Problem 26 - A Complete Resource Book in Chemistry for JEE Main

Question

On reacting with grignard reagent acetone gives (a) \(1^{\circ}\) alcohol (b) \(2^{\circ}\) alcohol (c) \(3^{\circ}\) alcohol (d) none of these

Step-by-Step Solution

Answer

(c) Acetone reacts with Grignard reagent to give a tertiary alcohol.

Step 1: Understanding Grignard Reagent Reaction

Grignard reagents, typically represented as RMgX, act as nucleophiles in reactions. When a Grignard reagent reacts with a carbonyl compound like acetone (a ketone), it adds to the carbonyl carbon, forming a new carbon-carbon bond. This results in an alkoxide intermediate, which is then protonated to form an alcohol.

Step 2: Structure of Acetone

Acetone is a ketone with the molecular formula \(CH_3COCH_3\). It has a central carbonyl carbon bonded to two methyl groups (\(CH_3\)). When a Grignard reagent adds to the carbonyl carbon, the structure around this atom influences the type of alcohol produced.

Step 3: Grignard Reaction with Acetone

When a Grignard reagent (RMgX) reacts with a ketone like acetone, it adds the R group to the carbonyl carbon. The reaction proceeds as follows: \(CH_3COCH_3 + RMgX ightarrow CH_3C(R)(OH)CH_3\). This intermediate is then protonated to form the corresponding alcohol.

Step 4: Identifying the Type of Alcohol

The product \(CH_3C(R)(OH)CH_3\) is a tertiary (\(3^{\circ}\)) alcohol. Each of the carbons attached to the carbon bearing the hydroxyl group corresponds to a different group: two methyl groups and one R group from the Grignard reagent, fulfilling the requirement for a tertiary alcohol.

Key Concepts

Tertiary Alcohol Formation

Tertiary alcohols are characterized by a carbon atom bonded to a hydroxyl group (OH) and three other carbon groups. In many chemical reactions, tertiary alcohols are formed through the addition of certain reagents, like Grignard reagents, to ketones such as acetone.
To form a tertiary alcohol, a Grignard reagent, typically expressed as RMgX, is used to attack a ketone or aldehyde. The reaction creates a new carbon-carbon bond, turning the initial carbonyl compound into an alcohol.

• The process begins with the Grignard reagent adding to the carbonyl carbon.
• This addition results in a new intermediate with an oxygen-containing alkoxide ion.
• Upon protonation, this structure converts into a stable tertiary alcohol.

Ultimately, the key to forming a tertiary alcohol is ensuring that the original compound, such as acetone, initially has two carbon groups attached to the central carbon, setting the stage for a Grignard addition.

Carbonyl Compounds

Carbonyl compounds are a major class of organic compounds distinguished by their carbon-oxygen double bond, known as a carbonyl group.
Examples of carbonyl compounds include aldehydes and ketones. The reactivity of these compounds in various reactions, like with Grignard reagents, stems from the polarized nature of the carbonyl bond.
Here's why this is important:

• The carbon atom in the carbonyl group has a partial positive charge; it's electrophilic.
• The oxygen atom holds a partial negative charge because of its higher electronegativity.
• When exposed to nucleophiles, like the Grignard reagent, carbonyl carbons are primed for attack.

Ketones, such as acetone, are particularly relevant because their central carbonyl group is bonded to two other carbon groups, enabling the formation of a tertiary alcohol product upon nucleophilic attack.

Nucleophilic Addition Reactions

Nucleophilic addition is a fundamental reaction type where a nucleophile donates an electron pair to an electrophile, forming a covalent bond.
This reaction is crucial when working with carbonyl compounds. Grignard reagents, known for being strong nucleophiles, are commonly used to perform nucleophilic additions.
Here's how it unfolds:

• The Grignard reagent approaches the carbonyl compound, targeting the carbon atom with its partial positive charge.
• The bond between carbon and oxygen in the carbonyl is broken, and a new carbon-carbon bond forms.
• The resulting alkoxide ion is then protonated in the presence of a suitable acid to yield an alcohol.

Nucleophilic addition reactions vary in product type depending on the nature of the carbonyl compound and the added group, like forming tertiary alcohols from ketones. Understanding this process is crucial to mastering reactions involving Grignard reagents.