Discussion
Hydration of fumaric acid gives malic acid as shown below. Assume that addition of water takes place specifically from A face or B face. The correct statement pertaining to stereochemistry of malic acid formed is
Answer: A
The correct statement is that addition specifically from the A face gives the S isomer of malic acid.
Fumaric acid is planar, and its two faces are enantiotopic: adding water to one face gives one enantiomer of malic acid, and adding it to the other face gives the other. In the drawing, the A face arrow is drawn bold, in front of the page, and the B face arrow hashed, behind it.
Take OH arriving from the A face at the carbon on the left, bonded to the HOOC group. The new stereocentre carries OH (priority 1), COOH (2), CH2COOH (3) and H (4). With the OH in front and the H in the plane of the page, pointing up and to the left, the sequence 1 \(\to\) 2 \(\to\) 3 viewed from the side opposite the hydrogen runs anticlockwise, so the centre is S. Addition from the B face therefore gives R.
That rules out 'A face gives R' and 'B face gives S'. Addition specifically from one face cannot give a racemic mixture, which needs both faces to react equally. (The enzyme fumarase does this addition stereospecifically and gives L-malic acid, which is S.)
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