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In the open-chain (linear) form of glucose, the number of stereocentres is X, and in its cyclic (pyranose) form the number of stereocentres is Y. What is the value of X + Y?

1. 7
2. 8
3. 9
4. 10

Correct answer: 9

Solution

Linear glucose has 4 stereocentres (C2, C3, C4, C5), so X = 4. Cyclisation forms a hemiacetal at C1, adding one more stereocentre, so Y = 5. X + Y = 9.

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