Opposite of an Elimination reaction?

CH3CH2Br ==> Ch2=CH2

The reagent for this is KOH in ethanol + Heat.

Say i want to go back to the reactant, 1-bromoethane, what would i react with it and what conditions would it be? I know it isn't just HBr.

Reply 1

icedsarcasm

Sure it's not just HBr? I'd say it was an electrophillic addition reaction going back the other way... It would be at room temperature and if the alkene was a liquid, you'd have to bubble the HBr gas through it...

Reply 2

Dr.Funk

I'm pretty sure its HBr gas at RTP. Electrophilic adition. That will turn ethene to 1 bromoethane.

Reply 3

manderton

Its an electrophillic addition reaction, and the only condition i can find is 'in the cold' lol.

I think you just have to know how to draw the mechanism.

the hydrogen, with the partial positive charge bonds to one side of the double bond which breaks homolytically (both electrons of one of the bonds go to the carbon the hydrogen bonds to) leaving the other carbon with a full positive charge, which attracts the intermediate bromide ion which has the negative charge - when the hydrogen bonds to the carbon, the hydrogen-bromide bond breakes with both electrons going to the bromide ion, giving it the full negative charge.

Reply 4

Prey

HBr is all you need, it's a strong acid. The reaction classified as electrophilic addition because the first step involves attack by H+.

Reply 5

Mos Def

Bah ... hmmm

Ok then what about when:

Ch3Ch2CH=CH2 ===> CH3CH2CHOHCH3 ?

Its a similar reaction, but the reagent is not just water now is it?

Reply 6

mtranter

that would be direct hydration of an alkene.
steam at 300C and 6.5x10^3 kPa in the presence of phosphoric acid catalyst.

Reply 7

Prey

I think it is conc. H2SO4 (catalyst) followed by water.

Edit: above industrial method also true.

Reply 8

Mos Def

Prey

I think it is conc. H2SO4 (catalyst) followed by water.

Edit: above industrial method also true.

Aye yours is right, but how did you know that an acid catalys was needed?

Reply 9

Prey

It's just a matter of memory at this level. I've just looked up the mechanism in my book, basically H+ from H2SO4 protonates the alkene across the C=C bond and attack by water acting as a nucleophile (OH-) follows. The H+ from water reforms the H2SO4.