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Author: Subject: Double bond formation location?

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[*] posted on 30-5-2019 at 09:09
Double bond formation location?

Given the first compound, when dehydrating the OH on the ethane chain would the double bond form from the carbon on the ring or further away from it? Im thinking away due to stability but not entirely sure if anyone could help? The doc has images of the structure in it =]


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[*] posted on 31-5-2019 at 15:23

The product likely depends on the reaction conditions.

Under thermodynamic control - e.g. with a mild base or in acid with heating - the reaction will have a late transition state. According to the Hammond postulate this will resemble the alkene product so you will favour the more substituted (more stable) alkene ("on the ring"). This is called the Zaitsev product.

Under kinetic control - e.g. with a strong base - the TS will be early and resemble a carbanion. Accordingly you will get (initially) the terminal alkene (called the Hoffmann product). It may be the case that prolonged reaction will cause isomerisation to the internal alkene.

In either case steric effects will be important too and so bulky bases will support formation of the terminal alkene (abstract the most sterically-accessible proton).

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