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D-Erythrose is an aldotetrose with the Fischer projection: CHO at top, C2 has H on left and OH on right, C3 has H on left and OH on right, CH2OH at bottom. Compare it with: Compound P: CHO, C2: H left/OH right, C3: H left/OH right, CH2OH (identical configuration at both centers) Compound Q: CHO, C2: HO left/H right, C3: H left/OH right, CH2OH Compound R: CHO, C2: H left/OH right, C3: HO left/H right, CH2OH Compound S: CHO, C2: HO left/H right, C3: HO left/H right, CH2OH How is each of P, Q, R, S related to D-Erythrose?

1. P is identical, Q is diastereomer, R is diastereomer, S is enantiomer
2. P is enantiomer, Q is identical, R is enantiomer, S is diastereomer
3. P is identical, Q is enantiomer, R is diastereomer, S is identical
4. P is diastereomer, Q is identical, R is identical, S is enantiomer

Correct answer: P is identical, Q is diastereomer, R is diastereomer, S is enantiomer

Solution

D-Erythrose: C2(OH right), C3(OH right). P: same — identical. Q: C2 inverted, C3 same — diastereomer. R: C2 same, C3 inverted — diastereomer. S: both inverted — enantiomer.

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