Question

1. Write out the balanced equation for the oxidation of hydrobromic acid by hydrogen peroxide.
2. How many stereoisomers of stilbene dibromide can exist?
3. A possible definition of a meso compound is: “while possessing 2 or more assignable
stereocenters, a meso compound is superimposable (identical to) its mirror image in at least
one conformation”. In other words, it is identical to its would-be enantiomer except for the fact
that the molecule contains… what? What physical characteristic arises in meso compounds
that results in the above definition?
4. Let us assume a cut-and-dry, black-and-white case that the mechanism does only one of 2
things when each molecule of product is produced: (1) goes entirely bromonium-ion/antiaddition;
or (2) goes through a fully-fledged planar carbocation intermediate with absolutely
no selectivity.
a. If you were to find by quantitative analysis that exactly 50% of your product mixture
was the meso stilbene dibromide and the other 50% was racemic stilbene dibromide,
then is mechanism case (1) or (2) favored? If so, qualitatively, by how much?
b. Think again. Question (a) is a mind-bender. I want to know what you originally /
immediately thought, and what you eventually came to. If they are not different, you
are either a genius or need to think more. Write out all the possibilities…
c. If you find, qualitatively, that >95% of the product is meso, what does this imply about
the mechanism? What percent of the time does the mechanism go by scenario (1)…
greater than what number?
5. Describe a more likely blend of mechanisms that could have produced your estimated product
distribution

Answer 1

This reaction is a green reaction of addition of trans stilbene with HBr in presence of H2O2 and the insitu produced bromine adds to stilbene to produce stilbene dibromide.

1) 2HBr + H₂O₂ ------> Br₂ + 2H₂O

2) Three steroisomers of stilbene dibromide exist, i) meso ii) the d and l pair

3) A meso compound has a plane of symmetry, which divides the molecule into two halves which are superimposable object and mirror image of each other. Thus the optical rotation of one half is nullified by other half resulting in no optical rotation. Thus a meso compound is achiral moleculecontaining chiral carbons and the inactivity is due to internal compensation.

4)a) If my product were meso it means the mechanism 1(anti addition ) is followed. If the product were racemic totally carbocation mechanism, (2nd) is followed.

However, here the product ananlysis shows 50% meso and 50% racemic which means both mechanisms operated equally bt 50% each.

b) The anti addition of bromine to trans stilbene gives meso isomer and through carbocation mechanism, the product would be racemic mixture.

c) If the product were >95% meso, it shows the reaction almost followed the bromonium ion intermediate and anti addition giving meso product as the major product,