Question

Write the steps involved in the following conversions? (a) Benzaldehyde to benzophenone (b) Benzaldehyde to 3 -phenylpropan-1-ol (c) Benzene to 2-phenyl ethanoic acid

Short answer

Question: Provide an overview of the conversion of benzaldehyde to benzophenone. Answer: The conversion of benzaldehyde to benzophenone involves a Friedel-Crafts Acylation reaction. This reaction takes place between benzaldehyde and benzene in the presence of aluminum chloride (AlCl3) as a catalyst. The aldehyde functional group of benzaldehyde is converted to a ketone group with the addition of a benzene ring, resulting in the formation of benzophenone.

Step-by-step solution

Identify the functional groups in the starting and target compounds

Benzaldehyde contains an aldehyde functional group (-CHO), while benzophenone is a ketone containing a carbonyl group (C=O) bonded to two benzene rings. The goal here is to convert the aldehyde group into a ketone group with an additional benzene ring.

Perform a Friedel-Crafts Acylation

To achieve this conversion, we can use a Friedel-Crafts Acylation reaction. React benzaldehyde with a benzene molecule and aluminum chloride (AlCl3) as a catalyst. The aluminum chloride will activate the carbonyl group of benzaldehyde and generate electrophilic acylium ion, which then reacts with benzene to form benzophenone. The balanced reaction is: Benzaldehyde + Benzene + AlCl3 -> Benzophenone + HCl + AlCl3 This completes the conversion of benzaldehyde to benzophenone. (b) Benzaldehyde to 3-phenylpropan-1-ol

Identify the functional groups in the starting and target compounds

Benzaldehyde contains an aldehyde functional group (-CHO), while 3-phenylpropan-1-ol is an alcohol containing a hydroxyl group (-OH) and a phenyl group. The goal here is to convert the aldehyde group to a hydroxyl group with an additional CH2 group.

Perform a Grignard Reaction

To achieve this conversion, we can use a Grignard reaction. React benzaldehyde with phenylmagnesium bromide (PhMgBr) to form 3-phenylpropan-1-ol. The balanced reaction is: Benzaldehyde + PhMgBr -> 3-Phenylpropan-1-ol + MgBrOH This completes the conversion of benzaldehyde to 3-phenylpropan-1-ol. (c) Benzene to 2-phenyl ethanoic acid

Identify the functional groups in the starting and target compounds

Benzene contains a phenyl group, while 2-phenyl ethanoic acid is a carboxylic acid containing a carboxyl group (COOH) bonded to a phenyl group. The goal is to convert benzene to a carboxylic acid.

Perform a Friedel-Crafts Acylation

First, we can perform a Friedel-Crafts Acylation to convert benzene to an acetylbenzene. To do this, react benzene with acetyl chloride (CH3COCl) in the presence of aluminum chloride (AlCl3) as a catalyst. The balanced reaction is: Benzene + CH3COCl + AlCl3 -> Acetylbenzene + HCl + AlCl3

Perform a Clemmensen Reduction

Now, we have acetylbenzene. We can convert this compound to ethylbenzene via a Clemmensen reduction. React acetylbenzene with zinc amalgam (Zn(Hg)) and hydrochloric acid (HCl). The balanced reaction is: Acetylbenzene + Zn(Hg) + HCl -> Ethylbenzene + H2O + ZnCl2

Oxidize ethylbenzene to 2-phenyl ethanoic acid

Finally, we need to convert ethylbenzene to 2-phenyl ethanoic acid. This can be achieved through oxidation using potassium permanganate (KMnO4) as the oxidizing agent and aqueous sulfuric acid (H2SO4). The balanced reaction is: Ethylbenzene + KMnO4 + H2SO4 -> 2-Phenyl ethanoic acid + MnSO4 + KHSO4 + H2O This completes the conversion of benzene to 2-phenyl ethanoic acid.

Key concepts

Friedel-Crafts Acylation

The Friedel-Crafts Acylation reaction is a crucial method in organic chemistry for introducing an acyl group into an aromatic ring. This reaction involves the reaction of an acid chloride with an aromatic compound in the presence of a Lewis acid catalyst, such as aluminum chloride (AlCl extsubscript{3}).

• Aluminum chloride acts by generating an electrophilic acylium ion from the acid chloride, which is a key player in the reaction.
• The electrophilic acylium ion then reacts with the aromatic ring, facilitating the substitution of a hydrogen atom in the aromatic system with the acyl group.

In our context, the Friedel-Crafts Acylation of benzaldehyde in the presence of benzene and AlCl extsubscript{3} results in the formation of benzophenone. It's a classic example wherein an aldehyde is transformed into a ketone by introducing an electrophilic aromatic substitution reaction.

Grignard Reaction

The Grignard Reaction is a staple in organic synthesis and is renowned for its ability to form carbon-carbon bonds. This reaction involves the use of a Grignard reagent, typically an organomagnesium halide, like phenylmagnesium bromide (PhMgBr).

• The Grignard reagent acts as a nucleophile that readily attacks electrophilic carbon atoms, such as those in aldehyde or ketone groups.
• In our specific transformation, benzaldehyde, which contains an aldehyde group, reacts with PhMgBr to form 3-phenylpropan-1-ol.

This reaction highlights the Grignard reagent's ability to convert a carbonyl group into an alcohol group while elongating the carbon chain. The versatility of the Grignard reaction makes it an invaluable tool in the arsenal of synthetic chemists.

Clemmensen Reduction

The Clemmensen Reduction is an organic reaction that reduces ketones (and aldehydes) to alkanes using zinc amalgam (Zn(Hg)) and hydrochloric acid (HCl). This method is particularly useful when dealing with compounds that are sensitive to harsh acidic conditions and other metal catalysts.

• In the traditional setup, zinc amalgam serves as the reducing agent which, in the presence of hydrochloric acid, facilitates the removal of the carbonyl oxygen in ketones.
• Within our exercise scenario, acetylbenzene undergoes a Clemmensen reduction to form ethylbenzene by stripping away the oxygen atom and converting the carbonyl to a simple hydrocarbon linkage.

The advantage of this reaction lies in its ability to eliminate carbonyl functionality without impacting the remainder of the sensitive aromatic framework.

Oxidation Reactions in Organic Chemistry

Oxidation reactions are fundamental in organic chemistry, enabling the transformation of alkanes, alkenes, and alcohols into more oxidized functional groups such as ketones, aldehydes, and carboxylic acids.

• These reactions often involve the loss of electrons or the gain of oxygen and are facilitated by oxidizing agents like potassium permanganate (KMnO extsubscript{4}).
• In the conversion of ethylbenzene to 2-phenyl ethanoic acid, KMnO extsubscript{4} is employed to oxidize the alkane side chain entirely, thereby forming a carboxylic acid group.

This method is essential due to its selectivity and efficiency in modifying functional groups while preserving the integrity of the aromatic structure.

Functional Group Conversions

Functional group conversions are the cornerstone of organic chemistry reactions, providing pathways to transform one chemical functional group into another. These transformations are vital as they enable the diversification and complexity needed in synthesizing various compounds.

• In the presented exercise, functional group transformation is central to each step.
• The Friedel-Crafts Acylation, Grignard Reaction, Clemmensen Reduction, and oxidation each showcase how different techniques are utilized to achieve desired molecular architectures.

Understanding these conversions allows chemists to manipulate and design target molecules, paving the way for the synthesis of complex organic compounds.