9. The specific rotation of (R)-(+)-glyceraldehyde is +8.7. If the observed spec

Question

9. The specific rotation of (R)-(+)-glyceraldehyde is +8.7. If the observed specific rotation of a mixture of (R)- and (S)-glyceraldehyde is +1.4, what percentage of (R)-glyceraldehyde is present and what is the enantiomeric excess?

10. Citrate synthase is one of the enzymes involved in the citric acid cycle, and it catalyzes the synthesis of citric acid from oxaloacetic acid and acetyl-CoA. If the synthesis is carried out with an isotopically labelled acetyl-CoA, the isomer shown is obtained.

a) Which enantiomer of citric acid is obtained?

b) Why is the other enantiomer not obtained?

c) If the acetyl-CoA used in the synthesis contains 12C instead of 14C, will the product be chiral? Explain

11. For the reaction below, using sawhorse structures predict the relative stereochemistry of the 2,3- dibromobutane intermediate (is it chiral or achiral?), and the stereochemistry of the 2-bromo-2- butene product.

Explanation / Answer

9.

Enantiomeric excess = 100 x observed specific rotation/ specific rotation=

                = 100 x 1.4/8.7 = 16 %

The percentages of

     R enantiomer is 50% + (16/2) % = 58 %

       S enantiomer is 50% - (16/2) % = 42 %

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