Answer to Question #167121 in Organic Chemistry for sally

Question

Explain the following statements:

(i) Inversion of configuration for product from SN2 reaction.

(ii) Tertiary alkyl halide undergoes SN1 reaction but primary alkyl halide favorites SN2

reaction

Answer 1

(i) Inversion of configuration usually happens when an organic compounds undergoes Nucelophilic substitution reaction by SN2 mechanism.

A Nucleophile (Electron rich species having affinity towards electron deficient centre) can attack the Stereocenter in two ways, like it can attack from frontside or from backside.

In the backside attack, the nucleophile attacks the stereocenter from the opposite side of the leaving group. This results in formation of the product having Inversion Of Configuration.

As the reactions between attacking Nucleophile (-OH) and Carbon starts, their bond strengthens & when this happens the configuration of Carbon atom under attack inverts in the same manner as an umbrella is turned inside out when caught in a strong wind. This process is called Inversion Of Configuration as can be seen the above equation.

(ii)An SN1 reaction makes a carbocation intermediate that has to be stable enough that it hangs around long enough for the nucleophile to find (by random motions and bumping around). Methyl, ethyl, etc. are considered to be electron donating groups so the more of them that are attached to the carbocation, the more stable it is. Hence. Tertiary substitution reactions will proceed via SN1.

An SN2 reaction does not create a carbocation intermediate so the extra methylation on the carbon at which the substitution is happening does not help. In fact, because SN2 reactions only proceed when the nucleophile (again, by random bumping around) latches onto the back side of the carbon (opposite side from the leaving group), the SN2 reaction will happen best if there are only hydrogens on that carbon because the tiny hydrogens won’t get in the way. A primary carbon has one methyl in it but SN2 can still proceed because the nucleophile still finds room to get in there. A secondary carbon has two methyl now but SN2 can still happen if the nucleophile is particularly good or the leaving group is particularly poor. Finally, the tertiary carbon has three methyl groups all crowding around the back side of the carbon from the leaving group so the nucleophile can almost never get into the carbon. This is called steric hindrance.

So a primary carbon will more likely undergo SN2 whereas a tertiary carbon will more likely do SN1 and a secondary carbon could go either way, depending on the relative strength of the nucleophile and the leaving group.