Question

The highest rate of reduction is observed when Grignard's reagent is treated with (A) Benzaldehyde (B) Benzophenone (C) Benzoyl chloride (D) Acetophenone

Short answer

Based on the reactivity of carbonyl groups and electrophilic nature of the substrates, Benzaldehyde (A) has the most electrophilic carbonyl carbon, making it the most reactive with Grignard's reagent. Therefore, the highest rate of reduction is observed when Grignard's reagent is treated with (A) Benzaldehyde.

Step-by-step solution

Understanding Grignard reagent and given compounds

Grignard reagent is an organometallic compound of the general formula R-Mg-X, where R is an alkyl or aryl group, and X is a halogen. Grignard reagents are strong nucleophiles and can form a variety of products depending on the electrophilic nature of the attacking substrate. The reactive part of a Grignard reagent is the carbon atom bonded to the Mg metal center. The given compounds are: (A) Benzaldehyde: it contains a carbonyl group (C=O) attached to a phenyl ring. (B) Benzophenone: it contains two phenyl rings attached to a central carbonyl group (C=O). (C) Benzoyl chloride: it contains a carbonyl group (C=O) attached to a phenyl ring and a chloride group. (D) Acetophenone: it contains a carbonyl group (C=O) attached to a phenyl ring and a methyl group.

Assessing the reactivity of the given compounds with Grignard reagent

Grignard reagent reacts with carbonyl groups and other electrophilic centers, and the rate of the reaction depends on the electrophilic nature of the substrate. (A) In Benzaldehyde, the carbonyl group can act as an electrophile. The Grignard reagent will attack the carbonyl carbon, resulting in a new alkoxide bond and the oxidation of the aldehyde to the alcohol. (B) In Benzophenone, the carbonyl group can also act as an electrophile. However, the presence of two phenyl rings increases electron density around the carbonyl carbon, making it less electrophilic than in benzaldehyde. (C) In Benzoyl chloride, the presence of a chloride group makes the carbonyl carbon even less electrophilic than in benzophenone. Furthermore, Grignard reagents also react with halides, leading to side-reactions and a decreased rate of reduction. (D) In Acetophenone, the carbonyl group can act as an electrophile like in benzaldehyde. However, the presence of an electron-donating methyl group increases electron density around the carbonyl carbon, making it less electrophilic than in benzaldehyde.

Identifying the compound with the highest rate of reduction

Based on the analysis, we find that Benzaldehyde (A) has the most electrophilic carbonyl carbon, and thus will be the most reactive with Grignard's reagent. The other compounds have substituents that either increase electron density around the carbonyl carbon or lead to side reactions. This makes them less reactive compared to benzaldehyde. So, the correct answer is (A) Benzaldehyde, as it will show the highest rate of reduction when treated with Grignard's reagent.

Key concepts

Electrophilic Substrates

When we talk about reactions involving Grignard reagents, we are often focusing on their interaction with electrophilic substrates. An electrophilic substrate is essentially a molecule that contains a positively polarized atom capable of attracting and forming bonds with electron-rich species, such as the negatively charged carbon in the Grignard reagent.

The rate at which a Grignard reagent reacts with an electrophilic substrate depends on the degree of the positive charge on the electrophiles. Substrates like aldehydes and ketones, with their partially positive carbonyl carbon atoms, are prime targets for attack by Grignard reagents. The more electron-withdrawing groups attached to the carbonyl carbon, the more electrophilic, and hence, the more reactive it becomes.

In our exercise, the correct answer, Benzaldehyde (A), lacks electron-donating groups, making its carbonyl carbon highly electrophilic and well-suited for attack by the Grignard reagent.

Reduction of Carbonyl Compounds

Carbonyl compounds, which include aldehydes, ketones, esters, and carboxylic chlorides, can be reduced to corresponding alcohols by the action of Grignard reagents. This process involves the breakage of the carbon-oxygen double bond (C=O) followed by the formation of a new carbon-carbon (C-C) bond.

During the reduction, the carbonyl group of aldehydes is transformed into a primary alcohol while ketones are converted into secondary alcohols. As a versatile tool for forming new C-C bonds, this reaction is a staple in organic synthesis. Aldehydes are generally more reactive in this context because they are less sterically hindered and have fewer electron-donating groups compared to ketones, which explains why Benzaldehyde (A) is the most amenable to reduction by a Grignard reagent among the options provided.

Organometallic Compounds

Grignard reagents are a class of organometallic compounds, meaning they contain a metal (magnesium in this case) directly bonded to a carbon atom of an organic group. These reagents are prepared by reacting metallic magnesium with an alkyl halide, resulting in a substance that has a highly polar bond between the carbon and the magnesium.

In Grignard reagents, the carbon side of the C-Mg bond has a partial negative charge, rendering it nucleophilic and ready to attack positively polarized or electrophilic carbon atoms. The unique reactivity of organometallic compounds like Grignard reagents comes from this polarity. They are not only pivotal in creating new carbon-carbon bonds but also serve diverse roles in organometallic chemistry, widely used for adding carbon skeletons onto molecules in the synthesis of more complex organic compounds.