Structures of four disaccharides are given below. Among the given disaccharides, the non-reducing sugar is :

Solution

Chemistry ma’am · 3 steps

Look for a free OH on an anomeric carbon: if no ring has one, the sugar cannot open up and reduce.

1. Reducing sugar test. The anomeric carbon is the ring carbon bonded to the ring oxygen, C1 of an aldose or C2 of a ketose. If it still carries a free OH, the ring can open to an aldehyde or ketone that reduces Tollens' or Fehling's reagent.
2. Check each structure. Find the anomeric carbon in each ring. Then ask whether the glycosidic oxygen has used it up, or whether it still carries a free OH.

   Option	Glycosidic link	Anomeric carbon of the second unit	Reducing?
   (a)	C1 of glucose to C2 of the five-membered fructose ring (sucrose type)	C2 is tied up in the glycosidic bond	No
   (b)	C1 to C4 (maltose type)	C1 has a free OH	Yes
   (c)	C1 to C4 (cellobiose type)	C1 has a free OH	Yes
   (d)	C1 to C4	C1 has a free OH	Yes

3. So. In (a) the bond joins C1 of glucose to C2 of fructose, so both anomeric carbons are used up. No ring can open, and (a) is the non-reducing sugar.

The idea to keep. A disaccharide is non-reducing only when its glycosidic bond joins two anomeric carbons, as in sucrose. A 1→4 link always leaves one hemiacetal OH free, so maltose, lactose and cellobiose all reduce.

Answer from the NTA final answer key. Solution by PrepWiser.