Question

7. For the substitution reaction 1-2+ the following rate constants were measured: k (x 104 s1) 0.3 2.0 0.8 0.6 H20 NH3 SCN Is this reaction associatively or dissociatively activated? Explain. 8. Explain the difference between the nucleophilic discrimination factor and the nucleophilicity parameter. Which is generally larger?

Answer 1

7. The reaction is a nucleophilic substitution reaction which can be of two types - A) Associative and B) Dissociative. For associative mechanism: reaction rate is sensitive to the variation of nucleophile as well as to the leaving group. But for dissociative mechanism: the reaction rate is only sensitive to variation of leaving group.

For the given reaction it is seen that the reaction rate varies on variation of of different nucleophiles. If the mechanism would proceed throgh dissociative mechanism, then the rate would be same in presence of different nucleophiles.

But as the rate of reaction varies on changing the nucleophiles, we can conclude that themechanism is associative.

8. nucleophilicity parameter is a nucleophile dependent parameter (N) , which is related to a second order rate constant as

log (k) = s(N+E)

where k is the the second rate constant of the reaction at 20 degree celcius, E is electrophilicity parameter and s is nucleophile dependent slope parameter

Here the parameter s which characterises the sensitivity of reaction rate to nucleophilicity parameter is called nucleophilic discrimination factor.

So nucleophilicity parameter only describes the change in rate of reaction on varying the nucleophiles. But nucleophilic dicrimination factor is a correlation between rate and nucleophilicity parameter.