CBSE Class 12-science Answered

Sn1 reaction is a substitution reaction in organic chemistry, where, Sn stands for nucleophilic substitution and 1 is the rate determining factor denoting that it is unimolecular.

Hydrolysis of tert-butyl bromide with water forming tert-butyl alcohol:

The S_N1 reaction takes place in three steps:

1. Tert-butyl carbocation is formed by separating a leaving group (a bromide anion) from the carbon atom. This step is slow and reversible.                         

2. Nucleophilic attack: the carbocation reacts with the nucleophile. A third step is required to complete the reaction if the nucleophile is a neutral molecule (a solvent). If the solvent is water, the intermediate is an oxonium ion. This is a fast step in the reaction.     

3. Deprotonation: Water which acts as a base removes a proton on the protonated nucleophile to form alcohol and a hydronium ion. This is a fast step as well. 

Sn² Reactions

   * The S_N2 reaction is also known as bimolecular nucleophilic substitution.

   * It is a type of nucleophilic substitution, where a lone pair from a nucleophile attacks an electron deficient electrophilic center and bonds with it, leaving behind what may be called as a leaving group. So, the leaving group is replaced by the incoming group in the very first step.

   * Such reactions are named as bimolecular nucleophilic substitution because two reacting species are involved in this process. It is a slow and rate-determining step.

   * The S_N2 reaction is known as the interchange mechanism among inorganic chemists.

Example:

Here, in this reaction, OH⁻ (nucleophile) attacks on a bromoethane (electrophile) resulting in ethanol, with the ejection of bromide (leaving group):

Distinction between S_N2 and S_N1reactions