What are reducing sugars?
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Solution and Explanation
Sugars are classified based on their ability to reduce mild oxidizing agents.
Step 2: Definition
Reducing sugars are those carbohydrates that can reduce mild oxidizing agents such as Tollen's reagent (ammoniacal $AgNO_3$) and Fehling's solution (alkaline $CuSO_4$). They must have a free anomeric carbon with a free $-$OH group (hemiacetal or hemiketal) that can open to regenerate the free aldehyde or ketone group.
Step 3: Examples
All monosaccharides (glucose, fructose, galactose) are reducing sugars. Among disaccharides, maltose and lactose are reducing sugars (they have a free anomeric carbon). Sucrose is a non-reducing sugar.
Final Answer: Reducing sugars are carbohydrates that can reduce Tollen's reagent and Fehling's solution. They possess a free anomeric $-$OH group capable of opening to give a free $-$CHO or $-$CO group. Examples: glucose, fructose, maltose, lactose.
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Give one chemical test to distinguish between the following pairs of compounds.
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(ii) Secondary and tertiary amines
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(v) Aniline and N-methylanilineAccount for the following:
(i) pKb of aniline is more than that of methylamine.
(ii) Ethylamine is soluble in water whereas aniline is not.
(iii) Methylamine in water reacts with ferric chloride to precipitate hydrated ferric oxide.
(iv) Although amino group is o– and p– directing in aromatic electrophilic substitution reactions, aniline on nitration gives a substantial amount of m-nitroaniline.
(v) Aniline does not undergo Friedel-Crafts reaction.
(vi) Diazonium salts of aromatic amines are more stable than those of aliphatic amines. (vii) Gabriel phthalimide synthesis is preferred for synthesising primary amines.
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