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What will be the major product of this elimination reaction?

The exocyclic product has more substituent groups on the alkene which should make it more stable but I have heard that exocyclic alkenes are less stable than their endocyclic counterparts. Why is this the case and is it a strong enough effect to outweigh the stability provided by a fourth alkyl substituent?

Additionally, what influence will kinetics have on this reaction? I think that the endocyclic product should be kinetically favoured because there is less steric hindrance for deprotonation of the intermediate cation at the ring position than at the isopropyl position.

bon
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2 Answers2

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What will be the major product of this elimination reaction?

In the acid-catalyzed dehydration of 1-methylcyclohexanol, both 1-methylcyclohexene and methylenecyclohexane are formed in ca. an 85:15 ratio. In your example we have two more substituents on the exocyclic double bond which will serve to further stabilize the exocyclic double bond and increase the amount of the exocyclic isomer. I don't know for sure how much it will push the equilibrium towards the exocyclic isomer, but I might guess that the product mixture would be at least 50:50, if not more of the exocyclic isomer - but I would expect a mixture.

what influence will kinetics have on this reaction?

Well you won't see the kinetics unless you go out of your way and look for it. Factors that influence which isomer is kinetically favored include:

  • statistical factors - there are 4 ring hydrogens that can be eliminated vs. only 1 on the isopropyl group; this favors kinetic formation of the endocyclic isomer
  • alignment of the hydrogen with the p-orbital on the carbocation carbon - the hydrogen on the isopropyl group can better align with the p-orbital than those hydrogens in the ring, however the cyclohexane ring can easily distort; so while this isn't a major factor it favors the exocyclic isomer
  • steric factors - I agree with your analysis, but would guess that the difference is small; still sterics would slightly favor the endocyclic isomer

It seems pretty close to call, but if I had to guess I'd say that the statistical factor wins out (and the steric argument further helps) and the endocyclic isomer would likely be the the kinetic product.

But again, in acid this is a fast equilibrium; the products will quickly equilibrate and the thermodynamically controlled product distribution will be observed. Maybe if you run the reaction at low temperature with a strong dehydrating agent ($\ce{POCl3}$) you could capture some meaningful kinetic information.

ron
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    Why is the exocyclic isomer usually disfavoured? Is it purely the statistical factor or is it thermodynamically less stable and if so why? – bon May 22 '15 at 10:29
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    The statistical factor will only matter if the reaction is kinetically controlled and generally, dehydrations with sulfuric acid are thermodynamically controlled. Take a look a this useful link showing alkene heats of hydrogenation. It shows methylenecyclohexene to be around 8 kJ/mol less stable than methylcyclohexene (about right for an 85: 15 ratio) as expected for a di- vs. tri-substituted double bond. Comparing tetramethyl ethylene to trimethylethylene we see that the former is about 1 kJ/mol more stable. – ron May 22 '15 at 14:45
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    This should push the product mix from 85:15 towards 50:50 – ron May 22 '15 at 14:46
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Please correct me if I'm mistaken, but in this case we would dehydrate to give the Zaitzev product (second listed product in the OP) as the major product. Consider also that adding heat as a reagent is common place to aid in dehydration.

Give a quick glance here for a refresher on alkene stability: https://en.wikipedia.org/wiki/Zaitsev%27s_rule

Alex
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  • Usually it would give the Zaitsev product but I have heard that the ring changes is and that the endocyclic product may be more stable – bon May 21 '15 at 21:15