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140
Viva Voce in
Pharmaceutical Chemistry
Q 734 If the melting point/boiling point of the product is not
matching the pure substance, what inference would you
draw?
Ans: The inference would be the product formed contains
impurities. Presence of impurities reduces the metting point/
boiling point.
Q 735 Why are stoppered flasks used in the synthesis of some
compounds?
Ans: Whenever the reactants have obnoxious odours or irritant
vapours are released by the reactants/product, stoppered
flasks are recommended.
Q 736 In which cases of synthesis, fuming cupboard is recom-
mended?
Ans: During synthesis, fuming cupboard is recommended when
toxic, irritant fumes are produced. The fume cupboard releases
the fumes outside the laboratory and does not harm the chemist.
PREPARATION OF BENZOIC ACID FROM BENZAMIDE
Q 737 How benzoic acid is prepared from benzamide?
Ans: Benzamide is subjected to alkaline hydrolysis, the amide
group undergoes hydrolysis to produce benzoic acid.
Q 738 What chemical reaction is occurring in the synthesis of
benzoic acid?
Ans:
Q 739 What are the uses of benzoic acid?
Ans: Benzoic acid is used in the manufacturing of perfumes, dyes,
topical medications as antibacterial, insect repellant. Sodium
benzoate is used as preservative and pH adjustor.
Q 740 What is the melting point of benzoic acid?
Ans: The melting point of benzoic acid is 121°C.
Preparation of Organic Compounds 141
PREPARATION OF PICRIC ACID FROM PHENOL
Q 741 How is picric acid prepared? Is direct nitration possible?
Ans: Picric acid is prepared by first heating phenol with conc.
sulphuric acid, which leads to formation of o- and p-phenol
sulphonic acids: Then this mixture is treated with nitric acid.
Nitration occurs at meta to sulphonic acid group and finally
sulphonic acid group is replaced by third nitro group, yielding
picric acid.
Direct nitration with nitric acid gives poor yield as it leads
to destruction of most of phenol by oxidation and resinous
products are formed.
Q 742 Why is picric acid called an acid when it does not contain
carboxyl group?
Ans: The nitro groups attached at o- and p- position are electron
withdrawing and thus the bond between O and H of –OH
group becomes weak and hydrogen can be easily donated.
The electron withdrawing nitro groups disperse the negative
charge in the 2,4,6-trinitrophenoxide anion, making it more
stable. Picric acid reacts with sodium bicarbonate and can
produce effervescence.
142
Viva Voce in
Pharmaceutical Chemistry
IDENTIFICATION TESTS
Q 743 What is the need of performing identification tests?
Ans: Identification tests as per pharmacopoeia are performed to
authenticate the drug sample. These are qualitative tests.
Q 744 Which type of tests are involved in identification?
Ans: Identification tests as per pharmacopoeia include:
1. Physical tests such as state, colour, odour and solubility
behaviour.
2. Chemical tests are carried out to get evidence in the form of
colour reactions which are based on the functional groups
present in the drug.
3. Spectral analysis to match the standard spectra of reference sample.
CHEMICAL TESTS FOR IDENTIFICATION OF ASPIRIN
Q 745 Describe the chemical tests for identification of aspirin?
Ans: First the aspirin is boiled with sodium hydroxide and then
cooled. Then dilute sulfuric acid is added to prevent the
reversibility of the reaction, a white crystalline precipitate of
salicylic acid is produced and the pungent smelling acetic acid
is obtained as the filtrate.
When sodium hydroxide is added, aspirin gets hydrolyzed
to produce salicylic acid and acetic acid, by hydrolysis of the
ester bond.
Identification and
Test for Purity of
Pharmaceuticals
7
142
Identification and Test for Purity of Pharmaceuticals 143
Q 746 How salicylic acid generated by hydrolysis of aspirin is
detected?
Ans: Ferric chloride solution is added to the solution obtained by
dissolving precipitate of the salicylic acid. A violet-colored
complex is formed by the reaction between the phenolic
hydroxy group of salicylic acid with ferric chloride.
Q 747 Aspirin is an odorless compound, but during chemical test
of aspirin a pungent smell is produced. Why?
Ans: During chemical test, aspirin undergoes hydrolysis forming
acetic acid and salt of salicylic acid. The acetic acid produced
is responsible for the pungent smell.
Q 748 How acetic acid generated by hydrolysis of aspirin is
detected?
Ans: Ethanol was added to the filtrate and then sulfuric acid was
added. A fruity odor of ethyl acetate is perceptible.
CHEMICAL TESTS FOR IDENTIFICATION OF CAFFEINE
Q 749 Describe the murexide test for identification of caffeine.
Ans: In murexide test, hydrochloric acid and potassium chlorate
is added to caffeine and evaporated to dryness on water bath.
The formed residue is then exposed to the vapours of dilute
ammonia solution. A purple colour would be produced
which disappeares on addition of fixed alkali solution.
144
Viva Voce in
Pharmaceutical Chemistry
Q 750 Why purple colour is produced in murexide test?
Ans: Caffeine is oxidised by potassium chlorate in presence of
hydrochloric acid yielding tetramethylalloxantine, which is
observed as a residue after evaporation. This residue on
reaction with ammonia forms murexide which is purple in
colour.
Q 751 Describe the tannic acid test for identification of caffeine.
Ans: To a saturated solution of caffeine add few drops of tannic
acid solution, white precipitate is obtained by interaction
between the nitrogen atoms of caffeine with tannic acid.
CHEMICAL TESTS FOR IDENTIFICATION OF SULPHANILAMIDE
Q 752 Describe the chemical tests of sulphanilamide along with
the reactions.
Ans: Three tests are performed for identification of sulphanilamide:
a. Take three test tubes. In the first test tube, the given substance
is dissolved in concentrated hydrochloric acid and water. In
the second test tube, a solution of sodium nitrite is taken. In
the third test tube, -naphthol is dissolved in sodium
hydroxide. Cool all solutions in the ice-cold water, add sodium
nitrite solution to the first test tube followed by addition of
-naphthol. Formation of yellow-orange-red dye confirmed
the presence of sulphanilamide.
Identification and Test for Purity of Pharmaceuticals 145
b. The substance is taken in a boiling tube and sodium
hydroxide solution was added. Then the contents were
boiled. Ammonia is evolved which gives dense white fumes
by introducing a glass rod dipped in hydrochloric acid to
the mouth of the test tube indicates the presence of sulpha-
nilamide.
c. In a boiling test tube, the substance was taken with
hydroxylamine hydrochloride and potassium hydroxide
solution. The mixture is boiled, cooled and few drops of ferric
chloride were added. Appearance of brick red colour
indicates the presence of sulphanilamide.
Q 753 What happens when sodium nitrite and
-naphthol are
added to an acidic solution of sulphanilamide?
Ans: The hydrochloric acid protonates the (free amino group of
sulphanilamide which reacts with sodium nitrite to form
diazonium salt. Then the -naphthol reacts with this
diazonium salt to produce an orange-red coloured azo dye
through the azo coupling reaction.
Q 754 Why does white fumes evolve when a glass rod dipped in
hydrochloric acid is brought to the mouth of test tube in
sulphanilamide detection?
Ans: When sulphanilamide is treated with sodium hydroxide,
hydrolysis occurs giving out ammonia and sulfanilic acid .
The released ammonia reacts with hydrochloric acid
producing white fumes of ammonium chloride.
146
Viva Voce in
Pharmaceutical Chemistry
Q 755 Describe the hydroxamic acid test for sulphanilamide along
with chemistry.
Ans: Sulphanilamide reacts with hydroxylamine hydrocholride to
give an oxo derivative which undergoes keto–enol tauto-
merism to give enol derivative which reacts with ferric
chloride to form a brick red coloured compound, the ferric
salt of corresponding hydroxamic acid.
Identification and Test for Purity of Pharmaceuticals 147
CHEMICAL TESTS FOR IDENTIFICATION OF PARACETAMOL
Q 756 Describe the chemical test of paracetamol along with the
reactions.
Ans: When paracetamol is boiled in hydrochloric acid, then water
is added and cooled. This does not produce any precipitation.
Potassium dichromate is then added which produces violet
colour that does not turn red.
The acetamino group of paracetamol is hydrolised to produce
p-amino phenol and acetic acid. This p-amino phenol is
oxidised by potassium dichromate to
148
Viva Voce in
Pharmaceutical Chemistry
PRELIMINARY EXAMINATION
Q 757 List out all organoleptic tests for an organic compound. What
do they suggest?
Ans: Observation of colour, odour, and physical state are the
organoleptic tests which give us clue about the identity of the
compound.
Q 758 Which preliminary tests can be performed for finding out
unsaturation in an organic compound?
Ans: Two tests can be performed to detect unsaturation:
1. Test with bromine: When bromine water is added to the
solution or suspension of a compound in carbon tetrachloride,
decolourisation of bromine water from yellow to colourless
indicates unsaturation. In case of amines and phenols,
precipitate is obtained.
2. Baeyer’s test: Test with alkaline KMnO
4
solution: When
alkaline KMnO
4
solution is added to solution of the substance
(in water or acetone) and on shaking, production of brown
precipitate of manganese dioxide (due to reduction), indicates
unsaturation.
Systematic
Qualitative Analysis
8
148
Systematic Qualitative Analysis 149
Q 759 Why do aromatic compounds give smoky flame on burning?
Ans: Aromatic compounds have higher percentage of carbon (as
compared to aliphatic compounds) which is not fully oxidised
and so burn with smoky flame, which has unburnt carbon
particles.
Q 760 Which aliphatic compounds burn with a smoky flame and
why?
Ans: The aliphatic compounds which have high percentage of
carbon, e.g. acetylene burn with smoky flame. The percentage
of carbon in acetylene and benzene is same. Chloroform and
carbon tetrachloride also burn with smoky flame because
during burning chlorine is produced due to oxidation. This
chlorine reduces the approach of oxygen to oxidise the carbon
to carbon monoxide and carbon dioxide. Thus, the unburnt
carbon particles give rise to smoky flame.
Q 761 If an organic compound burns with a green flame, what
inference is apparent?
Ans: When a compound burns producing a green flame, it is
indication of presence of halides. Violet flame indicates
iodoform.
Q 762 What would be the inference, when smell of ammonia is
observed during ignition test?
Ans: The inference would be, the compound may be nitrogenous
like an amide.
Q 763 Enumerate different odours and their inference.
Ans:
Odour Inference
Carbolic smell Phenols
Smell of bitter almonds Benzaldehyde or nitrobenzene
Pleasant/fruity smell Esters, ethers, acetate.
Spirituous smell Alcohols
Vinegar-like odour Acetic acid
Mouse-like odour Acetamide and acetonitrile
Pungent smell Formic acid, formalin, acetyl
chloride, benzoyl chloride
Offensive odour lsocyanides
Cucumber-like odour Chloral hydrate