Question

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 \(\left(\mathrm{PhCH}_{2} \mathrm{CO}_{2} \mathrm{H}\right)\) (d) Benzoic acid (e) Ethylbenzene (f) 1-Phenylethanol

Short answer

Use dehydration for styrene, oxidation with PCC for phenylacetaldehyde, further oxidation for phenylacetic acid and benzoic acid, hydrogenation of styrene for ethylbenzene, and Grignard reaction for 1-phenylethanol.

Step-by-step solution

Conversion to Styrene

To convert 2-phenylethanol to styrene, start by dehydrating 2-phenylethanol using an acid catalyst such as sulfuric acid (H₂SO₄) or phosphoric acid (H₃PO₄). This results in the elimination of water (H₂O) to form styrene (PhCH=CH₂). The reaction is a typical E1 elimination where the OH group is first protonated to form a good leaving group, followed by the formation of a carbocation and subsequent loss of a proton to give the double bond.

Conversion to Phenylacetaldehyde

To synthesize phenylacetaldehyde from 2-phenylethanol, carry out an oxidation reaction. Use a reagent like pyridinium chlorochromate (PCC), which will oxidize the primary alcohol group to an aldehyde. This avoids further oxidation to a carboxylic acid, stopping at phenylacetaldehyde (PhCH₂CHO).

Conversion to Phenylacetic Acid

Firstly, oxidize 2-phenylethanol to phenylacetaldehyde as in Step 2 using PCC. Subsequently, further oxidize phenylacetaldehyde to phenylacetic acid (PhCH₂CO₂H) using a strong oxidizing agent like potassium permanganate (KMnO₄) or Jones reagent (CrO₃). These reagents convert the aldehyde group to a carboxylic acid.

Conversion to Benzoic Acid

For converting 2-phenylethanol to benzoic acid, oxidize the alcohol group using a strong oxidizing agent like potassium permanganate (KMnO₄) under acidic or basic conditions. This will fully oxidize the molecule, removing a carbon to convert it to benzoic acid (PhCO₂H). Ensure that the reaction conditions are conducive for complete oxidation.

Conversion to Ethylbenzene

To make ethylbenzene from 2-phenylethanol, first, convert it to styrene through acid-catalyzed dehydration as described in Step 1. Then, perform a hydrogenation reaction by adding hydrogen gas (H₂) in the presence of a palladium or platinum catalyst, which reduces the alkene (C=C) bond into an alkane, forming ethylbenzene (PhCH₂CH₃).

Conversion to 1-Phenylethanol

Direct reduction of the 2-phenylethanol's primary alcohol to a secondary alcohol isn't directly possible, hence a workaround involves the Grignard reaction. Start with the alkylation of benzene to form acetophenone using a Friedel-Crafts reaction. Then, convert acetophenone to 1-phenylethanol by treating it with a Grignard reagent, like methylmagnesium bromide (CH₃MgBr), followed by hydrolysis to introduce the alcohol group.

Key concepts

Dehydration Reaction

A dehydration reaction is a common process in organic chemistry where water is removed from a molecule. This is typically achieved using an acid catalyst, like sulfuric or phosphoric acid. In the context of converting 2-phenylethanol to styrene, the dehydration process involves removing a molecule of water.
The reaction is categorized as an E1 elimination reaction, which involves several steps:

• First, the hydroxyl (-OH) group in 2-phenylethanol is protonated by the acid, making it a better leaving group.
• Next, a carbocation is formed as the water molecule departs.
• Finally, a proton is removed from the adjacent carbon, leading to the formation of a double bond to yield styrene.

This process is significant because it results in the transformation from an alcohol into an alkene. Dehydration reactions are not only crucial in synthetic chemistry but also in biological processes, indicating their wide-ranging importance.

Oxidation Reactions

Oxidation reactions in organic chemistry involve the increase of oxygen or the reduction of hydrogen in a molecule. They are a cornerstone in transforming functional groups.
For converting 2-phenylethanol to phenylacetaldehyde, oxidation is achieved by using pyridinium chlorochromate (PCC). This reagent is gentle enough to stop the oxidation at the aldehyde stage, preventing further transformation to a carboxylic acid. Steps in Oxidation:

• The alcohol group of 2-phenylethanol donates electrons, which PCC accepts, converting the alcohol to an aldehyde.
• This selective oxidation leaves the aldehyde group intact, an essential step for further transformations.

When oxidizing phenylacetaldehyde to phenylacetic acid, stronger agents like potassium permanganate (KMnO₄) or Jones reagent are used. These oxidize the aldehyde all the way to a carboxylic acid, demonstrating the power and breadth of oxidation chemistry in synthesizing functional groups.

Grignard Reaction

The Grignard reaction is a versatile method used to form carbon-carbon bonds, which is central to constructing more complex organic molecules. To understand how it can convert acetophenone to 1-phenylethanol, we need to look at the reaction details.
Grignard reagents are organomagnesium compounds such as methylmagnesium bromide (CH₃MgBr), which form when halides react with magnesium in a dry ether environment.
Here’s how the Grignard reaction proceeds:

• First, acetophenone, a compound with a carbonyl group (C=O), is treated with the Grignard reagent.
• The Grignard reagent acts as a nucleophile, attacking the electrophilic carbon of the carbonyl group.
• A new carbon-carbon bond forms, creating an alkoxide intermediate.
• Upon hydrolysis, the alkoxide converts into an alcohol, specifically 1-phenylethanol in this case.

This reaction allows chemists to elegantly add organic groups to existing structures, furthering the complexity and functionality of organic molecules.

Hydrogenation

Hydrogenation is a chemical reaction that involves the addition of hydrogen (H₂) to another compound, often turning unsaturated compounds into saturated ones. This process is key in converting styrene into ethylbenzene.
Using hydrogen gas in the presence of a metal catalyst, like palladium or platinum, hydrogenation proceeds smoothly:

• The catalyst provides a surface for the alkene bonds in styrene to interact with hydrogen gas.
• These double bonds (C=C) open up, allowing hydrogen atoms to add to the carbon atoms, reducing the alkene to an alkane.
• This results in the conversion of styrene to ethylbenzene, achieving the desired reduction.

Hydrogenation reactions are critical not only in synthetic organic chemistry but also in industrial applications, such as the production of margarine from vegetable oils through partial hydrogenation.

Friedel-Crafts Reaction

The Friedel-Crafts reaction is valuable for introducing alkyl or acyl groups into aromatic rings, enhancing their complexity and reactivity.
For the formation of acetophenone—a precursor in the synthesis of 1-phenylethanol—the Friedel-Crafts acylation reaction comes into play:

• An acyl chloride, such as ethanoyl chloride, reacts with benzene in the presence of a Lewis acid catalyst, like aluminum chloride (AlCl₃).
• The Lewis acid facilitates the formation of an acylium ion from the acyl chloride, which then attacks the electron-rich aromatic ring.
• The electrophilic aromatic substitution results in the formation of acetophenone.

This reaction is fundamental in organic synthesis, enabling the addition of carbon backbones to aromatic systems, thus paving the way for further chemical transformations, such as using the Grignard reaction to develop alcohols.