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Which sugar does NOT give a reddish-brown precipitate with Fehling's reagent?

1. Sucrose
2. Lactose
3. Glucose
4. Maltose

Correct answer: Sucrose

Solution

Fehling's test is positive for reducing sugars, which have a free anomeric OH group (or can form a free aldehyde/ketone in open-chain form). Glucose and maltose are reducing sugars (free anomeric OH). Lactose is a reducing sugar (free anomeric OH on the galactose unit). Sucrose has both anomeric carbons (C1 of glucose and C2 of fructose) linked together in the glycosidic bond, leaving no free anomeric OH. Therefore sucrose is non-reducing and does NOT give a positive Fehling's test.

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