Question

Treating 2-methyl-1-propanol with methylsulfonyl chloride in base, followed by reaction with pyridinium chlorochromate, and a final step in strong acid, will give an end product of: (A) 2-methyl-1-propanol. (B) 2-methylpropanal. (C) 2-methylpropanoic acid. (D) 2-methyl-1-propane.

Short answer

B) 2-methylpropanal

Step-by-step solution

- Understand the starting material

The starting compound is 2-methyl-1-propanol, which is an alcohol with the structure CH3-CH(CH3)-CH2OH.

- Reaction with methylsulfonyl chloride in base

The alcohol reacts with methylsulfonyl chloride (MsCl) in the presence of a base to form the corresponding mesylate (CH3-CH(CH3)-CH2-OMs). The mesyl group is a good leaving group.

- Reaction with pyridinium chlorochromate (PCC)

The mesylate reacts with PCC, which is an oxidizing agent. PCC will oxidize primary alcohols to aldehydes. The intermediate after this step is 2-methylpropanal (CH3-CH(CH3)-CHO).

- Reaction with strong acid

The final step involves reaction with a strong acid. Aldehydes are generally stable under acidic conditions, so the 2-methylpropanal will remain unchanged.

Key concepts

Mesylate Formation

Mesylate formation is a key step in the transformation of an alcohol to a better leaving group. The process involves reacting an alcohol with a methylsulfonyl chloride (MsCl) in the presence of a base, typically pyridine. This reaction replaces the hydroxyl (-OH) group of the alcohol with a mesylate (-OMs) group.

The mesylate group is much better at leaving during subsequent chemical reactions.

• Alcohol + MsCl + Base → Mesylate

The mesylate formation is particularly useful because the mesylate group is stable and does not participate in side reactions during the oxidation process.

This greatly improves the efficiency and selectivity of the desired reaction.

Use of PCC in Organic Chemistry

Pyridinium chlorochromate (PCC) is a versatile oxidizing agent used extensively in organic chemistry. One of its most common applications is the oxidation of primary alcohols to aldehydes.

PCC's mild nature is advantageous as it avoids over-oxidizing the aldehyde to a carboxylic acid. In the case of 2-methyl-1-propanol, reacting with PCC converts it to 2-methylpropanal.

The reaction can be summarized as:

• Primary Alcohol + PCC → Aldehyde

PCC also operates under mildly acidic conditions, which helps preserve the integrity of sensitive functional groups during the oxidation process. This makes PCC a preferred choice for selective oxidation in multi-step organic syntheses.

Aldehyde Stability Under Acidic Conditions

Aldehydes like 2-methylpropanal generally exhibit stability under acidic conditions, which is crucial for many organic reactions. In the context of this exercise, the final step involves exposing the aldehyde to a strong acid.

Aldehydes are less likely to undergo further reaction or degradation in acidic environments compared to other conditions.

• Aldehyde Stability: Aldehydes are sturdy and preserve their structure under acidic conditions.

This stability means that the intermediate product, 2-methylpropanal, will remain unchanged despite the presence of a strong acid.

This characteristic stability allows chemists to use aldehydes as intermediates in complex organic syntheses where acidic conditions are required for subsequent steps.