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160
Viva Voce in
Pharmaceutical Chemistry
Q 804 Complete the following:
Name B and C.
Ans: B would be an alcohol.
C would be sulphuric acid.
Q 805 How many carboxylic groups are present in succinic acid
and benzoic acid?
Ans: Benzoic acid has one carboxylic group while succinic acid has
two carboxylic groups.
Q 806 How is neutral solution of acid prepared during group analysis?
Ans: Place small quantity of the acid in a boiling tube and add a
slight excess of ammonia solution, i.e. until the solution is just
alkaline to litmus paper. Now, boil until the odour of ammonia
is completely removed and then cool.
Q 807 Which groups react with ferric chloride to give colour
reactions?
Ans: Colour reactions are produced when neutral ferric chloride
solution reacts with
1. Solutions of neutral salts of acids
2. Phenols and many of their derivatives
3. A few amines.
4. Hydroxamic acids.
Q 808 What are the different elements present in cinnamic acid?
Ans: Cinnamic acid is,
3-phenyl propanoic acid, C
6
H
5
–CH
2
–CH
2
COOH
It contains C, H, O elements.
Q 809 What happens when aromatic carboxylic acids are heated
with sodalime?
Ans: When aromatic acids are heated with sodalime, benzene or
phenol is produced from benzoic acid, salicylic acid,
Systematic Qualitative Analysis 161
respectively (with loss of CO
2
which is absorbed by sodalime).
Carboxylic group is eliminated.
Q 810 What happens when aliphatic acids like acetic acid or
succinic acid is heated with sodalime?
Ans: When acetic acid or succinic acid is heated with sodalime,
methane and ethane are evolved and CO
2
is absorbed by
sodalime. Carboxylic group is eliminated.
Q 811 What are the common derivatives which can be prepared
from carboxylic acid?
Ans: Benzyl thiouronium salts, amides and anilides can be easily
prepared from carboxylic acids.
Q 812 Why organic carboxylic acids are weak acids? or Why
carboxylic acids are weaker acids than mineral acids?
Ans: Smaller the numerical value of pK
a
, stronger is the acid and
carboxylic acids have pK
a
of the order of 5 which is greater
than that of mineral acids, e.g. pK
a
of HCl = –7 and pK
a
of
H
2
SO
4
= –9. Therefore, carboxylic acids are much weaker acids
than mineral acids.
Explanation:
In aqueous solutions, carboxylic acids ionize as
RCOOH H O RCOO H O
23
According to law of mass action,
K
[RCOO ] [H O ]
[RCOOH][H O]
3
2
As, water is in large excess. So, (H
2
O) will remain constant.
K[H O]
[RCOO ] [H O ]
[RCOOH]
2
3
K
a
[RCOO ] [H O ]
[RCOOH]
3
where,
K
a
is the dissociation constant of acid.
As, K
a
(H
3
O
+
)
162
Viva Voce in
Pharmaceutical Chemistry
Or
K
a
H
+
Greater the value of K
a
, greater is the tendency of acid to
ionize and hence it is stronger acid. As pK
a
= log K
a
, smaller
the value of pK
a
, stronger the acid.
Q 813 Name three carboxylic acids which exist as liquid.
Ans: Formic acid, acetic acid and lactic acid exist as liquid.
Q 814 Name two hydroxy carboxylic acids.
Ans: 1. Citric acid 2. Tartaric acid
Q 815 Name one amino carboxylic acid.
Ans: Anthranilic acid, H
2
N—C
6
H
4
—COOH.
Q 816 Give the general physical properties of carboxylic acids.
Ans: Most of the acids are colourless, crystalline solids except formic
acid, acetic acid and lactic acid.
Formic and acetic acid have pungent odour. Cinnamic acid
has a faint, pleasant odour. Aliphatic acids are all soluble in
cold water but readily soluble in boiling water.
Q 817 Give two acids having two carboxy groups.
Ans: Succinic acid, phthallic acid.
Q 818 Why does salicylic acid give violet colour with ferric chloride
solution?
Ans: Salicylic acid contains phenolic hydroxyl group. Phenolic
hydroxyl group reacts with ferric chloride to give corres-
ponding ferric salt which is violet in colour.
Q 819 How can aspirin and salicylic acid be differentiated by the
chemical test?
Ans: We need to perform ferric chloride test for both aspirin and
salicylic acid. Salicylic acid will give violet colour while aspirin
would give buff precipitate.
Q 820 Name the hydroxyl acid which appears as an intermediate
in Kreb’s cycle.
Ans: Citric acid appears in Kreb’s cycle.
Systematic Qualitative Analysis 163
Q 821 Why do some samples of aspirin give violet colour with ferric
chloride solution?
Ans: When aspirin is impure and contains salicylic acid as impurity,
it gives violet colour with ferric chloride solution. If aspirin is
not stored properly, it undergoes hydrolysis to give salicylic
acid.
ALCOHOLS/ALIPHATIC HYDROXY GROUP
Q 822 What are alcohols?
Ans: Alcohols are organic compounds of the general formula ROH,
where R is any alkyl or substituted alkyl group.
Q 823 Name the functional group present in alcohols.
Ans: Alcohols contain aliphatic hydroxyl group.
Q 824 Name five alcohols.
Ans: Methyl alcohol, ethyl alcohol, propanol, 2-propanol, butyl
alcohol, benzyl alcohol, etc.
Q 825 Describe physical properties of alcohols.
Ans: Alcohols are colourless liquids, completely miscible with
water, except benzyl alcohol and cyclohexanol. Alcohols have
faint odours, only benzyl alcohol has characteristic odour.
Q 826 State whether alcohols are acidic, basic or neutral.
Ans: Alcohols are neutral in reaction.
Q 827 What happens when a dry piece of sodium metal is added
to alcohol?
Ans: When a dry piece of sodium metal is added to alcohol,
vigorous reaction takes place with evolution of hydrogen gas
as effervescence.
Q 828 State what happens when alcohols are heated with acids in
presence of sulphuric acid.
Ans: Alcohols react with acids and form corresponding esters.
Q 829 What are primary, secondary and tertiary alcohols?
Ans: Alcohols are classified as primary, secondary or tertiary
according to the kind of carbon that bears the OH group. The
attached alkyl group can be primary, secondary or tertiary.
164
Viva Voce in
Pharmaceutical Chemistry
Q 830 How will you distinguish different types of alcohols?
Ans: We can distinguish primary, secondary and tertiary alcohols
using Luca’s test. Luca’s test is based upon rate of formation
of alkyl chlorides by 1°, 2° and 3° alcohols. When Luca’s
reagent is made to react with alcohol,
• Primary alcohol reacts very slowly and no separation of alkyl
chloride is observed
• Secondary alcohol—droplets of alkyl chloride may be seen after
one hour due to slow reaction.
• Tertiary alcohol—droplets of alkyl chloride are formed
immediately due to very fast reaction.
Q 831 Explain the ester formation test for alcohols.
Ans: Same as the ester test described under carboxylic acid. The
reagent is now carboxylic acid while the sample is an alcohol.
Q 832 Name the possible derivatives which can be prepared from
alcohols.
Ans: 3, 5-dinitrobenzoates, p-nitrobenzoates, phenylurethanes can
be prepared from alcohols. These are crystalline solids.
Q 833 Give the characteristic reactions of alcohol group.
Ans: Alcohols give following characteristic reactions:
1. Sodium Test: Alcohols react with sodium metal and produce
effervescence due to formation of hydrogen gas.
2. Ester Test: Alcohols react with acids to form esters which have
characteristic odour.
Systematic Qualitative Analysis 165
3. Reaction with acetyl chloride: Alcohols react with acetyl
chloride, producing an ester and hydrochloric acid gas. When
a rod dipped in ammonium hydroxide is brought in contact
with vapours, dense white fumes are produced.
4. Oxidation Test: Primary alcohols are oxidized to aldehydes
and then to acids when treated with oxidizing agent like
sodium dichromate in presence of conc. sulphuric acid. Green
colouration is observed due to formation of chromium
sulphate. The specific odour of aldehyde formed is perceptible.
Alcohol Aldehyde Odour
Methyl alcohol Formaldehyde Pungent
Ethyl alcohol Acetaldehyde Characteristic
n-butanol Butyraldehyde Pleasant
Benzyl alcohol Benzaldehyde bitter almond
Secondary alcohols form ketones after oxidation.
5. Benzoylation: Alcohols give crystalline esters with benzoyl
chloride or 3,5-dinitrobenzoyl chloride.
166
Viva Voce in
Pharmaceutical Chemistry
6. Iodoform Test: Alcohols containing the CH
3
CH(OH) group
give iodoform reaction.
PHENOLS/AROMATIC OR PHENOLIC HYDROXY GROUP
Q 834 What are phenols?
Ans: Phenols are organic compounds of the general formula ROH,
where R is phenyl, substituted phenyl or some other aryl group.
Q 835 Name the functional group present in phenols.
Ans: Phenols contain aromatic hydroxy group.
Q 836 Whether phenols are acidic, basic or neutral?
Ans: Phenols are mildly acidic in nature.
Q 837 Why are phenols acidic and while alcohols are neutral.
Ans: Alcohols as well as phenols contain hydroxyl group. After
ionization of the alcohol, alkoxide ion is formed while phenols
form phenoxide ions. The phenoxide ion is stabilized by
resonance more than the phenol itself and thus the ionization
is promoted giving hydrogen ions, while there is no
stabilization of the alkoxide ion and thus alcohols are neutral
and phenols are acidic.
Q 838 How would you arrange alcohol, phenols and carboxylic
acids in terms of acidity?
Ans: Phenols are more acidic than alcohols but are less acidic than
carboxylic acids.
Carboxylic acids > phenols > alcohols.
Q 839 How will you differentiate phenols from carboxylic acids?
Ans: Phenols do not produce effervescence when made to react
with sodium bicarbonate while acids produce the effer-
vescence.
Q 840 What is the chemical name of picric acid? What makes it
more acidic than phenol? Why is picric acid called acid, when
chemically it is a phenol?
Ans: The chemical name of picric acid is 2,4,6-trinitro phenol. The
three electrons withdrawing nitro groups make the bond
R/H
Systematic Qualitative Analysis 167
between oxygen and hydrogen (of the hydroxy group )
weaker. Thus, the hydrogen can be easily donated forming
2,4,6-trinitro phenoxide ion. This anion is stabilized more
because the electron withdrawing nitro groups disperse the
negative charge in the anion. The ability to donate the
hydrogen ion increases making it much more acidic. Thus, it
is called an acid. It is more acidic than phenol because the
trinitro phenoxy anion is more stable than phenoxy ion of
phenol.
Q 841 How can phenols be distinguished from alcohols?
Ans: Phenols give positive phthalic anhydride test, Libberman’s
test and azo dye test while alcohols do not give positive
response in these tests.
Q 842 What is liquified phenol IP?
Ans: Liquified phenol IP contains 80% w/w of phenol.
Q 843What are the different elements present in paracetamol?
Ans: Paracetamol is nothing but para-acetamino phenol.
Thus, the elements present in paracetamol are nitrogen,
oxygen, carbon and hydrogen.
Q 844 Explain different qualitative tests for identification of
phenols.
Ans: Different qualitative tests for identification of phenols are:
1. Bromination Test: When bromine water is gradually added
to aqueous solution of phenol, initially decolourisation of
bromine water occurs and on adding excess, a white or
yellowish precipitate is formed.
168
Viva Voce in
Pharmaceutical Chemistry
2 . Ferric Chloride Test: When ferric chloride solution is added
to aqueous solution of phenol, violet colouration is observed.
3. Phthalein Test: To the organic compound, phthalic anhydride
is added followed by 0.5 ml of conc. sulphuric acid. Heat and
cool. Add excess of sodium hydroxide. A pink red colour is
produced due to formation of phenolphthalein.
4. Liebermann’s Nitroso Test: Dissolve an organic compound
in conc. H
2
SO
4
and add few crystals of sodium nitrite. A blue-
green or blue-violet colour is produced which changes to red
on dilution with water. This colour again changes to green,
Systematic Qualitative Analysis 169
blue or violet on addition of NaOH solution. This test is
performed with organic compounds having free phenolic
groups at para position.
5. Azo dye Test: To aniline, add conc. HCl, cool the solution to
0–5°C. Add chilled aqueous solution of sodium nitrite (0–5°C),
followed by solution of organic compound in sodium
hydroxide (0–5°C), orange red-coloured azo dye is produced.