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150
Viva Voce in
Pharmaceutical Chemistry
Q 764 An organic compound dissolves in 5% sodium hydroxide
solution. What kind of compound it may be?
Ans: The compound may be carboxylic acid or phenol which is acidic
in nature and would dissolve in a base like sodium hydroxide.
Q 765 If an organic compound dissolves in hot dilute sodium
hydroxide and gives off ammonia, what is the inference?
Ans: The inference is that, the compound is an amide or ammonium
salt.
Q 766 What inference will you draw if an organic compound is
found to dissolve in 5% hydrochloric acid?
Ans: The compound may be amine which is basic in nature and
thus dissolves in 5% HCl.
Q 767 Which classes of organic compounds are acidic to litmus but
do not give effervescence with sodium bicarbonate?
Ans: Phenols are acidic to litmus but do not give effervescence with
sodium bicarbonate.
Q 768 Arrange the following class of organic compounds in
increasing order of acidic nature, carboxylic acids, phenols,
nitrophenols.
Ans: Phenols < nitrophenols < carboxylic acids.
Q 769 What is Beilsten’s test?
Ans: In Beilsten’s test, the copper gauze is heated and then the hot
copper oxide formed is dipped into small quantity of an
organic compound and now the copper wire/gauze is inserted
into the vertical edge of the flame. A transient bright green
colouration indicates halogen in an organic compound. Hot
copper oxide decomposes a small quantity of the organic
compound with the formation of corresponding copper
halide. The copper halide is volatile and gives the green
colouration. However urea, thio-urea and some pyridine
compounds give volatile cuprous cyanide, which is volatile
and thus gives green colouration. Therefore, it can be said
that a negative result for halogen is decisive, whereas the
positive result means that halogen is probably present.
Q 770 If a compound decolourises bromine water with formation
of precipitate, what is the possible compound?
Ans: The possible compound would be aromatic amine like aniline
or phenol.
Systematic Qualitative Analysis 151
EXTRA ELEMENTAL DETECTION
Q 771 What is Lassaigne’s test?
Ans: It is the test in which elements in an organic compound are
converted into their ionisable inorganic forms by fusion of
the compound with metallic sodium. These ionisable
inorganic compounds are used for detection of elements by
performing chemical reactions.
Q 772 Lassaigne’s test is used for which purpose?
Ans: Lassaigne’s test is used to detect the presence of extra elements
like nitrogen, sulphur and halogens in an organic compound.
Q773 Give the chemistry of Lassaigne’s test.
Ans: In Lassaigne’s test the organic compound is heated with
metallic sodium to produce sodium cyanide, sodium sulphide,
sodium thiocyanide, sodium halide, depending upon the
element present in the organic compound.
Q 774 What is the procedure for Lassaigne’s test?
Ans: Cut a small piece of sodium metal and dry it by pressing
between the folds of the filter paper. Put it in a clean and dry
ignition tube. Heat the ignition tube gently until the sodium
152
Viva Voce in
Pharmaceutical Chemistry
metal forms a bead. Remove the ignition tube from fire and
add a small quantity of the given organic compound so that
it comes into contact with the sodium metal. Continue heating
until the tube is red hot. Plunge the red hot tube into 20 ml of
distilled water contained in a china dish. Crush the tube with
glass rod, if not broken. Stir the contents and boil for 5 minutes
and filter. The filtrate obtained is Lassaigne’s extract.
Q 775 Why must ignition tube be dried in fusion test?
Ans: Ignition tube must be dried because the sodium metal would
react with water, liberating hydrogen gas.
The metal would not be available for reaction with the extra
elements.
Q 776 Name the alternative method for Lassaigne’s test.
Ans: Sodium carbonate-zinc method can be used for detection of
extra elements. When an organic compound is heated with a
mixture of zinc powder and sodium carbonate, the nitrogen
and halogens are converted into sodium cyanide and sodium
halides, respectively and the sulphur into zinc sulphide which
is insoluble in water. The sodium cyanide or sodium halide is
extracted with water and detected as in Lassaigne’s test. While
the zinc sulphide in the residue is decomposed with dilute
acid and the hydrogen sulphide is identified with lead acetate
paper. The test for nitrogen is not affected by the presence of
sulphur.
Q 777 For which compounds Lassaigne’s test is not satisfactory?
How can we overcome this difficulty?
Ans: Lassaigne’s test for detecting nitrogen in organic compounds
is unsatisfactory with explosive compounds like diazonium
salts, nitro-alkanes, organic azides, azo compounds, polynitro
compounds and certain volatile nitrogenous substances. This
difficulty can be overcome by mixing the compound with pure
naphthalene or sucrose or by mixing the substance with
sodium and placing a layer of soda lime above the mixture.
Q 778 Why do not we do any-test for the ‘C’, ‘H’ and ‘O’ in detection
of element in elemental analysis?
Ans: Organic compounds are made up of carbon and hydrogen
and thus the presence of ‘C’ and ‘H’ is assumed.
Systematic Qualitative Analysis 153
If oxygen is present, its presence becomes known during
detection of functional groups.
Q 779 What are the common constituting elements of organic
compounds?
Ans: The common constituting elements of organic compounds are
carbon, hydrogen, oxygen, nitrogen, sulphur and halogens.
Very rarely phosphorous is present.
Q 780 Name five compounds containing nitrogen.
Ans: Aniline, nitrobenzene, urea, benzoquinone, pyridine,
diphenyl amine, etc.
Q 781 Name three compounds containing sulphur.
Ans: Sulphosalicylic acid, p-toluene sulphonyl chloride, phenol
sulphonic acid, etc.
Q 782 Name five compounds containing nitrogen and sulphur
both.
Ans: Thiourea, sulphanilamide, tolbutamide, frusemide,
sulphanilic acid, penicillin.
Q 783 Name three compounds containing halides.
Ans: Chlorobenzene, bromobenzene, iodoform.
Q 784 How will you detect the presence of nitrogen in Lassaigne’s
extract?
Ans: We add a pinch of ferrous sulphate crystals to the extract. If
dirty green precipitate is not obtained, add a drop of sodium
hydroxide. Heat to boiling and then add dilute H
2
SO
4
.
Development of prussian blue colour/ppt indicates the
presence of nitrogen.
Q 785 Write chemical equations for detection of nitrogen.
Ans: The chemical reactions involved in detection of nitrogen
are:
154
Viva Voce in
Pharmaceutical Chemistry
Q 786 How will you detect the presence of sulphur in Lassaigne’s
extract. Give the chemical reactions involved.
Ans: We can perform two chemical tests:
1. Lead acetate test: When lead acetate solution is added to
acidified (with acetic acid) sodium extract black precipitate
of lead sulphide is formed.
2. Sodium nitroprusside test: When a few drops of freshly
prepared sodium nitroprusside solution are added to sodium
extract, violet coloured sodium thionitroprusside is formed.
Q 787 How halogens are detected in organic compounds? Write
the reactions involved.
Ans: The halogens are detected by addition of silver nitrate solution
to the fusion extract, which has been acidified with dilute nitric
acid. Silver halides are precipitated.
Q 788 How will you differentiate the chloride, bromide and
iodide?
Ans:
1. The silver chloride (formed by reaction between sodium
extract and silver nitrate) would dissolve in ammonium
hydroxide, silver bromide would partially dissolve and silver
iodide would not dissolve in excess ammonium hydroxide.
Systematic Qualitative Analysis 155
2. Take sodium extract, add dilute HNO
3
, then carbon
tetrachloride or benzene and then add few drops of chlorine
water. Chloride will impart no colour, bromide would impart
yellowish-brown colour and iodide would impart violet/
magenta colour to carbon tetrachloride layer.
When we add chlorine water, it liberates bromine or iodine
from corresponding sodium halide. This liberated bromine
or iodine dissolves in carbon tetrachloride and imparts pale
yellow or magenta colour, respectively.
Q 789 What precaution will you take for detection of halogens in
elemental analysis, if compound contains ‘N’ and/or ‘S’?
Ans: If nitrogen and/or sulphur are also present along with halides,
then the addition of silver nitrate to the acidified fusion
solution will precipitate silver cyanide and/or silver sulphide
in addition to silver halides. This problem can be overcome
by adding dilute nitric acid to fusion extract and boiling it.
The gaseous hydrogen cyanide or hydrogen sulphide is
removed by boiling.
Q 790 Give the test for detection of N and S together in elemental
analysis.
Ans: The sodium fusion extract would contain sodium thiocyanide
when both N and S are present in an organic compound.
Acidify the fusion extract with hydrochloric acid and add
ferric chloride solution, formation of blood red colour indicates
presence of N and S together.
156
Viva Voce in
Pharmaceutical Chemistry
FUNCTIONAL GROUP DETECTION
Q 791 An organic compound does not contain any extra element.
To what chemical classes the compound may belong?
Ans: The compound may belong to acids, alcohols, phenols,
aldehydes, ketones, esters, acid anhydrides, carbohydrates,
quinones, ethers, hydrocarbons, etc.
Q 792 What type of functional groups may be present in an organic
compound containing nitrogen?
Ans: Amines, amides, imides, ammonium salts, nitriles, anilides,
amino acids, nitro compounds, etc.
Q 793 What type of functional groups may be present in an organic
compound containing sulphur?
Ans: Sulphonic acid, alkali sulphate compounds of aldehydes and
ketones, sulphonyl, sulphamoyl, thioxo groups may be
present.
Q 794 Which functional groups may be present when an organic
compound contains halogens?
Ans: Acid halides, halogeno hydrocarbons and chloral hydrate.
Q 795 If an organic compound contains oxygen, which functional
groups may be present?
Ans: Acids, alcohols, phenols, aldehydes, ketones, esters, ethers,
acid anhydrides, etc.
Q 796 Give significance of derivative.
Ans: After performing the tests for determination of the functional
class of the compound, it is necessary to prepare the derivative
to prove the identity of the compound with one of the
members of the class.
Q 797 Give ideal characteristics of derivative.
Ans:
1. The derivative should be easily and quickly prepared in good
yield by an unambiguous reaction and be easily purified.
2. The derivative should be solid with melting point in between
50–250°C.
Systematic Qualitative Analysis 157
3. The derivative should be reproducible under similar
experimental conditions.
4. The physical and chemical properties of derivatives should
be markedly different from those of the original compound.
CARBOXYLIC ACIDS
Q 798 Name five carboxylic acids and write their structures.
Ans:
• Acetic acid, CH
3
COOH
• Benzoic acid
• Salicylic acid
• Succinic acid
• Acetyl salicylic acid
Q 799 How will you identify presence of carboxylic acid functional
group in an organic compound or which tests are given by
carboxylic group or Explain the tests for carboxylic group?
158
Viva Voce in
Pharmaceutical Chemistry
Ans: We will identify the presence of carboxylic group by
performing following tests:
1. Sodium Bicarbonate Test: Production of effervescence by
reaction between sodium bicarbonate solution and organic
sample indicates the presence of carboxylic acid.
2. Ester Test: Production of banana/apple like odour after reaction
between organic acid and ethyl alcohol in presence of conc.
H
2
SO
4
, indicates the presence of acid (mostly acetic acid).
Salicylic acid reacts with methyl alcohol in presence of conc.
H
2
SO
4
to produce methyl salicylate which has odour of oil of
wintergreen (balm like odour).
3. Ferric Chloride Test: Neutralise the organic substance using
ammonia solution until the contents are just alkaline. Boil the
solution gently to remove excess ammonia. Cool it and then
add few drops of neutral ferric chloride solution. A deep red/
buff/violet colouration/ppt indicates the presence of acetic
acid, benzoic acid, salicylic acid, respectively.
4. Benzyl Thiouronium Test: When aqueous solution of benzyl
thiouronium chloride is added to neutral solution of the
organic sample, production of white precipitate indicates the
presence of carboxylic acid.
Systematic Qualitative Analysis 159
Q 800 How will you differentiate between acetic acid and salicylic
acid?
Ans:
1. Ferric chloride test gives red colour for acetic acid and violet
colour for salicylic acid.
2. In ester formation test, salicylic acid reacts with methyl alcohol
to form methyl salicylate which has characteristic odour of
oil of wintergreen while acetic acid reacts with ethyl alcohol
to give ethyl acetate which has fruity odour.
Q 801 Which groups give positive ester test?
Ans: Carboxylic acids and alcohols give ester test positive.
Q 802 Why is sulphuric acid added in ester test?
Ans: Sulphuric acid acts as a dehydrating agent and thus checks
the reversibility of the reaction.
Q 803 Write the structure of organic acid containing phenolic
hydroxyl group and give its name.
Ans: