Question

Discuss how the Hammond postulate can be used in explaining the basis for the Markovnikov’s rule.

Explain the mechanism of catalytic hydrogenation of alkenes using appropriate diagrams and reaction arrows.

Explain why the hydroboration-oxidation reaction shows non-MKV regiochemistry

Explain why hydrohalogenation and hydration obeys the MKV (Markovnikov) rule.

Explain why the alkene halogenation and the alkene halohydrin reaction are stereoselective, i.e., the trans isomer is produced but not the cis isomer.

Answer 1

1. Hammond's postulate states that the transition state of a reaction resembles either the reactants or products, to whichever it is closer in energy. In Markovnikov’s reaction, a carbocation will be formed as an intermediate and formationof carbocation is endergonic. The transition state resembles the tertiary carbocation is more stable than primary carbocation hence tertiary carbocation forms faster

2. Mechanism of catalytic hydrogenation of alkenes involves the breaking and making of bonds between alkenes and H2 simultaneously on the surface of the metal catalyst. The mechanism of fornation is

3. hydroboration-oxidation reaction shows anti-MKV regiochemistry due to the fact that hydrogen attaches to the alkene as hydride ion(nucleophile) rather than proton.

4. hydrohalogenation and hydration obeys the MKV (Markovnikov) rule because those reactions proceeds via attack of proton (an electrophile) and involves the formation of stable carbocation

5. alkene halogenation and halohydrin reaction are stereoselective because the reaction proceeds via the formation of cyclic internediate that facilitates the anti attack of nucleophile on the cyclonium intermediate.Thus it forms exclusively trans product