Comments on: Alkene Stability

By: CHANDRU

CHANDRU — Thu, 22 Feb 2024 23:07:26 +0000

WHY TRANS ISOMER HAS A HIGHER ENTROPY? SIR

By: Garuda

Garuda — Thu, 27 Apr 2023 00:42:38 +0000

Why 1,1 di substituted alkenes are more Stable than 1,2 trans substituted alkene

By: James Ashenhurst

James Ashenhurst — Fri, 21 Apr 2023 20:36:53 +0000

In reply to Jake.

Yes – it seems somewhat contradictory that the more substituted alkene would be the most reactive in reactions such as epoxidation and also in addition of HX to alkenes.

In reactions with electrophiles (like mCPBA) the alkene is the nucleophile. Generally, the greater the number of substituents attached to the alkene, the more nucleophilic it is (electron-rich). The transition state for an epoxidation reaction has several electron-deficient carbon atoms. Greater substitution on the alkene by additional alkyl groups helps to donate electron density to the electron poor carbon atoms, lowering the energy of the transition state and leading to a faster reaction.

By: Jake

Jake — Wed, 19 Apr 2023 11:15:25 +0000

Then why in epoxidation reaction, the more substituted alkene is more reactive?

By: A

Thu, 09 Feb 2023 05:39:20 +0000

excuse me, in the molecular orbital diagram why does it say pi (C-H) + pi* and not sigma (C-H) + pi* for the molecular orbitals, when it’s a C-H sigma bond donating the electron pair? Am I missing something again?

By: James Ashenhurst

James Ashenhurst — Mon, 23 Jan 2023 15:59:06 +0000

In reply to Hashimoto Ai. IUPAC naming does not permit 2,2-dimethyl-3-pentene. It could be 2,2-dimethyl-2-pentene (which is trisubstituted) or 4,4-dimethyl-2-pentene (which is disubstituted, could be cis or trans). Offhand, without checking the data, I would confidently say that the trisubstituted alkene is more stable by about 1 kcal/mol over the trans disubstituted.

By: Hashimoto Ai

Hashimoto Ai — Sat, 21 Jan 2023 17:02:47 +0000

Hey, it is stated that “the heat of hydrogenation is quite consistent for a series of linear, (((non-branched))) monosubstituted alkenes.” What if it’s branched? Let’s say compare the stability between 2-methyl-2-butene and 2,2-dimethyl-3-pentene. Do we have to still prioritize the number of substituents as the most consideration?

By: James Ashenhurst

James Ashenhurst — Mon, 22 Aug 2022 14:43:07 +0000

The data presented in this post is all from the same paper, a study of the enthalpies of hydrogenation of the hexenes.
https://www.sciencedirect.com/science/article/abs/pii/0021961487900589 .
The heat of hydrogenation of the geminal and the trans are comparable to the first decimal place in this case.

By: James Ashenhurst

James Ashenhurst — Wed, 20 Jul 2022 20:10:02 +0000

In reply to Adam. Darn it. Thank you for bringing that to my attention, I'll have to look into fixing that.

By: Tom Chan

Tom Chan — Mon, 09 Aug 2021 13:33:59 +0000

Hi, for the appendix 1, does this mean that the sigma(C-H) orbitals can donate e- density to both pi and pi* orbitals in C=C for stabilization? And for the donation to pi* antibonding orbital, may I ask will this weaken the C=C pi bond (since I heard that donation to antibonding orbitals weaken the bonding orbitals)? thanks a lot :)