Question

Explain why primary alkyl halide reacts with nucleophile by sn2 mechanism and with tertiary alkyl halide the mechanism is sn1

Answer 1

Answer:

In SN2 reaction, attack of nucleophile takes place from backward direction. Therefore, steric hindrance is one of the major factor in SN2. Since primary alkyl halide is the least sterically hindered among primary,secondary and tertiary alkyl halides. Therefore, Primary alkyl halides undergo SN2 mechanism.

SN1 reaction depends upon the stability of the carbocation formed during substitution reaction. Since among primary, secondary, and tertiary carbocations, Primary carbocations are least stable. Therefore, it doesn’t undergo SN1 reaction.

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Answer 2

Answer:

There are two kinds of reactions of haloalkanes naming SN1 And SN2 Reaction.

Substitution Nucleophilic unimolecular (SN1)

This reaction is carried out in polar protic solvents such as water, alcohol, acetic acid etc. This reaction follows first order kinetics. Hence, this is named as substitution nucleophilic unimolecular. This reaction takes place in two steps as described below:

 

In step 1, the bond between carbon and halogen breaks due to presence of a nucleophile and formation of carbocation takes place. It is the slowest and the reversible step as huge amount of energy is required to break the bond. The bond is broken by solvation of the compound in protic solvent, thus this step is slowest of all. The rate of reaction depends only on haloalkane not on nucleophile.

 

In step 2, the nucleophile attacks the carbocation formed in step 1 and the new compound is formed.

Since, the rate defining step of the reaction is formation of carbocation, hence greater the stability of formation of intermediate carbocation, more is the ease of the compound undergoing substitution nucleophilic unimolecular or SN1 reaction. In case of alkyl halides, 3O alkyl halides undergo SN1 reaction very fast because of the high stability of 3O carbocations. Hence allylic and benzylic halides show high reactivity towards the SN1 reaction.

 

Substitution Nucleophilic bimolecular (SN2)

This reaction follows second order kinetics and the rate of reaction depends upon both haloalkane as well as participating nucleophile. Hence this reaction is known as substitution nucleophilic bimolecular reaction. In this reaction, the nucleophile attacks the positively charged carbon and the halogen leaves the group. Both the formation of carbocation and exiting of halogen take place simultaneously. In this process, unlike SN1 mechanism the inversion of configuration is observed. Since this reaction requires the approach of the nucleophile to the carbon bearing the leaving group, the presence of bulky substituents on or near the carbon atom has a dramatic inhibiting effect. So opposite to SN1 reaction mechanism, this is favoured by mostly by primary carbon, then secondary carbon and then tertiary carbon.

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