3
$\begingroup$

It is well-known that for electrophilic addition across an alkene, the reactivity increases when intermediate carbocation is more stable (eg. addition of $\ce{HBr}$ or acid-catalyzed addition of $\ce{H2O}$) i.e. when the electron density on carbon atom is more.

For nucleophilic addition on a carbonyl group the reactivity is opposite, i.e. more the positive charge (less electron density) on carbonyl carbon, more is the reactivity. The relation between reactivity order and electron density is opposite in the two cases. Why is this so?

I am just finishing high school, and I have been taught that the reason for this is as follows:

For any reaction the overall rate is determined by the slowest step, called the rate determining step (r.d.s).

For electrophilic addition, the r.d.s. is the formation of carbocation. Formation of carbocation is faster if the carbocation is stable. Hence rate increases and reactivity increases for stable carbocation.

For nucleophilic addition the r.d.s. is attack of the nucleophile, which forms a tetrahedral intermediate. Attack of nucleophile is faster if the carbonyl carbon is electron deficient. Hence for carbon with greater electron deficiency rate and reactivity increases.

However, when I posted this as an answer in this question, a comment on my answer mentioned that the concept of rate determining step is not applicable and basically useless. I am open to the fact that what I have learnt might be an oversimplification of a more detailed concept (similar to how Newton's laws are taught in school first, as a simplified version of relativity at low speeds).

Since what I know is obviously wrong, I would like to know the real reason behind the discrepancy of the order of reactivity.

I searched on the site to see if a similar question has been asked before, but couldn't find any.

TL;DR - For nucleophilic addition on a carbonyl group the reactivity is opposite, i.e. more the positive charge (less electron density) on carbonyl carbon, more is the reactivity. The relation between reactivity order and electron density is opposite in the two cases. Why is this so?

$\endgroup$
8
  • $\begingroup$ A more stable intermediate means that it has a greater chance to exist. The probability of any collision involving a more stable intermediate is greater than a collision with a rarer intermediate. Whereas in case of carbonyls the electron deficiency pulls the nucleophile towards itself. $\endgroup$ Jun 10 at 4:44
  • $\begingroup$ I don't know for sure but shouldn't this be considered a duplicate of the mentioned question? $\endgroup$ Jun 10 at 4:45
  • $\begingroup$ @NisargBhavsar The mentioned question does not have a correct answer so far and it's closed, so even if it is a duplicate, there is still no answer. $\endgroup$
    – TRC
    Jun 10 at 5:58
  • $\begingroup$ @NisargBhavsar Regarding the answer you mentioned in your comment (about stable intermediate and pulling the nucleophile), isn't it the same as what I already know (and what I've mentioned in my question?) $\endgroup$
    – TRC
    Jun 10 at 6:00
  • $\begingroup$ @TRC You might want to put your final question in bold or put a TL;DR in bold so the readers don’t get confused. $\endgroup$
    – Rishi
    Jun 10 at 10:45
3
$\begingroup$

After not having any answer for over a week, I wrote about this question in the comments of an answer I had posted to another question. There another user confirmed that the reasoning I have given in my question is not wrong, and is valid -

For any reaction the overall rate is determined by the slowest step, called the rate determining step (r.d.s). Reactivity is determined by rate of reaction, since more reactive implies it reacts faster.

For electrophilic addition, the r.d.s. is the formation of carbocation. Formation of carbocation is faster if the carbocation is stable. Hence rate increases and reactivity increases for stable carbocation. Thus the reactivity of electrophilic addition increases with increasing electron density.

For nucleophilic addition the r.d.s. is attack of the nucleophile, which forms a tetrahedral intermediate. Attack of nucleophile is faster if the carbonyl carbon is electron deficient. Hence for nucleophilic addition, reactivity increases with decreasing electron density.

$\endgroup$
0

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.