stereospecific reduction of benzoin with NaBH4

Aim
Synthesis of Meso-1,2-diphenyl-1,2-ethanediol with focus on stereoselectivity
Method
Please refer to the CEM3005W practical manual for details on experimental procedure.

Results and Analysis
{Limiting reagent} Mass benzoin = 2.0050g (0.00945 mole)
Mass sodium borohydride = 0.4059g (0.0107 mole)
Mass compound A used to form compound B = 1.0647g

Results
Mass (g) moles Molecular formula Yield (%)
Mp (pure)
Compound A
1.7458
0.00815
C14H14O2
86.22
138-139
Compound C
0.4255
0.001988
C14H14O2
40.02
136-138

Spectral Analysis
IR
Absorption (cm-1)
Assignment
Benzoin
3411, 3374
OH

2961, 2903
C-H aromatic

2358
C-O

1678
Ketone
Compound A
3372
OH stretch

3310
OH stretch

3085, 3060, 3030
C-H stretch

2900
C-H aromatic

1451
OH bend
Compound B
3371, 3309
C-H

2900, 2855
C-H stretch, methyl

1451
Ar C=C

1278
Acetal

1H NMR
Chemical shift (δ)
Multiplicity and Integration
Assignment
Compound B
1.6401
S, 3H
C-CHa3

1.8602
S, 3H
C-CHb3

5.5459
S, 2H
C(ph)Hc

7.0352
M, 10H
Ar-Hd

Compound A:

Meso-hydrobenzoin
Compound B:

Meso-1,2-diphenyl-1,2- ethanediol
Discussion and Conclusion
One can ascertain from the H NMR the stereoselectivity taking place in the reaction. Since there is not coupling of the Hc’s on product B, one can assume that they are in a similar environment, thus deducing that they are on the same side, forming a meso molecule. The doublet of doublet formed by coupling of the methyl groups shows that they are in different environments too, strongly implying that the phenyl groups are likely both on one side, supporting the stereoselectivity of the product.

References
1. Clayden, Jonathan., Greeves, Nick., Warren,Stuart G., Organic chemistry, Oxford University Press, Oxford; New YorK, 2012.

References: 1. Clayden, Jonathan., Greeves, Nick., Warren,Stuart G., Organic chemistry, Oxford University Press, Oxford; New YorK, 2012.