SN1 and SN2 Reaction of Haloalkanes

Haloalkanes are converted into alcohols using hydroxide ion in aqueous media through S_N1 and S_N2 Reactions. Alcohols can efficiently be prepared by substitution of haloalkanes and sulfonic esters with good leaving groups. The choice of reagents and reaction conditions for the hydrolysis is important because competitive elimination reactions are possible especially at high temperatures leading to alkenes.

The hydrolysis of haloalkanes depends on the structure of the haloalkanes, primary haloalkanes typically undergo S_N2 reactions whereas tertiary haloalkanes react an S_N1 mechanism for tertiary haloalkanes or tertiary alkyl halides. There are two kinds of reactions of haloalkanes naming S_N1 And S_N2 Reaction. 

S_N1 Reaction 

The S_N1 reaction is a substitution nucleophilic unimolecular reaction. It is a two-step reaction. In the first step, The carbon-halogen bond breaks heterolytically with the halogen retaining the previously shared pair of electrons. In the second step, the nucleophile reacts rapidly with the carbocation that was formed in the first step.

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.

Step-1:

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

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 the formation of a carbocation, hence greater the stability of formation of an intermediate carbocation, more is the ease of the compound undergoing substitution nucleophilic unimolecular or S_N1 reaction.
• In the case of alkyl halides, 3^o alkyl halides undergo S_N1 reaction very fast because of the high stability of 3^o carbocations.
• Hence allylic and benzylic halides show high reactivity towards the S_N1 reaction.

S_N2 Reaction

This reaction follows second order kinetics and the rate of reaction depends upon both haloalkane and 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.

It is a one-step reaction. Both the formation of carbocation and exiting of halogen take place simultaneously. In this process, unlike the S_N1 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 S_N1 reaction mechanism, this is favoured mostly by primary carbon, then secondary carbon and then tertiary carbon. Nucleophilic substitution reaction depends on a number of factors. Some important factors include.

• Effect of the solvent
• Effect of the structure of the substrate
• Effect of the nucleophile
• Effect of leaving-group.

Comparing S_N1 and S_N2 Reactions

The solvent in which the nucleophilic substitution reaction is carried out also has an influence on whether an S_N2 or an S_N1 reaction will predominate. Before understanding how a solvent favours one reaction over another we must understand how solvents stabilize organic molecules.

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