Why benzyl chloride is highly reactive in SN1 reaction in spite of primary alkyl halide ?

Why benzyl chloride is highly reactive in SN¹ reaction in spite of primary alkyl halide?

 In spite of primary alkyl halide benzyl chloride is sufficiently active in SN¹ reaction. This abnormal behavior of benzyl chloride can be explained on the basis of stability of benzyl carbocation.

The benzyl carbocation is formed by the slow dissociation of benzyl chloride in the first step of SN¹ reaction.

Now, benzyl carbocation under goes resonance with the pi electrons of benzene ring . So , in spite of primary carbocation, it gains enough stability.

Consequently, benzyl chloride is highly reactive in SN¹ reaction. The resonance of benzyl carbocation are as follows, 

How the rate of SN¹ and SN² reaction depends on the nature of leaving  group?

The rate of both SN¹ and SN² reaction depends on the nature of leaving  group. Because, the expulsion of leaving group occurs in both cases.

Weak bases, that is, the anions are generated from the strong acids are best as a leaving group.The increasing order of ability of anions as a leaving group are,

The ability of halide ions as a leaving group depends on their C – X bond dissociation enthalpy .

Now, the lowest bond dissociation enthalpy of C – I bond is 234 KJ / mole and the highest bond dissociation enthalpy of C – F bond which is 452 KJ / mole. 

Hence I ^– ion act as a good leaving group and F ^– ion behave as a worst leaving group among the halide ions. The order is F ^– <  Cl ^– <  Br ^– <  I ^{– .}

Which one is more active between 1-iodo butane and 1-chloro butane towards SN² reaction ?

Between 1-iodo butane and 1-chloro butane ,1-iodo butane shows more ability towards SN² reaction. Because, the SN² reaction mechanism depends on on the ability of leaving group.

Now, between iodine and chlorine, iodide is a good leaving group than chloride.

Hence, 1-iodo butane is  more reactive than 1-chloro butane.

Why vinyl chloride is inactive in SN² reaction?

The SN² reaction is a one step reaction process . In this reaction mechanism the nucleophile attack the carbon atom from the opposite side of the leaving group . That is ,bond making and bond breaking in SN² reaction occurs simultaneously .

Now, vinyl chloride have a ethylenic double bond and chlorine has three lone of electrons . 

So, this lone pair of electron on chlorine atom participate in resonance with the pi electrons. Hence ,C – Cl bond gains partially double bond character.

Therefore, the breaking of C – Cl bond is not so easy.For the above reason, vinyl chloride is inactive in SN² reaction mechanism.

Why chloro benzene is inactive in SN² reaction?

The SN² reaction is a one step reaction process. In this reaction mechanism the nucleophile attack the carbon atom from the opposite side of the leaving group .

That is ,bond making and bond breaking in SN² reaction occurs simultaneously .

Now, in case of chloro benzene ,the lone pair on chlorine atom under goes resonance with the pi electrons of benzene ring.

So, C –Cl bond of chloro benzene gets double bond character. Hence, the breaking of C – Cl bond is not so easy.Consequently, chloro benzene is inactive in SN² reaction mechanism. 

Summary 

Why benzyl chloride is highly reactive in SN¹ reaction in spite of primary halide?

 How the rate of SN¹ and SN² reaction depends on the nature of leaving  group ? 

Which one is more active, 1-iodo butane or 1-chloro butane towards SN² reaction ?

Why vinyl chloride is inactive in SN² reaction ?

Why chloro benzene is inactive in SN² reaction ?

What is Dow process ? Click here.