more examples of addition of hydrogen halides to alkenes: all symmetric

What if alkene is asymmetric? Which product forms?

only tert-butyl chloride forms

regiochemistry explained by mechanism

constitutional isomers

more stable carbocation

what does the TS look like?

Hammond postulate: TS most resembles species which is closest in energy

TS III resembles productTS I resembles reactant

tert-butyl cation forms faster – carbocation formation is rds

electrophilic addition reactions are regioselective

Markovnikov rule: electrophile adds to sp2 carbon that is bonded to greater # of carbons

not regioselective:(draw carbocation intermediates)

CF3

HBr

F3C H

Br

anti-Markovnikov addition of HBr

or

will give mix of 2 and 3-bromo product

2-bromo only

synthesis problem:

acid-catalyzed addition of water to alkenes

catalyst regenerated

acid-catalyzed addition of alcohol

What’s happening here?

carbocation rearrangementhydride shift

alkyl (methyl) shift

ring expansion

addition of bromine to alkenes

more stable than carbocation

haohydrin formation

explaining regioselectivitythis carbon has more + character

more variations:

oxymercuration-reduction: adding water without rearrangements

source of :H-

converting alkenes to epoxides

concerted reaction:

hydroboration-oxidation: anti-Markovnikov addition of water

(don’t worry about mechanism of oxidation step, Bruice p. 187)

catalytic hydrogenation of alkenes

heats of hydrogenation can be used to compare stability of alkenes

notice: more substituted – more stable

also: trans more stable than cis

should be able to show how to do all of these transformations: