The decreasing order of reactivity towards dehydrohalogenation (E$$_1$$) reaction of the following compounds is:

An $$\text{E}1$$ elimination reaction proceeds via a two-step mechanism where the first and rate-determining step (RDS) is the loss of the leaving group ($$\text{Cl}^-$$) to form a carbocation intermediate:

$$\text{Reactivity toward E1} \propto \text{Stability of the Carbocation intermediate}$$

The general order of carbocation stability is: Allylic / Benzylic (resonance stabilized) > Tertiary ($$3^\circ$$) > Secondary ($$2^\circ$$) > Primary ($$1^\circ$$).

Step-by-Step Analysis of the Generated Carbocations:

• Compound (B): Primary Allylic Chloride

  Loss of $$\text{Cl}^-$$ yields a primary allylic carbocation ($$\text{CH}_2=\text{CH--CH}_2^+\text{--CH}_2\text{--CH}_3$$ or similar chain system). This carbocation is highly stable because the positive charge is directly delocalized via resonance with the adjacent carbon-carbon double bond. Hence, it is exceptionally reactive.




• Compound (D): Secondary Allylic Chloride

  Loss of $$\text{Cl}^-$$ forms a secondary allylic-like carbocation. While it has resonance stability, it undergoes minor structural or steric constraint compared to the primary unhindered linear conjugation of B, making it slightly less favored than B but far more stable than non-conjugated carbocations.




• Compound (C): Secondary Alkyl Chloride

  Loss of $$\text{Cl}^-$$ creates a simple secondary ($$2^\circ$$) carbocation ($$\text{CH}_3\text{--CH}^+\text{--CH}_2\text{--CH}_3$$). It is stabilized purely by hyperconjugation and inductive effects, which are weaker than resonance. It is less reactive than the allylic systems (B and D).




• Compound (A): Primary Alkyl Chloride

  Loss of $$\text{Cl}^-$$ yields a primary ($$1^\circ$$) carbocation ($$\text{CH}_3\text{--CH}_2\text{--CH}_2\text{--CH}_2\text{--CH}_2^+$$). Primary carbocations are extremely unstable and difficult to form under standard conditions. Thus, it is the least reactive compound.

Conclusion:

Arranging the compounds based on the stability profile of their respective carbocations yields the following decreasing order of reactivity:

$$\mathbf{B > D > C > A}$$

Answer: Option — B > D > C > A