Question

Identify the major product formed when 2-Methylhexan-3-ol is heated with concentrated sulphuric acid.

• A.

  

• B.

  

• C.

  

• D.

  

Hint:

Always watch out for carbocation rearrangements when dehydrating secondary alcohols near a branched carbon! A secondary carbocation right next to a $\mathrm{-CH(CH_3)-}$ node will always execute a 1,2-hydride shift to form a tertiary carbocation, ensuring the double bond ends up at the more branched position.

Answer 1

The Correct Option is A

Step 1: Understanding the Question:
The problem asks for the major structural product obtained when the secondary alcohol 2-methylhexan-3-ol is heated with concentrated sulfuric acid ($\mathrm{H_2SO_4}$), which acts as a strong dehydrating agent.

Step 2: Key Formula or Approach:
Heating an alcohol with concentrated acid triggers an acid-catalyzed dehydration reaction ($-\mathrm{H_2O}$), following an $\mathrm{E1}$ elimination mechanism through a carbocation intermediate. The final alkene distribution is determined by Zaitsev's Rule (also known as Saytzeff's Rule), which states that the most highly substituted, conjugated, and thermodynamically stable alkene will form as the major product.

Step 3: Detailed Explanation:
1. Structure of reactant: 2-Methylhexan-3-ol is $\mathrm{CH_3-CH(CH_3)-CH(OH)-CH_2-CH_2-CH_3}$.
2. Carbocation Formation: Protonation of the $-\mathrm{OH}$ group followed by loss of a water molecule produces a secondary carbocation at the C3 position:
$$\mathrm{CH_3-CH(CH_3)-\overset{+}{C}H-CH_2-CH_2-CH_3}$$ 3. Carbocation Rearrangement: This secondary carbocation is adjacent to a tertiary carbon (C2). A 1,2-hydride shift ($\sim\mathrm{H^-}$) occurs spontaneously from C2 to C3 to transform it into a significantly more stable tertiary carbocation:
$$\mathrm{CH_3-\overset{+}{C}(CH_3)-CH_2-CH_2-CH_2-CH_3}$$ 4. Deprotonation (Elimination): A proton ($\mathrm{H^+}$) is eliminated from the adjacent C3 position to establish a highly substituted double bond ($\mathrm{C=C}$) between C2 and C3. This gives the most stable alkene structure:
$$\mathrm{CH_3-C(CH_3)=CH-CH_2-CH_2-CH_3}$$ The systematic IUPAC name for this rearranged, tetrasubstituted/trisubstituted major framework is 2-methylhex-2-ene.

Step 4: Final Answer:
The major product formed is 2-Methylhex-2-ene, matching option (A).