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I recently came across a problem of choosing which product to be made in case of pyrolytic elimination reactions. I was taught that usually Hoffman products are formed in such reactions. For e.g.:

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In this reaction, I sort of got the chills in my spine to make Zaitsev product because of the aromaticity obtained in the product but of course I obeyed the Hoffman rule that was taught to me and made the open ring structure of the 5 member ring of nitrogen connected at bottom. The answer is of course what I have illustrated but I need to understand when do we decide these. Another example is:

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Can someone please explain how do I go about this i.e. how do I know when it is Hoffman and when it is Zaitsev?

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  • $\begingroup$ I think you should also mention the temperature at which this happens. Cope elimination gives hoffman around $100^o C$ also relevant en.wikipedia.org/wiki/Ei_mechanism#General_Features the second example is a zaitsev product. There is clearly an $\alpha-H$ benefit threshold for the preferred product which is tipped by temperature $\endgroup$
    – napstablook
    Commented Jun 24, 2021 at 9:49
  • $\begingroup$ I think cope reactions exept xanthate esters occur at high temp $\endgroup$
    – davacd
    Commented Jun 24, 2021 at 10:22
  • $\begingroup$ OK, but the 2 examples in the middle were in a research paper right? what was the temperature for those? they are definitely cope. $\endgroup$
    – napstablook
    Commented Jun 24, 2021 at 10:24
  • $\begingroup$ Yeah but we usually don't decide these products on basis of alpha hydrogens I have seen reactions that give hoffman even at 450°C so I think temp is kot an issue $\endgroup$
    – davacd
    Commented Jun 24, 2021 at 10:24
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    $\begingroup$ @davacd it is not a cut and dry case. Hofmann isn't always obtained. Hofmann is only obtained in temperature ranges which vary case-to-case. But in context of competitive exams you should choose hofmann regardless unless +R effect is present. As they more often than not just want you to recognize the reaction and may not mention temperature. $\endgroup$
    – napstablook
    Commented Jun 24, 2021 at 10:57

1 Answer 1

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Cope elimination is a pericyclic reaction that always prefers syn-elimination. Due to hydroxylamine being a poor leaving group, the reactions often tend to prefer hofmann products.

  • However the reason for preference towards hofmann products is the stability of carbanion formed due to less $\ce{\alpha-H}$ and consequently less destabilization by +H effect. (since leaving group is poor, the acidic-H departs before the leaving group departs inducing negative charge at the $\ce{\beta-C}$)

  • The carbanion's stability is enhanced by the resonance effect in your first and last examples, and we know that resonance effect is often more stabilizing than hyperconjugation hence the preference of zaitsev product. Resonance effect almost always gets preference even if the reaction often produces hoffmann product. (since the leaving group can always be more bulkier there are edge cases)

  • Regarding your 2nd and 3rd examples they are probably giving zaitsev out of the temperature range for hofmann product in cope elimination. Where resonance effect is not in play, the determination between hofmann and zaitsev becomes a balance between the thermodynamic stability of the final product and the kinetic stability of the carbanion transition state.

  • All of the above examples at very high temperature will provide zaitsev product as thermodynamically favored product is formed. However in a certain temperature range these reactions give the hofmann product where the kinetic stability is dominating. Cope elimination produces hofmann product near $ \ce{T \approx 100^o C}$ pyrolysis of xanthates produces hofmann near $\ce{200^o C}$ and esters give pyrolytic syn elimination near $\ce{500^o C}$.*

  • The conclusion is that unless temperature is given, it is not exactly clear that you need to produce hofmann which is also stated in wikipedia

There are many factors that affect the product composition of Ei reactions, but typically they follow Hofmann’s rule and lose a β-hydrogen from the least substituted position, giving the alkene that is less substituted (the opposite of Zaitsev's rule).1 Some factors affecting product composition include steric effects, conjugation, and stability of the forming alkene.

The example of 5 member T.S. in wikipedia also supports zaitsev products. Basically the selectivity is not very good, and for correct answer we can only refer to data given to us. This is why temperature is important to mention.

*Source: Peter Sykes 6th edition section 9.9 (Pyrolytic syn elimination)

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  • $\begingroup$ Thankyou for that informative answer, so to sum it up at higher temp we will obviously see thermodynamic stability before finalizing product and also if In an exam say temp is not mentioned for some reason then I think we should take decesion on the basis of resonance>hyperconj and that it should be minium as instability caused due to it(while simultaneously respecting the peri planar config needed) and also check for -I effect for stabalization of carbanaion $\endgroup$
    – davacd
    Commented Jun 24, 2021 at 11:17
  • $\begingroup$ Can you confirm that in the case temp is not given then which should I give more prefferce inductive effect or less hyperconjugatuion? For eg if some withdrawing groups like halogens are there and there is also a choise of least hyperconjugatuion so which should I choose $\endgroup$
    – davacd
    Commented Jun 24, 2021 at 11:23
  • $\begingroup$ Also can you please check that I have correctly understood your point while summing my thing up $\endgroup$
    – davacd
    Commented Jun 24, 2021 at 11:24
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    $\begingroup$ @davacd from a purely exam point of view if temperature is not mentioned you should choose hofmann product and zaitsev if there is resonance to be had. If hyperconjugation effect is same for $\beta-H$ options then go for checking -I effect. You shouldn't worry about inductive effect dominating hyperconjugative effect, that decision can only be made with the relevant data. So rigidly follow the order M>H>I . $\endgroup$
    – napstablook
    Commented Jun 24, 2021 at 11:28

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