Chem 31.1 Post Lab

Chem 31.1 Post Lab 3
CARBONYL COMPOUNDS AND CARBOHYDRATES

HYDROLYSIS OF POLYSACCHARIDES

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Susceptible to both nucleophiles and electrophiles
Presence of C=O

TEST FOR ALDEHYDES AND KETONES
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Reaction with 2,4-DNP (Dinitrophenylhydrazine)
Positive: Formation of yellow precipitate

IODOFORM TEST
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Carbonyl compounds: Methyl ketones will give a positive result
Postive: Yellow precipitate (CHI3)

MOLISCH’S TEST for carbohydrates
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TOLLEN’S TEST – for aldehydes
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Test for easily oxidizable compounds (ex.
Aldehydes)
Positive: Silver mirror deposit

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Polyhydroxyaldehydes (aldoses)
Polyhydroxyketones (ketoses)

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Classifications:

Test for reducing sugars
Reagent: CuSO4, sodium citrate and sodium carbonate in a mildly basic solution
Positive test: Brick red precipitate

OSAZONE TEST

1. Simple - monosaccharides
2. Complex – disaccharides, polysaccharides

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Hemiacetals/Hemiketals
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Dehydration test to identify carbohydrates from non-carbohydrates Reagent: Molisch reagent (10% α-napthol in ethanol) and conc. H2SO4
Positive test: Purple ring at the interface of two layers for all carbohydrates.

BENEDICT’S TEST

CARBOHYDRATES
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Starch  Glucose
- Starch is a polymer of D-glucose units bound by glycosidic linkages.
Cellulose  Glucose
- Breakage of the β-1,4-glycosidic bonds by acids leads to the hydrolysis of cellulose polymers, resulting in the sugar molecule glucose or oligosaccharides.

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Intramolecular reaction: C = O group and one of the
–OH groups

Test for carbohydrates; time for the formation of osazone gives an idea for the identity of the sugar.
Reagent: phenylhydrazine-HCl/NaCH3COOH
Positive test: yellow crystals

ACYL COMPOUNDS, SOAPS AND DETERGENTS
SIMPLE SUGARS (monosaccharides)
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Cannot be converted into smaller sugars by hydrolysis Glucose, fructose

Undergo nucleophilic substitution

Benzoic Acid - Soluble in hot water,