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In many organic chemistry questions, we use mechanisms that favours thermodynamically stable product (like aromatic rings), compromising the rate by some factor. For example, if there were two pathways

  1. Where final product is benzoid structure,
  2. Where final structure is not aromatic but forms faster.

Generally, we say that the benzoid structure is the major product.

Since equilibrium constant does not depend on the kinetics, after the reaction is complete, the major product must be governed by equilibrium constant. Therefore, after a very long time when the reaction is complete, the major product must be that product which is most stable thermodynamically.

But why do we consider rate in some reactions like E2 elimination? We say Hoffman product is more stable for a poor leaving group/bulky base, but $\Delta H$ for the Hoffman product is more (as Hoffman product is less stable). So, in the end, according to equilibrium constant, Zaytsev product must be major.

So, while finding the major product, do we consider rate, or $\Delta H$?

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  • $\begingroup$ I think reworking this q. as "limitations of H. & Z. rules" would help it. See, they're relevant pretty much only in the simplest scenarios, when there is only one relevant functional group. If a benzene ring can form, invoking them is a misunderstanding. $\endgroup$
    – Mithoron
    Commented Jun 20 at 12:56
  • $\begingroup$ Major products are defined either kinetically either thermodynamically, depending on which factor is decisive. If relevant reactions are slow enough, preferred is the product being formed faster. If they are fast enough, preferred is the TD preferred product. $\endgroup$
    – Poutnik
    Commented Jun 21 at 11:10
  • $\begingroup$ Right. If the reaction is slow, the observation would be that the reaction with higher rate will form major product. Thank you, I got it! $\endgroup$
    – EagerToLearn
    Commented Jun 24 at 9:33

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Major products are not "defined" they are determined by reaction conditions

You claim "Since equilibrium constant does not depend on the kinetics, after the reaction is complete, the major product must be governed by equilibrium constant". (my highlight). But this is wrong.

An extreme case would be the conversion of diamond to graphite, which is thermodynamically favoured. Lucky for the diamond industry this reaction does not occur under normal conditions.

Chemists choose reaction conditions to maximise the products they want to get, when possible. Sometimes the desired product is the kinetic product, other times it is the thermodynamic product. And the specific conditions of a reaction can alter which is the major product.

For example, high and low temperature reactions of HBr with butadiene gives different products. At low temperature the equilibrium between the final product and the intermediate is slow and the major product is the 3-bromo-1-butene (because the bromide ion reacts faster at that site in the intermediate); at higher temperatures the equilibrium is faster so the thermodynamically more stable 4-bromo product dominates.

Chemists can often manipulate reaction conditions to get either the thermodynamically favourable product or the kinetically favoured one. The major product is determined by the conditions and chemists need to be aware of both possibilities.

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  • $\begingroup$ So, whenever we use rate as predominant factor for major product, does it mean that the reaction is slow? Since, if the reaction was fast, it would attain equilibrium fast and give thermodynamically stable product, right? $\endgroup$
    – EagerToLearn
    Commented Jun 21 at 11:10

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