This preview shows pages 1–3. Sign up to view the full content.
This preview has intentionally blurred sections. Sign up to view the full version.View Full Document
Unformatted text preview: Chem 2261 Notes Reactions of Alkenes (Bruice, Chapters 4) 1 Regioselectivity of Electrophilic Addition Reactions Consider the following reaction: HCl R e g i o s e l e c t i v i t y 2 possible products, yet ONLY ONE IS OBSERVED + How can we account for this selectivity? We have two competing reactions: + CH 2 G 1 G 2 CH 2 H + CH 2 + H Cl H Cl Rxn Coord Free energy tertiary carbocation primary carbocation FAVORED HCl The favored pathway is the one which proceeds through a more stable carbocation intermediate. (This is true for the reactions of other alkenes as well.) Carbocation Stability: 3 > 2 > 1 > methyl H 3 C H 3 C CH 3 + > H H 3 C CH 3 + > H H CH 3 + > H H H + attached alkyl groups stabilize the carbocation MOST STABLE LEAST STABLE This is because alkyl groups stabilize carbocations through hyperconjugation: + H + H + H + + orbital picture resonance structures charge delocalization H The energy of what species on the reaction coordinate diagram determines the overall rate of the hydrogen halide addition reaction? Is it the carbocation? Chem 2261 Notes Reactions of Alkenes (Bruice, Chapters 4) 2 It's not the energy of the carbocation but the energy of the transition state leading to the carbocation that determines the overall rate of the hydrogen halide addition reaction. The Hammond postulate (see p.164-165 of your text) tells us: The transition state will be more similar in structure to the species that it is more similar to in energy. So exergonic reactions have early, reactant-like transition states and endergonic reactions have late, product-like transition states. Transition States Predicted by Hammond Postulate early TS exergonic late TS endergonic Rxn Coord Free energy resembles reactants resembles products What species does the transition state resemble for the rate-determining step of the hydrogen halide addition reaction? Since the transition state leading to a carbocation should bear some resemblance to the carbocation, we can make the following prediction: The more stable the carbocation, the more stable the transition state leading to it. So more stable carbocations should be formed at faster rates than less stable ones. From this, we can account for the rule given above: The favored pathway is the one which proceeds through a more stable carbocation intermediate....
View Full Document