Question

6)Draw perspective formulas for the acetonide formed from the meso and racemic hydrobenzoin (use models or PC Spartan if necessary to visualize these compounds).

7)Based on your structures in question 6.

a)How many H NMR signals should be observed for the benzyl hydrogens for the meso and racemic acetonide? Explain your answer

b)How many H NMR signals should be observed for the methyl groups for the meso and racemic acetonide? Explain your answer.

c)Explain how H NMR can be used to establish the stereochemistry of the hydrobenzoin (meso or racemic) - formed in the reduction of benzil with NaBH₄.

Answer 1

Let us draw the structures first for the given problem

Benzil to hydrobenzoin to acetonide

Now the acetoide formed will be:

6) So the perspective formulas are drawn above

7) Bases on the structures the NMR for benzyl hydrogen will be same for both the types

in both the compounds the protons will show similar peaks near to 7-7.5 ppm

b) The H-NMr for meso and racemic mixture

(i) In case of meso derivative we will obtain two kinds of methyl proton shifts.

The methyl groups which are on the same side of phenyl group will be more deshielded as compared to the other two methyl groups

The deshielded methyl protons will show peaks downfield with higher chemical shift.

(ii) In case of racemic mixture we will not observe these two kinds of methyl proton shift

c) H NMR can be used to establish the stereochemistry of hydrobenzoin formed in redcution of benzil with NaBH4 only when they are further coverted to acetnoide derivative as then it will give two distinct peaks of methyl hydrogen.