JEE Main · 2019 · 10 Apr 2019 · Shift 2 · Biomolecules

Count the stereocentres in each form. In open-chain D-glucose the carbonyl carbon $C1$ is flat and achiral, leaving $C2$, $C3$, $C4$ and $C5$ as the four stereocentres. When the chain closes into the pyranose ring, $C1$ becomes the anomeric carbon bonded to four different groups and turns into a new stereocentre, so the cyclic form carries five. The two counts are therefore four in the linear form and five in the cyclic form.