Question

1. (a) The addition of acetic acid to sodium borohydride will cause the displacement of one or more hydrides from each boron atom with an acetate ion. The resulting borohydride reducing agent is less reactive than the parent sodium borohydride. What does tartaric acid do in the sodium borohydride reduction to cause a chiral reaction environment for the ketone reduction reaction? Illustrate with a structure.

(b) What are the specific rotations of pure (R)-ethyl 3-hydroxybutanoate, (S)-ethyl 3-hydroxybutanoate and (L)- tartaric acid?

(c) Assume that the reduction converted 80% of the starting material, ethyl acetoacetate, to give a product mixture of (R)-ethyl-3-hydroxybutyrate and (S)-ethyl-3-hydroxybutyrate. Using a 1 dm long polarimeter cell, the optical rotation of the reaction mixture, which includes unreacted ethyl acetoacetate to be +10.5°. The concentration is 0.5 g/mL (total material, not just product). Find the TOTAL percent of the enantiomer that is in excess.

(d) Could you use LiAlH4 for the reduction of ethyl acetoacetate rather than NaBH4 and obtain the same products? Why?

Answer 1

Solution:

part (a) The reaction of acetic acid with sodium borohydride yields the product of alcohol with racemic mixture while chiral reagents like bakers yeast are used in reduction of selective enantiomers, in usage of tartaric acid with sodium borohydride blocks the racemic formation instead gives single reduced molecule with non-enantiomeric molecule.

part (b) The formula used in computing specific rotation is as below,

(R)-ethyl-3-hydroxybutyrate = -46^o (anti-clockwise)

(S)-ethyl-3-hydroxybutyrate = +43^o (clockwise)

(L)- tartaric acid = +12.4^o (clockwise)

part (c) substiututing the given vakues in the below expression we get,

+10.5 = () / (1*0.5)

= 5.25

% mixture = (5.25/10.25)*100% = 50% in excess

part (d)

LiAlH₄ is highly reactive than NaBH₄, both are reducing agent of H- but LiAlH₄ produces hydrogen gas in polar solvents hence they have to used with the presence of polar aprotic solvents like THF (or) diethyl ether, in general LiAlH₄ can be used as reducing agent under the influence of polar aprotic solvents.