Problem 104 - A Complete Resource Book in Chemistry for JEE Main

Question

From which of the following tertiary butyl alcohol is obtained by the action of methyl magnesium iodide? (a) \(\mathrm{CO}_{2}\) (b) \(\mathrm{HCHO}\) (c) \(\mathrm{CH}_{3} \mathrm{CHO}\) (d) \(\mathrm{CH}_{3} \mathrm{COCH}_{3}\)

Step-by-Step Solution

Answer

Tertiary butyl alcohol is obtained from (d) \( \text{CH}_3\text{COCH}_3 \).

Step 1: Understanding Tertiary Butyl Alcohol

Tertiary butyl alcohol, also known as 2-methyl-2-propanol, has the chemical formula \[ \text{(CH}_3\text{)}_3\text{COH} \]. To obtain this compound via a reaction with methyl magnesium iodide, a Grignard reagent, we need a carbon atom that can host three methyl groups (\(\text{CH}_3\)) and an additional \(\text{OH}\) group.

Step 2: Analyzing the Options

Each option represents a different carbonyl compound. Methyl magnesium iodide (\(\text{CH}_3\text{MgI}\)) will add a \(\text{CH}_3\) group to the carbon atom of the carbonyl compound, forming an alcohol:- (a) \(\text{CO}_2\) forms a carboxylic acid on reaction, not an alcohol.- (b) \(\text{HCHO}\) reacts to form a primary alcohol.- (c) \(\text{CH}_3\text{CHO}\) reacts to form a secondary alcohol.- (d) \(\text{CH}_3\text{COCH}_3\), with two methyl groups already attached, reacts to form a tertiary alcohol.

Step 3: Concluding the Correct Reaction

For \( \text{CH}_3\text{COCH}_3 \), the action of methyl magnesium iodide will attach an extra \(\text{CH}_3\) group, resulting in \( \text{CH}_3\text{C(OH)(CH}_3\text{)_3} \), which is tertiary butyl alcohol. Hence, (d) \( \text{CH}_3\text{COCH}_3 \) is the correct reagent for the reaction.

Key Concepts

Understanding Grignard Reagents

Grignard reagents are a fascinating class of organometallic compounds, widely used in organic chemistry for forming carbon-carbon bonds. They are typically formulated as RMgX, where R represents an organic group, such as an alkyl or aryl group, and X is a halogen, like iodine, bromine, or chlorine.
These reagents are incredibly reactive. They act as strong nucleophiles, which means they have an electron-rich center that can donate an electron pair to other chemical species to form new bonds.

• Grignard reagents are formed by reacting magnesium metal with an organic halide in an anhydrous ether environment.
• They play a crucial role in the formation of alcohols from carbonyl compounds through nucleophilic addition reactions.

When Grignard reagents encounter a carbonyl compound, such as a ketone or aldehyde, a new carbon-carbon bond is formed. This often results in the production of an important class of compounds known as alcohols. This feature makes them invaluable in synthesizing complex organic molecules.

Exploring Carbonyl Compounds

Carbonyl compounds, characterized by the presence of a carbon-oxygen double bond (C=O), are a central group in organic chemistry. The carbonyl group is highly polar, with oxygen being more electronegative than carbon, making the carbon atom a great site for nucleophilic attack.
These compounds can be classified into different types, such as aldehydes, ketones, carboxylic acids, and esters, each with unique reactivity.

• Ketones, like acetone (\(\text{CH}_3\text{COCH}_3\)), have two alkyl or aryl groups attached to the carbonyl carbon.
• Aldehydes feature at least one hydrogen atom connected to the carbonyl carbon, like formaldehyde (\(\text{HCHO}\)).

In reactions with Grignard reagents, carbonyl compounds undergo a transformation where the Grignard reagent forms a bond with the carbonyl carbon, turning the C=O into a C-OH group, or an alcohol. The nature of the alcohol—whether primary, secondary, or tertiary—depends on the starting carbonyl compound and the Grignard reagent used.

Role of Methyl Magnesium Iodide

Methyl magnesium iodide is a specific type of Grignard reagent that stands out due to its simplicity and effectiveness. It is composed of a methyl group (\(\text{CH}_3\)) bonded to magnesium and an iodide ion. This reagent is primarily used to add a methyl group to a carbonyl compound, creating a new carbon-carbon bond.

• Formed by treating methyl iodide with magnesium, usually in an ether solvent, methyl magnesium iodide is highly reactive.
• It is particularly effective in transforming carbonyl compounds into alcohols by attaching a methyl group to the carbon of the carbonyl group.

When methyl magnesium iodide reacts with a carbonyl compound, a crucial step occurs: the methyl group acts as a nucleophile. This group targets the electrophilic carbonyl carbon and forms a new bond, converting the double-bonded oxygen into an alcohol (OH-group). For instance, reacting it with acetone results in tertiary butyl alcohol, showcasing its utility in forming tertiary alcohols.