Chemical Forums
Chemistry Forums for Students => Organic Chemistry Forum => Topic started by: Billqaz3 on June 08, 2016, 01:38:04 PM
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My book seems to imply that because of Hammond's postulate endothermic reactions have a large difference in product ratios, while exothermic reactions tend to have smaller differences in product ratios. Can someone explain this to me.
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My understanding: It's not necessarily whether the overall reaction is endothermic or exothermic. It's whether the step that controls the selectivity is exothermic or endothermic. Often the step that controls the selectivity involves formation of two different reactive intermediates, for example two different carbocations. Since formations of the carbocations are endothermic, the transition states leading to the carbocations also resemble the two possible carbocations. So one is a "secondary" transition state and another is a "tertiary" transition state. So the relative rates of carbocation formation reflects their relative stability.