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JEE Advanced levelOne or more correct optionsDifficulty 3 of 3st109

Which statements about the open-chain D-aldopentoses (CHO at the top, at the bottom, OH on the right at C4 for the D-series) are correct?

  1. AThere are eight aldopentose stereoisomers in total.
  2. BD-Ribose and D-xylose are diastereomers.
  3. CD-Ribose and L-ribose are enantiomers.
  4. DReducing D-ribose (CHO to ) gives an optically active alditol.

Answer: A, B, C

Three stereocentres give aldopentoses, 4 in each series. D-Ribose and D-xylose differ only at C3, so they are diastereomers, and D-ribose and L-ribose are mirror images. Reducing D-ribose gives ribitol with OH on the right at C2, C3 and C4; with identical ends it has an internal mirror plane and is meso, so the last statement is false.
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