carbohydrate reaction

Question 1

aldehyde portion of glucose (bromine water)

Answer 1

• mild oxidation with bromine water
• aldehyde->carboxylic acid
• HC==O -> COOH
• product: gluconic acid

Question 2

aldehyde portion of glucose (NH2OH)

Answer 2

• nucleophilic addition-elimination (substitution) with hydroxylamine(NH2OH)
• aldehyde->oxime
• HC==O -> HC==NOH
• O is removed, replaced with NOH
• oxime is pesticide, toxic, poisonous

Question 3

aldehyde portion of glucose and terminal alcohol (HNO3 -[O} )

Answer 3

-vigorous oxidation with warm dilute nitric acid HNO3 -[O]
-aldehyde–> carboxylic acid
-(primary) alcohol–> carboxylic acid
-HC==O -> COOH
-CH2OH -> HC==O -> COOH
-product:glucaric acid(aldaric acid)
ald=specify it’s originally aldehyde

Question 4

aldehyde portion of glucose (NHBH4)

Answer 4

-reduction with sodium borohydride (NHBH4)
-aldehyde–> (primary) alcohol
-HC==O -> CH2OH
-product: sorbitol (polyalcohol/alditol)
ald=specify it’s originally aldehyde

Question 5

Test for reducing or non-reducing sugar

Answer 5

-tollen’s reagent (silver mirror, Ag reduced)
-Fehling reagent (brick red precipitate, Cu reduced)
-Benedict’s reagent (brick red precipitate, Cu reduced)
*positive test indicates sugar is oxidised
*reducing sugar can undergo oxidation
*aldehyde can further oxidised while ketone cant
*therefore, all aldoses are reducing sugar while ketoses are non-reducing
#exception: fructose. it reduces tollen’s because toleen’s has NH3 which is basic. Fructose is readily isomerized to an aldose in basic solution