Question

8.8 g of monohydric alcohol added to ethyl magnesium iodide in ether liberates 2240 cm³ of ethane at STP. This monohydric alcohol when oxidised using pyridinium-chlorochromate, forms a carbonyl compound that answers silver mirror test (Tollens’ test). The monohydric alcohol is :

• A. butan-2-ol
• B. 2, 2-dimethyl propan-1-ol
• C. pentan-2-ol
• D. 2, 2-dimethyl ethan-1-ol

Answer 1

The Correct Option is B

Reaction with Ethyl Magnesium Iodide (Grignard reagent):

The alcohol reacts with ethyl magnesium iodide (C₂H₅MgI) to liberate ethane (C₂H₆). The volume of ethane liberated is 2240 cm³ at STP.

• This indicates that the alcohol is a primary alcohol, as primary alcohols react with Grignard reagents to produce alkanes.
• Oxidation with Pyridinium-Chlorochromate (PCC):

The alcohol is then oxidized with Pyridinium chlorochromate (PCC), which oxidizes primary alcohols to aldehydes.

• The aldehyde formed in this reaction must be able to react with Tollens’ reagent to form a silver mirror, indicating it is an aldehyde that undergoes oxidation to form a carboxylic acid. Only aldehydes can give a positive Tollens' test (silver mirror test).

Identification of the Alcohol:

• The monohydric alcohol must be 2, 2-dimethyl propan-1-ol, as it is a primary alcohol that fits the reaction with Grignard reagent and gives an aldehyde upon oxidation that passes the Tollens’ test.
• 2, 2-dimethyl propan-1-ol is a branched primary alcohol, and oxidation of the alcohol forms 2, 2-dimethylpropanal (a non-hindered aldehyde), which reacts with Tollens’ reagent.

Thus, the monohydric alcohol is 2, 2-dimethyl propan-1-ol, making option (B) the correct answer.

Answer 2

Pyridinium chlorochromate (PCC) is a mild oxidizing agent that selectively oxidizes primary alcohols to aldehydes and secondary alcohols to ketones.

• Butan-2-ol (a secondary alcohol) can be oxidized by PCC to a ketone.
• 2,2-dimethyl ethan-1-ol (a primary alcohol) can be oxidized to an aldehyde using PCC.
• 1-butanol is also a primary alcohol, and while it can be oxidized by PCC, it leads to the formation of aldehydes.
• 2,2-dimethyl propan-1-ol is a tertiary alcohol and is generally not oxidized by PCC under normal conditions.