Which of the following is correct structure of α-anomer of maltose?

Solution

Chemistry ma’am · 3 steps

First confirm both rings are real glucose, then look at the free OH on the right ring's C1: down means α.

1. What maltose is. Maltose is two α-D-glucose units joined by an α(1→4) glycosidic link. The C1 of the left ring bonds through O to the C4 of the right ring. The right ring keeps a free OH at its C1, so it is the reducing end and can be α or β.
2. Glucose in Haworth form. In a D-glucopyranose ring the CH₂OH is up. The OH groups sit down at C2, up at C3, down at C4. At C1, an OH down (opposite side to CH₂OH) is α, and an OH up is β.

   Option	C3 OH in both rings	Left C1 link to O	Right ring C1 OH	Gives you
   (a)	up, glucose	down, α	up	β-maltose
   (b)	down, not glucose	down, α	up	not maltose
   (c)	up, glucose	down, α	down	α-maltose
   (d)	down, not glucose	down, α	down	not maltose

3. So. Only (c) has two correct glucose rings, the α(1→4) link, and the free C1 OH pointing down. The answer is (c).

The idea to keep. Remember glucose in Haworth form as down, up, down for the OH groups at C2, C3, C4. The anomer of a disaccharide is decided only at the free C1 OH of the reducing ring.

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