Question

What is the effect of changing the following factors in a nucleophilic substitution reaction of alkyl halides: nature of the halide, nature of the solvent, relative concentrations of the reactants, temperature of the reaction, and nature of the nucleophile?

Answer 1

The effect of the changing following on the rate of the reaction in alky halides:

a) Naturature of the halide :

When the hologen atom is better leaving the rate of the reaction will be faster both in SN1 & SN2 reaction as the C-halogen bond is broken in the slow step of the reaction. (Reactivity order ,RI>RBr>.RCl )

b)Nature of the solvent

Polar solvent favours SN¹ reaction as it passes through cabocation formation . Carbocation is stabilsed by solvation by the solvent with high polarity . There is less effect of solvent polarity on SN²reaction as the negative charge is dispersed in T.S. & no. new charge is created.

c)Relative Concentration

As the concentration of the substrate increases, both of substrate & reagent , the rate of the reaction increases in SN² reaction. However, in SN1 reaction rate increases by increasing substrate i.e. halide concentration but no. effect of effect of reagent i.e. alkali concentration. Reagent is not involved in the slow step.

d)Effect of temperature

With increase in temperature the rate of the both SN¹ & SN² increases because of increase of molecular collusions.

e)Nature of the nucleophile

The stroner nucleophilic reagent favours SN² reaction as the nuccleophile enters in the slow step of the reaction while there is less effect of the nature of the nucleophile on SN1 reaction as ii enters in the substrate in the fast step of the reaction& it not involved in the rate limiting step.