GC Question

[DrHJ]

Hello all,
I have two questions:
1. Is NH2 an electron withdrawing or donating group?

2. This is a part of question No. 20 of Destroyer:
I answered this question correctly, but in the answers it is written that choice b is the most unstable between choices b and d: (See attached)
Doesn't the gauche interaction between Br and CH3 make it less stable than gauche interaction between Br and Br?

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[qkrdpsk]

NH2 is an EDG and idk about 2nd question lol I thought it was because Br was larger but I'm wrong

 

[Likkriue]

on the 2016 destroyer that is question 29, and the solution says Br-Br is preferred in terms of gauche over ch3-br. But remember the two you listed aren't gauche interactions anymore but eclipsed which may have different values, Orgoman would probably be able to answer this better. I have no idea what the eclipsed values for energy is.

 

[DrHJ]

NH2 is an EDG and idk about 2nd question lol I thought it was because Br was larger but I'm wrong

I think NH2 is electron donating too. But in Q24 of destroyer (I am attaching the question), it explains that choices B, C and E all have electron withdrawing groups. But choice B has NH2 which is electron donating.

 

[Likkriue]

It's saying the phenyl is electron withdrawing not the amine group. Comparing basicitys you want to focus on the amine group. The more electron donating groups the more basic the amine because the conjugate acid is more stable with electron donating groups. A is correct cause the cycloalkyl group stabilizes the conjugate acid of the amine. All the other choices are full of electron withdrawing groups. Electron donating groups stabilize the positive charge on the NH3+(Conjugate acid) Meaning there will be more OH- ions available. Electron withdrawing groups like phenyl will destabilize the positive charge as it is already lacking electrons causing the conjugate acid to be less stable and preferred shifting the equilibrium left and thus less OH- will be available.

Remember to compare basicitys you have to look at the stability of the conjugate acid. Vice versa to compare aciditys you have to look at the stability of the conjugate base. This is the one rule that must be stuck on your head, as it will help you solve these problems in less than a second.