bromination, epoxdation, dihydroxylation Flashcards
what is the stereochemistry of additions
the sp2 centre can be attacked by nu from either top or bottom forming enantiomers in racemate, no difference between them so no energetic preference
why is only trans product formed when mixing an alkene with bromine water
bromonium ion is formed forming other Br atom to the opposite side.
Bromonium ion - positive ion formed where 1 Br bonds to both C atoms of double bond.
Can also be argued to be due to sterics
describe formation trans Bromine product
Alkene pi orbital attacks sigma* on Br2 , Br-Br bond broken, forming Bromonium ion,
Nu attacks electrophile, opening the bromonium ion. Br nu orbital attacks C-Br sigma*, opening C-Br bond-forming dibromide products.
Backside attack as Br atom must come in from opposite side to C-Br bond.
how can bromonium ions be intercepted
with suitable nucleophilic solvent eg MeOH, opens up ion attacks more substituted position
NUCLEOPHILE ATTACKS MOST SUBSTITUTED END OF BROMONIUM ION
what is the stereospecificity of E and Z alkene bromination
E - meso products (mirror plane not chiral)
Z - forms racemate enantiomers chiral with no mirror image.
what is an epoxide
a three membered ring containing O which can be isolated in contrast to bromonium
Are electrophiles which are opened by nu and quenched by water forming anti product
what makes epoxide more reactive than an ethee
Ring strain
how are epoxides formed
electron-rich alkenes react with peroxy acid in concentrated syn addition
more electron rich alkenes are epoxidized faster.
what is the process of dihydroxylation
Alkene reacts with OsO4 osmium tetroxide in concerted (single step) syn cycloaddition giving 1,2-diols after hydolysis
what are the issues with OsO4
toxic and volatile
