MCQEasyJEE 2024Alkanes (Properties, Halogenation)

JEE Chemistry 2024 Question with Solution

Number of alkanes obtained on electrolysis of a mixture of CH3COONa\text{CH}_3\text{COONa} and C2H5COONa\text{C}_2\text{H}_5\text{COONa} is:

  • A

    22

  • B

    33

  • C

    44

  • D

    55

Answer

Correct answer:B

Step-by-step solution

Standard Method

Given: Electrolysis of a mixture of CH3COONa\text{CH}_3\text{COONa} and C2H5COONa\text{C}_2\text{H}_5\text{COONa}.

Find: Number of alkanes formed.

From CH3COONa\text{CH}_3\text{COONa}, the radical formed is

CH3COONaCH3\text{CH}_3\text{COONa} \rightarrow \cdot \text{CH}_3

From C2H5COONa\text{C}_2\text{H}_5\text{COONa}, the radical formed is

C2H5COONaC2H5\text{C}_2\text{H}_5\text{COONa} \rightarrow \cdot \text{C}_2\text{H}_5

Now these radicals can combine as follows:

2CH3CH3CH32\cdot \text{CH}_3 \rightarrow \text{CH}_3\text{CH}_3

This gives ethane.

2C2H5CH3CH2CH2CH32\cdot \text{C}_2\text{H}_5 \rightarrow \text{CH}_3\text{CH}_2\text{CH}_2\text{CH}_3

This gives butane.

CH3+C2H5CH3CH2CH3\cdot \text{CH}_3 + \cdot \text{C}_2\text{H}_5 \rightarrow \text{CH}_3\text{CH}_2\text{CH}_3

This gives propane. The solution labels this as propene once, but the product formula shown is CH3CH2CH3\text{CH}_3\text{CH}_2\text{CH}_3, which is propane.

Therefore, the alkanes obtained are ethane, propane, and butane. Hence, the number of alkanes formed is 33.

The correct option is B.

Listing All Possible Couplings

Given: A mixture of acetate and propionate salts undergoes Kolbe electrolysis.

Find: Count all distinct alkane products.

The radicals formed are CH3\cdot \text{CH}_3 and C2H5\cdot \text{C}_2\text{H}_5.

Possible radical couplings are:

  1. CH3\cdot \text{CH}_3 with CH3\cdot \text{CH}_3
2CH3C2H62\cdot \text{CH}_3 \rightarrow \text{C}_2\text{H}_6
  1. C2H5\cdot \text{C}_2\text{H}_5 with C2H5\cdot \text{C}_2\text{H}_5
2C2H5C4H102\cdot \text{C}_2\text{H}_5 \rightarrow \text{C}_4\text{H}_{10}
  1. CH3\cdot \text{CH}_3 with C2H5\cdot \text{C}_2\text{H}_5
CH3+C2H5C3H8\cdot \text{CH}_3 + \cdot \text{C}_2\text{H}_5 \rightarrow \text{C}_3\text{H}_8

Thus, the distinct alkanes are C2H6\text{C}_2\text{H}_6, C3H8\text{C}_3\text{H}_8, and C4H10\text{C}_4\text{H}_{10}. Therefore, the number of alkanes is 33.

Common mistakes

  • Assuming only identical radicals combine. That is wrong because cross-coupling between CH3\cdot \text{CH}_3 and C2H5\cdot \text{C}_2\text{H}_5 also occurs. Always count both self-coupling and mixed-coupling products.

  • Confusing the product CH3CH2CH3\text{CH}_3\text{CH}_2\text{CH}_3 with propene. That is incorrect because CH3CH2CH3\text{CH}_3\text{CH}_2\text{CH}_3 is a saturated alkane, propane. Identify the product from the structural formula, not the mislabeled name.

  • Counting radicals instead of final alkanes. Radicals are intermediates formed during electrolysis, but the question asks for stable alkane products. List the final coupled hydrocarbons only.

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