Question

The decreasing order of reactivity of the following alkenes towards HBr addition is: \(\text{CH}_3-\text{CH}=\text{CH}_2 \, (\text{I}), \quad \text{CF}_3-\text{CH}=\text{CH}_2 \, (\text{II}), \quad \text{MeOCH}=\text{CH}_2 \, (\text{III}),\) , \((\text{IV}) \)

• A. \(I>II>III>IV\)
• B. \(II>IV>I>III\)
• C. \(III>IV>I>II\)
• D. \(III>I>II>IV\)

Answer 1

The Correct Option is D

The reactivity of alkenes towards HBr addition depends on the stability of the carbocation intermediate formed during the reaction:

• \(\text{- MeOCH=CH}_2\) (III) forms a resonance-stabilized carbocation due to the electron-donating methoxy group, making it the most reactive.
• \(\text{- CH}_3\text{-CH=CH}_2\) (I) forms a secondary carbocation, which is moderately stable.
• \(\text{- CF}_3\text{-CH=CH}_2\) (II) forms a destabilized carbocation due to the electron-withdrawing CF₃ group.
• \(\text{- CH}_3 - \overset{\text{O}}{\overset{||}{\text{C}}} - \text{CH=CH}_2\) (IV) forms a sterically hindered carbocation, making it the least reactive.

Thus, the order of reactivity is III>I>II>IV.