more examples of addition of hydrogen halides to alkenes: all symmetric
Post on 21-Dec-2015
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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: