Carbohydrates
Reducing sugars can reduce tollen’s, benedic’ts
or fehling’s reagents. They have free aldehyde
or ketone group. All monosaccharides and most
disaccharides are reducing sugars.
Non reducing sugars, sucrose is a non-reducing
sugar.
Molisch’s test
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10% alcoholic solution of alphanaphthol
Concentrated sulfuric acid
Detect carbohydrates
Carbs go dehydration by sulfuric acid to
form furfural (furfuraldehyde) that
reacts with alpha naphthol
Forms violet colored product
Fehling’s test
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Detect reducing sugars
A; copper sulphate solution B;
potassium sodium tartrate
The solution is heated in water bath
Carb is reduces deep blue solution of
copper 2 ions to red precipitate of
insoluble copper oxide
Forms red precipitate
Copper ions present is +3 state reduced
to +2 oxidation state and in alkaline
medium is precipitated as red cuprous
oxide
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Tollen’s test
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Detect reducing sugar
Contains blue copper 3 ions that are
deuced to copper 2 ions by carbs
They precipitate as red colored
cuprous oxide
In alkaline medium, sodium carbonate
converts glucose to enediol which
reduce cupric to cuprous forming
cuprous hydroxide.
Ammoniacal silver nitrate solution
Detects reducing sugars
Forms silver mirror
Carbs react with tollens reagent on the
inner wall of the test tube
Silver mirror is precipitating out of the
solution
Silver ions are reduced to metallic silver
Iodine test
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Detect presence of starch
Iodine is not soluble in water, so iodine
solution is prepared by dissolving iodine
in water in the presence of potassium
iodine.
Reacts with starch to form starch/iodine
complex to form blue black color to the
reaction mixture.
Heating blue color and cooling makes
clue color reappear
Forms blue solution
Fats and oils
Solubility test
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Benedict’s test
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Solution is kept in sodium citrate
Forms red precipitate
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Detects presence of all lipids
Check if miscible or immiscible in polar
or non-polar solvents
Miscible in non-polar solvents like
chloroform,
partially in a polar solvent like ethanol
and
immiscible in polar solvent like water.
Positive result lipids are soluble
Translucent spot test
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lipid will not wet the filter paper
it will form greasy spot and will not
disappear.
Acrolein test
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detect glycerol or fat
dehydrating agent like potassium
biphosphate
the glycerol portion of the molecule is
dehydrated to from unsaturated
aldehyde
Arcolein that is smelly
Potassium biphosphate is a strong acid
too.
When potassium biphosphate is
heated with fat, hydrolysis occurs and
the glycerol produced is dehydrated to
form acrolein
Baudouin test
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Detect presence of sesame oil
Gives rose red color
Hydrochloric acid and furfural solution
Hubble’s test
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Used to detect degree of unsaturation
Alcoholic solution of iodine that
contains some mercuric chloride
Violet color of iodine fades away if
unsaturated, if saturated the color
persists
Iodine/halogen reacts with the double
bond site at the c=c to produce di
halogenated single bond
Amount of iodine reacted is determined
by adding potassium iodine
It causes remaining iodine
monochloride to form molecular iodine,
it is then titrated with standard solution
of 0.1 solution of sodium thiosulfate
Saturated will not get halogenated
reaction
If iodine number is 0-70 it is a fat
If it is above it is an oil.
Starch is used as the indicator for this
reaction, it will react with the liberated
iodine to give purple product.