Question

Which of the following alkenes on hydration gives a tertiary alcohol

• A. 2-Butene
• B. Isobutylene
• C. Ethene
• D. $\alpha$-Butylene

Answer 1

Answer: (B)

Isobutylene

Answer 2

Hydration of alkenes follows Markovnikov's rule. For an alkene to yield a tertiary alcohol, the carbon atom of the double bond that receives the -OH group must be bonded to three other carbon atoms (or two carbon atoms and one hydrogen atom, which then becomes three carbon atoms after the addition of -OH).

1. 2-Butene ($CH_3-CH=CH-CH_3$): Hydration gives Butan-2-ol (secondary alcohol).
2. Isobutylene ($(CH_3)_2C=CH_2$): Hydration gives 2-Methylpropan-2-ol ($(CH_3)_3C-OH$), which is a tertiary alcohol. The carbon atom with the double bond that receives the -OH group is bonded to two methyl groups and one $CH_2$ group (which becomes $CH_3$ after H addition), making it a tertiary carbon.
3. Ethene ($CH_2=CH_2$): Hydration gives Ethanol (primary alcohol).
4. $\alpha$-Butylene (1-Butene, $CH_2=CH-CH_2-CH_3$): Hydration gives Butan-2-ol (secondary alcohol).

Therefore, Isobutylene is the correct answer.