[Anthasci]

I am preparing for an exam and I'd need some extra confirmation for the following problems:

Firstly, there is the arranging of carbocations in an (increasing) order of stability. We have (all para)

a) +CH₂-Ph-propyl
b) +CH₂-Ph-OCH₃
c) +CH₂-Ph-F
d) +CH₂-Ph-i-propyl
e) +CH₂-Ph-N(CH₃)₂

My reasoning went as followed - least stable is c), because F is more -I than +M and seems to be the most EWG group here. Next should then be d), since Me₂CH is electron donating, but unable to resonate (subquestion - why do I have it written as +M/+I? Which electrons can it possibly donate? Next in line is a) because the alkyl chain is more spread and thus +I effect is lesser.
Then, I come at a dilemma - both b) and e) are more +M than -I, and I'd think nitrogen is more happy with a positive charge than oxygen. However, R₃N⁺ is -M/-I and that would be destabilizing to the carbocation. Is there something to this or am I just using circular logic here?

To sum it up, this is what I believe to be correct (increasing order of stability): c, d, a, b, e
Am I missing something here?

I may need more clarification later on.

[orgopete]

The difference between a) and d) should be small as the substitution is separated by an additional atom. However, let's rethink the order. As inferred, carbon is a better electron donor than hydrogen. Then what effect would you expect if you were to compare Me, Et, Pr, iPr, and t-Bu? Except for propyl, they are a methyl group in which successive hydrogen atoms are replaced with methyl groups. I would have predicted a similar effect as substituting a hydrogen on ammonia with an alkyl group. However, there is a notorious reversal with triethylamine not being the most basic. I cannot say this does or does not happen with the alkyl substitutions I was alluding to. (I didn't search for any benzyl halide solvolysis rates.)

The reasoning is correct in predicting e) to be more stable than b). You need to be careful in comparing -NR3(+) though as it is not the same as -NR2. If you wished to compare b) and e) on that basis, you would have to compare -OR2(+) to -NR3(+). The ammonium looks more stable, though not a group to stabilize a benzylic carbocation. Neither of these are present and do not apply. The correct analysis should be the -NR2 with does stabilize the carbocation by formation of =NR2(+). That is different.

Just a technical detail of a solvolysis. If a benzyl alcohol is formed, then HX is also formed. With an aniline, the aniline could capture the HX and form an -NHMe2(+) and reverse the stabilization effect of an amino group. Presumably, the experiments by kinetics or buffering avoid this complication.

[Anthasci]

Thanks Pete, that was quite informative. Yeah, I just realized my mistake of associating =NR2(+) with -NR3(+), so that's that.

I have, however another problem at hand. I have to determine which one in the following pair is a better candidate for E2 HX elimination (X is Cl):



Now in my opinion, these two stereoisomers are not enantiomers - there's a C2 symmetry and moreover, the mirror images are indentical. Still, I went and drew both cyclohexane chair conformers for both compounds. As it turns out, only one conformer of each has an axial Cl and in both compounds, this same conformer also has the axial methyl group. This seems to confirm my same-compounds hypothesis. My final answer therefore would be that both eliminations would proceed at a basically equally slow rate, since both appropriate conformers are the less stable version of another (axial methyls). Is this correct?

[Anthasci]

Good, thanks. What about this reaction?



The only way I see this reaction going is the transfer of the C=O bond to O, which is then a prime target for the proton, whereas the (-)OMe is able to bond to the electrophyllic carbocation. I realize this might be a bit of an easy question, but a friend of mine had MeOH in place of OMe and since I transcribed it from him (they did this example on a seminar and I was absent due to unfortunate events), I can't imagine the professor telling them the wrong course of the reaction. Unless of course, he copied it wrong.

[discodermolide]

Surely this must give the dimethyl acetal via protonation of the carbonyl oxygen?