Question

If there are two different reactants in a nucleophilic substitution reaction that have two very different rates when getting to the products, what would be the reason for the difference in rate between the two reactants be?

Answer 1

The rate of a reaction can be affected by anumber of reasons like,

1. Nucleophilicity

It is the ability of a nucleophileto attack an electrophilic centre. It varies with the nucleophile. Nucleophiicity is calculated as,

logk.k₀ = S_n

where k = rate constant of the reaction, k₀ = rate constant when water is the nucleophile and  S_n = relative nucleophilicity.

2. Solvent Effect

The reaction rate depends on the extend to which the nucleophile gets solvated. Ths depending on the solvents, reaction rate can vary.

3. Steric Hinderence

The reaction rate also depends on the steric hinderence involved with the electrophile. For eg, in case of S_N1 reactions, primary halides are the ones that undergo the reaction faster due to the least hinderence involved.

4. Stability of the intermediate involved

The reaction rate can also be affected by the intermediates involved. For eg, in S_N2 type of reactions, the rate is higher for teritiary halides due to the stability of the intermediate involved. Thus more the stability of the intermediate, more is the reaction rate.