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There are a few questions in thermodynamics (with unverified data) that the addition reaction of ethene to give ethyl chloride has a +ve enthalpy change, thereby elimination to give alkene must be endothermic. The latter seems to be correct as in (ideal) E2 mechanism the TS is known to stabilized by alkene character. This, as per Hammond's postulate lead us to the idea that alkenes must be closer to TS in energy coordinate.

But, there are different sources which claim it otherwise include LG Wade - that it is exothermic

Exothermic E2

What was expected and given in some other sources like Prof. William Reusch chem.libretexts.org suggests otherwise - which corresponds with Hammond's postulate is the second image.Endothermic E2

I understand, it may change with the nature of substrate and the base up to certain extent, but there could be some generalization, or at least the data about the reaction: ethyl chloride giving ethene under E2 conditions.

Could someone please verify the two opposite claims? Thanks.

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  • $\begingroup$ I have the reasoning of positive entropy favouring these reactions at high temperature, making dG more negative. Also, most of these energy profiles have been wrt dG and not dH. They do have here the energetics of electrophilic addition of alkenes, which is exothermic (actually they gave it as exoergic, dE may be dG, I guess): chem.libretexts.org/Bookshelves/Organic_Chemistry/… $\endgroup$
    – Che Mistry
    Jan 23 at 3:29

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