Reverse of optical isomerism in a reaction

[Kushal]

Homework Statement

when chiral 2-bromobutane reacts with aqueous hydroxide ions, the optical activity of the product is reversed. Suggest why the optical isomerism is reversed.

The Attempt at a Solution

i have no idea... but i don't know if the fact that the hydroxide ion will attack the electron deficient carbon (from C - Br) away from the Br atom to avoid repulsion may lead to the answer...

thanks!

 

[Borek]

You are on the right track. Try to imagine how the reaction proceeds. Molecule model may come handy.

 

[Kushal]

ermmm well i tried to imagine but i still can't see how a dextro or leavo enantiomer could become the opposite?!

because the Br^- will go and the OH^- will just attach to the electron deficient carbon.

does that have something to do with the S_N2 mechanism?

 

[Borek]

does that have something to do with the S_N2 mechanism?

Yes.

 

[Kushal]

i had a look at the mechanism but i can't find how there could be a reversal in chirality. a hint would be well appreciated.

thnks

 

[Borek]

You start with R1, R2, R3 and Br. You end with R1, R2, R3 and OH. During reaction R1, R2 and R3 are first "made flat" (when there exist intermediate, 5 substituded carbon) and then they are "switched" to the other side, as if they were reflected in the mirror.

 

[Kushal]

so you mean that when the carbanion forms a trigonal bipyramidal structure, the R1, R2 and R3 lie on one plane with the Br and OH perpendicular to that plane.

but how does the switching occur?! does it occur after the Br^- leaves?

thnks

 

[epenguin]

I remember an old textbook (Ingold) likened it to an umbrella inverting. Imagine a 3-ribbed umbrella - not a perfect analogy but may help visualise.

 

[Borek]

so you mean that when the carbanion forms a trigonal bipyramidal structure, the R1, R2 and R3 lie on one plane with the Br and OH perpendicular to that plane.

That's how it looks. Not sure if its OK to call it carbanion, I am called Mr.pH, not Mr.Organic

but how does the switching occur?! does it occur after the Br^- leaves?

For the OH^- to attach to carbon, R1, R2 and R3 must move to make place, then when Br^- leaves, they have new place where they can move - at this moment they have to change symmetry. They behave like a hat turned inside out.

 

[Borek]

I remember an old textbook (Ingold) likened it to an umbrella inverting. Imagine a 3-ribbed umbrella - not a perfect analogy but may help visualise.

Pretty good example Somehow I missed the moment you posted the answer.

 

[Kushal]

niice analogies lol... thank you borek and epenguin...

i can get the picture now:)