Some Orgo Questions

[ANEShopeful2018]

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Hi guys, I was doing a Kaplan Subject test and there were a few points that I wasn't too sure about. Thanks for the help in advance!

1. For carbocations, more substitution = more stability while for carbanions, less sub. = more stable. However, is it true that benzyl/allyl groups stablize both? (I guess more resonance is always helpful, but the substitution depends on the nature of the group?)

2. For comparison of melting/boiling point, is more branching equal to lower MP as well as BP?

3. Referring to the above, does increased MW always increases MP and BP?

4. For amines, does more substitution mean less BP/MP because of the reduced H bonding?

Thanks! 👍

 

[loveoforganic]

1. Yes to all
2. Yes for BP, MP is tricky, and I never feel comfortable commenting on it. I think the rule they want you to memorize is what you said though.
3. I hate to say always, not knowing everything, but I'll say essentially always for analogous structures.
4. You have three main effects - induction, sterics, and MW. I would think it's too much going on to reasonably guess without more specific info.

 

[axo86]

Hi Andrew,
For your first question, I can explain more clearly. Consider the different mechanisms by which alkyl groups and allyl/benzyl groups offer stabilization. The alkyl group works primarily by inductive effects, so the trend you offered about more and less substitution is pretty straightforward. However, the allyl and benzyl groups offer stabilization through the extended conjugation of the pi system to a p orbital. The overlap here is a geometric factor of the p orbitals of the pi system being aligned with the p orbital in question, so it doesn't matter if the p orbital is empty (as in the allyl carbocation), half-filled (as in the allyl radical), or completely filled (as in the allyl carbanion) in order to get stabilization.
Hope that helps! Anyone on these boards feel free to let me know if you have more organic qs as I've had the unfortunate experience of working in ochem for a couple years 😉