Question

8.8 g of monohydric alcohol reacts with ethyl magnesium iodide, releasing 2240 $cm^3$ of ethane at STP. This monohydric alcohol forms a carbonyl compound that answers Tollen’s test. The alcohol is:

• A. Butan-2-ol
• B. 2, 2-Dimethylpropan-1-ol
• C. Pentan-2-ol
• D. 2, 2-Dimethylethan-1-ol

Hint:

2,2-Dimethylpropan-1-ol reacts with ethyl magnesium iodide, releasing ethane and oxidizes to 2,2-dimethylpropanal, which gives a positive Tollen's test.

Answer 1

The Correct Option is B

To determine which monohydric alcohol is involved, let's analyze the problem step by step.

1. The reaction given is between a monohydric alcohol and ethyl magnesium iodide, leading to the release of ethane. This reaction indicates the alcohol is a primary alcohol because only primary alcohols release ethane when reacted with Grignard reagents like ethyl magnesium iodide.
2. The release of ethane (C₂H₆) at STP is provided as 2240 cm³. At STP, 1 mole of any gas occupies 22400 cm³. Thus, the moles of ethane released can be calculated as: \[\text{Moles of } C_2H_6 = \frac{2240 \, \text{cm}^3}{22400 \, \text{cm}^3/\text{mol}} = 0.1 \, \text{mol}\]
3. This reaction implies that 0.1 mol of alcohol reacted, because each mole of alcohol will produce one mole of C₂H₆.
4. The molar mass of the alcohol can be determined using the weight of alcohol that reacted, which is 8.8 g. Using the formula: \[\text{Molar mass of alcohol} = \frac{8.8 \, \text{g}}{0.1 \, \text{mol}} = 88 \, \text{g/mol}\]
5. From the options given, we identify which alcohol has a molecular weight of approximately 88 g/mol. Option '2,2-Dimethylpropan-1-ol' (C₅H₁₂O) has the molar mass: \[(5 \times 12) + (12 \times 1) + 16 = 88 \, \text{g/mol}\]
6. Furthermore, the alcohol forms a carbonyl compound that answers Tollen’s test. Tollen's test is used to identify aldehydes. Oxidizing 2,2-Dimethylpropan-1-ol gives an aldehyde, which confirms it is the correct alcohol.
7. Thus, the correct answer is 2,2-Dimethylpropan-1-ol.