January 15, 2025, 10:30:28 AM
Forum Rules: Read This Before Posting


Topic: Substitution Reactions (positions of substitution)  (Read 2351 times)

0 Members and 1 Guest are viewing this topic.

Offline wilson

  • Regular Member
  • ***
  • Posts: 51
  • Mole Snacks: +0/-0
  • Gender: Male
Substitution Reactions (positions of substitution)
« on: March 03, 2007, 04:44:03 PM »
Hi, I have read about nucleophilic, electrophilic and free radical substitutions but I have a few doubts:

1. Consider phenol.
We know that under an EAS (specifically Friedel Crafts Alkylation), we can methylate phenol to become o-cresol and p-cresol.
In the reaction we understand that the 2nd or 4th carbon on the ring has been attacked and the hydrogen is substituted by Me. We know that -OH is an ortho and para activator.
Now, can methylation take place over the -OH itself since the O has a high electron density? And can other substitution reactions like free radical substitution take place at the H atom of the -OH group itself to get -OCH3?
Does this have anything to do with intermediate stability or bond enthalpies?

2. In Nucleophilic Aromatic Substitution (addition-elimination), we can react p-nitrochlorobenzene with sodium methoxide to substitute the Cl with OCH3. Now again, why is Cl substituted and not the H on the benzene ring or the nitro group instead?

From the 2 examples above, I find that the activators are not substituted. Am I right? Why is this so?

3. In SN2, if we have a poor leaving group such as -OH, and a poor nucleophile such as Cl-, will the reaction take place? I think this would mean competition of substitution and elimination. We have to compare the nucleophile and leaving groups to determine whether the reaction is favourable, right?

This is the overarching question:
In substitution reactions, how do I know when will the substitution proceed and more importantly, which atom will be substituted?

I apologize if the answer comes clearly from the textbook. But from what I read, it didn't. I just hope that someone can clear this mess in my head.

Sponsored Links