Conflicting information in TBR Orgo

[typicalindian]

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This is from an older copy of TBR and while I was reading a newer copy it states

bond a is stronger than bond c, despite both sharing an sp2 hybridized and an sp3 hypridized carbon, because bond c contains the more highly substituted carbon. Choice B is the best answer

can someone please clarify which explanation is right?

edit: its from page 11 of orgo chapter 1 in TBR if you want to look at it.

 

[MedPR]

This is from an older copy of TBR and while I was reading a newer copy it states



can someone please clarify which explanation is right?

edit: its from page 11 of orgo chapter 1 in TBR if you want to look at it.

I can't see the picture, but I think I've done this problem and had an issue with it as well. Is the question about an alkene and the bonds in question are methyl and an isopropyl? If so, check out this thread.. It's about the same question.
http://forums.studentdoctor.net/showthread.php?t=876613

 

[typicalindian]

I can't see the picture, but I think I've done this problem and had an issue with it as well. Is the question about an alkene and the bonds in question are methyl and an isopropyl? If so, check out this thread.. It's about the same question.
http://forums.studentdoctor.net/showthread.php?t=876613

you are absolutely right that is the one lol I guess i'll just skip over it now 🙁

 

[MedPR]

you are absolutely right that is the one lol I guess i'll just skip over it now 🙁

Here's the explanation my gen chem teacher emailed me, though I don't think it is pertaining to the exact question:

Teacher

Of course I am willing to help you however I can.

I assume that we are talking about the second carbon of the isopropyl group which is, of course, a tertiary carbon versus a methyl group which is a primary carbon. Think about the stability of the resulting carbocation that would form if the isopropyl group leaves versus the methyl group leaving.

Me

The question is asking specifically about the sp2-sp3 bonds formed between C1 of the alkene and the substituents. I guess the question could be worded this way: Which substituent - methyl, or isopropyl - would require less energy to cleave from the alkene.

Teacher
I believe I am still correct. 🙂

When the leaving group cleaves from the alkene, it will do so as a carbocation. This means that the leaving group which can stabilize the positive charge best will be more likely and easier to cleave away. Isopropyl can stabilize a positive charge much easier than can a methyl group (which really can't).

If, on the other hand, for some reason the leaving group cleaves as a carbanion, then the opposite would be true of the stability. The isopropyl group is less stable as a carbanion than is the methyl group.


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Now that I'm reading this again, it seems as though the answers in the original thread are right.. The whole 5+1 and 3+3 idea.

 

[Pisiform]

To me, choice D looks correct because it makes sense that more substituted carbon would be much more stable and so would have stronger bonds and greater bond dissociation energy.

 

[ljc]

If, on the other hand, for some reason the leaving group cleaves as a carbanion, then the opposite would be true of the stability. The isopropyl group is less stable as a carbanion than is the methyl group.

Can someone elaborate on this? Isn't it always better to have more room to spread out a charge, regardless of what that charge is?

 

[MT Headed]

Can someone elaborate on this? Isn't it always better to have more room to spread out a charge, regardless of what that charge is?

Yes, but alkyl substituents are electron donating and not electron withdrawing. So if you have a carbanion, and then this damn isopropyl group is donating even more electron dentisy to the carbon, ugh! Spreading the minus charge around would be good, if it had something willing to take up the minus charge (even partially).

 

[ljc]

Yes, but alkyl substituents are electron donating and not electron withdrawing. So if you have a carbanion, and then this damn isopropyl group is donating even more electron dentisy to the carbon, ugh! Spreading the minus charge around would be good, if it had something willing to take up the minus charge (even partially).

Let me get this straight. We've already agreed that cleaving the isopropyl, which is now positively charged (leaving a carbanion on the remaining molecule) is better than cleaving the methane, which would also be positively charged (again, leaving a carbanion on the remaining molecule). The other option is to cleave the isopropyl as a negative carbanion (leaving a carbocation on the remaining molecule), or a methyl as a negative carbanion (again, leaving a carbocation on the remaining molecule). Why is isopropyl carbanion+carbocation molecule worse than methyl carbanion+carbocation molecule? Why can methyl better hold a negative charge than isopropyl?

Are you saying CH3(-) is more stable than C3H7(-)?

 

[MedPR]

Let me get this straight. We've already agreed that cleaving the isopropyl, which is now positively charged (leaving a carbanion on the remaining molecule) is better than cleaving the methane, which would also be positively charged (again, leaving a carbanion on the remaining molecule). The other option is to cleave the isopropyl as a negative carbanion (leaving a carbocation on the remaining molecule), or a methyl as a negative carbanion (again, leaving a carbocation on the remaining molecule). Why is isopropyl carbanion+carbocation molecule worse than methyl carbanion+carbocation molecule? Why can methyl better hold a negative charge than isopropyl?

Are you saying CH3(-) is more stable than C3H7(-)?

the isopropyl carbanion + carbocation is worse for the same reason that the isopropyl carbocation + carbanion is better.

Carbocations are stabilized by induction (and hyperconjugation), so the more/bigger alkyl group, the more electron donation + stabilization. As you know, carbocation stability is tertiary>secondary>primary>methyl.

Carbanions on the other hand, are basically the exact opposite. You know that delocalization of electrons is good for stability. A carbanion has a surplus of electrons and doesn't want anymore negative charge on the carbon. Since alkyl groups are electron donating, they are aiding in the localization of that negative charge, which makes the carbanion even more unstable. Since hydrogens aren't really electron donating or withdrawing (as far as I know) they are, in a sense, the lesser of two evils when compared to alkyl groups on a carbanion.

 

[ljc]

Got it. Thank you!