Page 103 - Chemistry - XI

Proteins
Proteins are the building blocks of plant systems. They are made up of amino acids and have –NH amine and
2
–COOH carboxylic group in their monomer. They are joined by peptide bond. They usually contain sulphur
and sometimes phosphorus.

Objective
To analyse the presence of carbohydrates, proteins and fats in various fruits and vegetables.

Materials Required

Test tubes, burner, test tube holder, water bath, tripod stand, fruit and vegetable juice, universal indicator,
conc. H SO , conc. HNO , iodine, distilled water, Molisch reagent, Millon’s reagent, and common reagent
2 4 3
Procedure
1. Test for Carbohydrates: Carbohydrates can be identifi ed by Molisch test, Fehling test, Tollen’s
reagent or by Benedict test. They are given below.

(i) Molisch Reagent: This test is common for all types of carbohydrates and is used most widely. It
is carried out by adding 2–4 drops of 10% alcoholic solution of α-naphthol (Molisch reagent) to
the aqueous solution of carbohydrates under test. Concentrated sulphuric acid is slowly poured
along the sides of the test tube. A purple ring is formed at the junction of the two layers. The
colouration is accounted for by the splitting of the carbohydrate molecule by conc. sulphuric acid
with the formation of furfural or its derivatives (among other reaction products) that takes part in
the condensation reaction with α-naphthol to give coloured compounds.

(ii) Reduction Tests:
(a) Fehling’s test: Fehling’s solution contains cupric ions. Reducing sugars reduce these to
cuprous ions which under alkaline conditions form red insoluble cuprous oxide.

RCHO + 2Cu + 5OH RCOO + Cu O + 3H O
–
–
2+
2 2
Red ppt.
(b) Tollen's test: In this test, an aqueous solution of carbohydrates is treated with ammoniacal
silver nitrate (Tollen’s reagent) and the resulting mixture is warmed in a hot water bath. A
shining silver mirror is formed on the inner side of the test tube. A reducing sugar reduces Ag
to metallic silver which forms a shiny deposit on the inner side of the test tube.
RCHO + 2[Ag(NH ) ] + 5OH → RCOO + 2Ag + 2H O + 4NH 3
–
+
–
3 2
2
(c) Benedict’s test: In this test, an aqueous solution of carbohydrate is treated with Benedict’s
reagent and the mixture is strongly heated for 2–3 minutes. A red precipitate of Cu O is
2
formed. Cupric ions present in Benedict’s reagent are reduced by reducing sugars to cuprous
ions which in alkaline medium form red Cu O.
2
2. Test for Fats: Fats or lipids are compounds with generally low boiling points. They have a greasy
touch and are insoluble in water, but soluble in organic solvents.
(i) Solubility test: If fat is mixed with ether or chloroform in a dry test tube, it dissolves giving a clear
If fat is mixed with ether or chloroform in a dry test tube, it dissolves giving a clear
solution.

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