The sugar shown is oxidised with bromine water and the acid formed loses one molecule of water intramolecularly on heating to give a six-membered lactone. What is the degree of unsaturation (rings plus pi bonds) of that lactone?

Answer: C

Bromine water oxidises the CHO of D-glucose to COOH (gluconic acid,

C X_{6} H X_{12} O X_{7}

). Intramolecular esterification with loss of

H X_{2} O

gives glucono-delta-lactone,

C X_{6} H X_{10} O X_{6}

. Degree of unsaturation

= (2 \times 6 + 2 - 10) /2 = 2

: one ring plus one C=O.