Problem 8 - Arihant Objective Approach to Organic Chemistry for IIT JEE and NEET all Medical Entrances Part 4

Question

1-Phenyl ethanol can be prepared by the reaction of benzaldehyde with: \(\quad\) |CBSE 1997| (a) methyl bromide (b) ethyl iodide and magnesium (c) methyl iodide and magnesium (d) methyl bromide and aluminium bromide

Step-by-Step Solution

Answer

(c) methyl iodide and magnesium

Step 1: Understand the Reaction Type

1-Phenyl ethanol can be prepared from benzaldehyde using a Grignard reaction. This reaction involves the addition of an organomagnesium halide to a carbonyl compound, such as an aldehyde, leading to the formation of an alcohol.

Step 2: Identify the Reagent

To form 1-Phenyl ethanol, benzaldehyde must react with a Grignard reagent. The Grignard reagent is typically formed by the reaction of an alkyl or aryl halide with magnesium metal. In this case, the target compound (1-Phenyl ethanol) indicates that a Grignard reagent made from an appropriate halide should react with benzaldehyde.

Step 3: Determine the Suitable Grignard Reagent

For the production of 1-Phenyl ethanol, the specific Grignard reagent needed is formed from methyl iodide and magnesium. This reagent, methyl magnesium iodide, will add the methyl group to the benzaldehyde, producing 1-Phenyl ethanol.

Step 4: Choose the Correct Option

Reviewing the options, **methyl iodide and magnesium** is the correct combination to form the required Grignard reagent for this synthesis, matching option (c).

Key Concepts

Organomagnesium Halide

This is a key component used in many chemical reactions, allowing for powerful transformations in organic chemistry. Organomagnesium halides are often referred to as Grignard reagents, after the chemist Victor Grignard who discovered them. These reagents are formed by the reaction of an alkyl or aryl halide with magnesium metal. The resulting compound, typically represented as RMgX, where R is an organic group and X is a halogen, is known for its nucleophilicity.
Grignard reagents are highly reactive and are used to create new carbon-carbon bonds in molecules. They play a crucial role in processes that convert simple organic molecules into more complex structures.
When handling these reagents, it's important to maintain anhydrous conditions to prevent reactions with water, which can deactivate the reagent.

• Formation: React an alkyl or aryl halide with magnesium in an anhydrous solvent like ether.
• Reactivity: Known for their strong nucleophilic properties.
• Usage: Essential for forming carbon-carbon bonds.

Carbonyl Compound

Carbonyl compounds are organic molecules that contain a carbon-oxygen double bond, or a carbonyl group (C=O). Typical examples include aldehydes and ketones. In the context of Grignard reactions, aldehydes usually serve as the carbonyl compound for reactions to synthesize alcohols. Their structure allows them to act as electrophiles, meaning they attract nucleophiles like Grignard reagents.
The reaction between a Grignard reagent and a carbonyl compound involves the nucleophile attacking the electrophilic carbon atom within the carbonyl group. This process effectively transforms the C=O group into a C-OH group, resulting in an alcohol.

• Types: Includes aldehydes, ketones, carboxylic acids, etc.
• Reactivity: Serve as electrophiles in nucleophilic addition reactions.
• Role in Synthesis: Key for producing alcohols when reacting with Grignard reagents.

1-Phenyl Ethanol Synthesis

1-Phenyl ethanol is a useful organic compound synthesized via the Grignard reaction. This process involves the reaction of benzaldehyde, a carbonyl compound, with a Grignard reagent, specifically methyl magnesium iodide. To achieve this synthesis, you start with benzaldehyde and add methyl magnesium iodide. The methyl magnesium iodide supplies a methyl group, which attaches to the carbon atom of the aldehyde. This forms an intermediate alkoxide, which upon protonation, results in the formation of 1-Phenyl ethanol.
This transformation showcases the versatility of the Grignard reaction, enabling the addition of alkyl groups to form secondary alcohols.

• Starting Materials: Benzaldehyde and methyl magnesium iodide.
• Intermediate: Alkoxide formed during the reaction.
• Final Product: Yields 1-Phenyl ethanol after protonation.