Why methyl chloride (CH3Cl) is inactive in SN1 reaction and (CH3)3C-Cl ( t-butyl chloride) is inactive in SN2 reaction ?

Why methyl chloride (CH₃Cl) is inactive in SN¹ reaction ?

The SN¹ reaction occurs in two step . First step is the slow ionization of C – Cl bond to form carbocation. In second step ,the nucleophile attack the carbocation to give the expected product. 

Now , methyl chloride is a primary alkyl halide . It produced methyl carbocation which is a primary carbocation . 

Since the primary carbocation is very much less stable than the secondary or tertiary carbocation, so there is no chance to occur the second step of  SN¹ reaction.

Consequently, methyl chloride (CH₃Cl) is inactive in SN¹ reaction.

Why (CH₃)₃C-Cl ( t-butyl chloride) is inactive in SN² reaction ?

The SN² reaction is one step process.The nucleophile attack the carbon atom from the opposite side of the leaving group.That is in SN² reaction ,back side attack takes place. 

Now, t-butyl chloride is a tertiary alkyl halide. Due to steric crowding of three methyl groups back side attack on carbon atom does not occur.

Consequently, (CH₃)₃C-Cl ( t-butyl chloride) is inactive in SN² reaction .

How the rate of SN¹ and SN² reaction depends on the concentration and activity of  nucleophile ?

The SN¹ reaction takes place in two step. In first step, slow ionization of alkyl halide  occurs . This is the rate determining step .

In second step, nucleophile attack the carbocation to form products . Now, the rate of SN¹ reaction does not depends on the nature and concentration of the concerned nucleophile.

Because , nucleophile does not participate in the rate determining step.

On the other hand , the rate of SN² reaction mechanism depends on the natute and concentration of the concerned nucleophile .

Because, SN² reaction is a one step process . So, in SN² reaction both substrate ( alkyl halide ) and nucleophile participate simultaneously .

Now, if the attacking atom of different nucleophile are same , on that case  strong bases are acted as strong nucleophile , such as CH3O ^– > HO ^– > CH3COO ^– etc.

But ,if the attacking atoms of nucleophile are different , in that case the nucleophile which contains less electronegative ,big size and more polarized atom, act as a strong nucleophile . Such as ,  NH2 ^– > HO ^– > I^– > Br ^– > Cl ^– .

Practice problem 

• Why methyl chloride (CH₃Cl) is inactive in SN¹ reaction ?
• Why  (CH₃)₃C-Cl ( t-butyl chloride) is inactive in SN² reaction ? 
• How does the rate of SN¹ and SN² reaction depends on the concentration and activity of  nucleophile ?

What is Borodine Hunsdiecker reaction ? Click here .