈧�) are common choices for this purpose.
Steps involved in the process:
Steps involved in the process:
- First, the acid protonates the alcohol group, turning the OH into a better leaving group.
- Next, a carbocation forms, allowing the elimination of water.
- This results in the formation of a double bond, yielding styrene (PhCH=CH鈧�).
Phenylacetic Acid Formation
Phenylacetic acid can be synthesized from 2-phenylethanol through a two-step oxidation process. Initially, the alcohol is oxidized to phenylacetaldehyde, which is then further oxidized to form the acid.
Here's how to achieve this transformation:
Here's how to achieve this transformation:
- Start with the oxidation of 2-phenylethanol to phenylacetaldehyde using pyridinium chlorochromate (PCC) or Dess-Martin periodinane (DMP).
- Subsequently, apply a strong oxidizing agent such as potassium permanganate (KMnO鈧�) or chromium trioxide (CrO鈧�) to oxidize the aldehyde to phenylacetic acid (PhCH鈧侰OOH).
Benzoic Acid Synthesis
Converting 2-phenylethanol to benzoic acid requires breaking down the entire side chain through oxidation. This specific reaction involves using robust oxidizing agents that can cleave the side chain completely.
Consider these steps:
Consider these steps:
- Employ a strong oxidizing agent such as potassium permanganate (KMnO鈧�) or sodium dichromate (Na鈧侰r鈧侽鈧�) in an acidic medium.
- This cleavage results in the formation of benzoic acid (C鈧咹鈧匔OOH).
Ethylbenzene Production
Producing ethylbenzene from 2-phenylethanol involves reduction and alkylation steps. The goal is to remove the hydroxyl group and form an ethyl group.
Here's the breakdown:
Here's the breakdown:
- Start by reducing the hydroxyl group of 2-phenylethanol to hydrogen, using hydrogen gas with a palladium catalyst (Pd/C) or lithium aluminum hydride (LiAlH鈧�).
- The reaction directly replaces the OH with a hydrogen atom, yielding ethylbenzene (C鈧咹鈧匔H鈧侰H鈧�).
Benzaldehyde Synthesis
To synthesize benzaldehyde from 2-phenylethanol, selective oxidation is essential. The reaction must stop at the aldehyde stage without progressing to a carboxylic acid. Manganese dioxide (MnO鈧�) is an effective reagent for this selective oxidation.
Here's how it works:
Here's how it works:
- Apply MnO鈧� to 2-phenylethanol to oxidize the primary alcohol specifically to an aldehyde.
- This yields benzaldehyde (C鈧咹鈧匔HO), an essential aromatic compound frequently used in creating perfumes, dyes, and flavoring agents.
1-Phenylethanol Preparation
The preparation of 1-phenylethanol from 2-phenylethanol involves rearrangement and reduction steps. This process includes transforming an alcohol into a ketone and then back to a different alcohol.
Here鈥檚 the method:
Here鈥檚 the method:
- First, oxidize 2-phenylethanol to acetophenone, a ketone, using an oxidizing agent like chromic acid.
- Then, reduce the ketone back to an alcohol using lithium aluminum hydride (LiAlH鈧�) to achieve 1-phenylethanol.
