An alkene which is least reactive towards electrophilic...

An alkene which is least reactive towards electrophilic addition, among the following is (a) $\mathrm{H}_{2} \mathrm{C}=\mathrm{CH}-\mathrm{Cl}$ (b) $\left(\mathrm{CH}_{3}\right)_{2} \mathrm{C}=\mathrm{CH}_{2}$ (c) $\left(\mathrm{CH}_{3}\right)_{2} \mathrm{C}=\mathrm{C}\left(\mathrm{CH}_{3}\right)_{2}$ (d) $\mathrm{ClCH}_{2} \mathrm{CH}=\mathrm{CH}_{2}$

An alkene which is least reactive towards electrophilic addition, among the following is
(a) $\mathrm{H}_{2} \mathrm{C}=\mathrm{CH}-\mathrm{Cl}$
(b) $\left(\mathrm{CH}_{3}\right)_{2} \mathrm{C}=\mathrm{CH}_{2}$
(c) $\left(\mathrm{CH}_{3}\right)_{2} \mathrm{C}=\mathrm{C}\left(\mathrm{CH}_{3}\right)_{2}$
(d) $\mathrm{ClCH}_{2} \mathrm{CH}=\mathrm{CH}_{2}$

Key Concepts

Electrophilic Addition Reaction

This fundamental reaction mechanism in organic chemistry involves the addition of an electrophile to the electron-rich carbon-carbon double bond of an alkene. The process typically forms a carbocation intermediate, which is then attacked by a nucleophile. The overall reactivity is strongly influenced by the electron density of the alkene, making the initial electrophilic attack a key step in determining the reaction rate.

Substituent Effects on Alkene Reactivity

Substituents attached to the alkene can either donate or withdraw electron density through inductive effects or resonance, significantly affecting the alkene’s reactivity toward electrophiles. Electron-donating groups enhance the electron density of the double bond, generally increasing its reactivity in electrophilic addition, while electron-withdrawing groups decrease the electron density, rendering the alkene less reactive.

Alkene Substitution Patterns

The nature and extent of alkene substitution, such as whether the alkene is monosubstituted, disubstituted, or more substituted, play a crucial role in determining its stability and reactivity. More highly substituted alkenes are generally more stable due to effects like hyperconjugation, but this stabilization can sometimes result in lower reactivity towards electrophiles as the double bond becomes less electron-rich.

Inductive Effect and Hyperconjugation

These electronic phenomena are critical in assessing alkene reactivity. The inductive effect refers to the transmission of electron density through sigma bonds, affected by substituents, while hyperconjugation involves the delocalization of electrons from adjacent C–H bonds into the pi system of the double bond. Both effects can modulate the electron density and stability of alkenes, thereby influencing their susceptibility to electrophilic attack.

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