Question

Aqueous solution of sodium salt of 2-methylbutanoic acid on Kolbe electrolysis yields P (major product). P is

• A. 2-methylhexane
• B. 3,4-dimethylhexane
• C. 2-methylpropane (isobutane)
• D. Butane

Answer 1

Answer: (B)

3,4-dimethylhexane

Answer 2

Kolbe electrolysis is a reaction that converts the sodium (or potassium) salt of a carboxylic acid into an alkane. The general mechanism involves the oxidative decarboxylation of the carboxylate ion at the anode, followed by radical dimerization.

1. Identify the starting material: The starting material is the sodium salt of 2-methylbutanoic acid.

2. Determine the alkyl radical (R•): In Kolbe electrolysis, the carboxylate ion ($R-COO^−$) loses an electron to form an acyloxy radical ($R-COO^\bullet$), which then decarboxylates to form an alkyl radical ($R^\bullet$) and carbon dioxide ($CO_2$). The R group in this case is:

      CH₃
      |
CH₃ - CH₂ - CH -

So, the alkyl radical formed is:

      CH₃
      |
CH₃ - CH₂ - CH•  (This is a sec-butyl radical or 1-methylpropyl radical)

3. Dimerization to form the major product (P): Two identical alkyl radicals combine to form a symmetrical alkane. $2R^\bullet \rightarrow R-R$ Therefore, two sec-butyl radicals will combine:

      CH₃        CH₃
      |          |
CH₃ - CH₂ - CH•  +  •CH - CH₂ - CH₃

This dimerization yields the product P:

      CH₃   CH₃
      |     |
CH₃ - CH₂ - CH - CH - CH₂ - CH₃

4. Name the product P: To name the product, find the longest continuous carbon chain and number it to give the lowest possible locants to the substituents. The longest chain has 6 carbons (hexane). The methyl groups are located at positions 3 and 4. Thus, the name of the product P is 3,4-dimethylhexane.