Question:medium

Which of the following carbohydrates is a NON-reducing sugar and therefore gives a NEGATIVE Benedict’s (and Fehling’s) test?

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Which disaccharide locks BOTH of its anomeric carbons in the glycosidic bond?
Updated On: Jun 25, 2026
  • Sucrose
  • Maltose
  • Lactose
  • Glucose
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The Correct Option is A

Solution and Explanation

A positive Benedict’s test needs a sugar that still has a free anomeric (reducing) carbon able to donate electrons and reduce $\text{Cu}^{2+}$ to red $\text{Cu}_2\text{O}$. So the question reduces to: which option has NO free anomeric carbon?

Glucose is a monosaccharide aldose with an open-chain aldehyde - clearly reducing. Maltose joins two glucose units by an $\alpha\text{-}1,4$ bond and lactose joins galactose to glucose by a $\beta\text{-}1,4$ bond; in both, one anomeric carbon stays free, so both reduce Benedict’s reagent.

$\textbf{Sucrose}$ is unique here: its glucose and fructose are united by an $\alpha,\beta\text{-}1,2$ glycosidic bond that engages BOTH anomeric carbons simultaneously. With no free reducing group left, sucrose cannot reduce cupric ion and the test stays blue (negative). It only turns positive after acid hydrolysis ("invert sugar") frees the monosaccharides.

\[\boxed{\text{Sucrose} = \text{non-reducing sugar, Benedict negative}}\]
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