Question 28 of 32intermediate🔧 ApplyShort Answer2 marks

Write structures of all the alkenes which on hydrogenation give 2-methylbutane.

Correct Answer

The alkenes which on hydrogenation give 2-methylbutane must have the same carbon skeleton as 2-methylbutane. The double bond can be placed at different positions in this skeleton.

The possible structures are:

  1. 2-Methylbut-1-ene: CH2=C−CH2−CH3\text{CH}_2 = \text{C} - \text{CH}_2 - \text{CH}_3 with a CH3\text{CH}_3 branch on the second carbon.
  2. 2-Methylbut-2-ene: CH3−C=CH−CH3\text{CH}_3 - \text{C} = \text{CH} - \text{CH}_3 with a CH3\text{CH}_3 branch on the second carbon.
  3. 3-Methylbut-1-ene: CH3−CH−CH=CH2\text{CH}_3 - \text{CH} - \text{CH} = \text{CH}_2 with a CH3\text{CH}_3 branch on the third carbon.

Hydrogenation of these alkenes adds hydrogen across the double bond, yielding 2-methylbutane.

Exercise: EXERCISES | Q: 9.21 | (Page Page 33)
For More Understanding

Explanation

The question requires identifying all positional isomers of alkenes that share the carbon skeleton of 2-methylbutane. By placing the double bond at different valid positions (C1-C2, C2-C3, and C3-C4) in the 2-methylbutane skeleton, we get three distinct alkenes: 2-methylbut-1-ene, 2-methylbut-2-ene, and 3-methylbut-1-ene. Upon hydrogenation, the double bond is saturated to a single bond, giving 2-methylbutane in all three cases.

Solution Steps

  1. Step 1: Identify the carbon skeleton of 2-methylbutane, which is a four-carbon chain with a methyl branch at carbon 2.

  2. Step 2: Place the double bond between different carbon atoms in this skeleton to form possible alkenes.

  3. Step 3: The resulting structures are 2-methylbut-1-ene, 2-methylbut-2-ene, and 3-methylbut-1-ene.

PreviousNext