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170
Viva Voce in
Pharmaceutical Chemistry
Q 845 Explain phthalein test for phenols.
Ans: As described in answer 844, point 3.
Q 846 Explain ferric chloride test for phenols.
Ans: As described in answer 844, point 2.
Q 847 Explain Liebermann’s nitroso test for phenols.
Ans: As described in answer 844, point 4.
Q 848 Explain azo dye test for phenols.
Ans: As described in answer 844, point 5.
Q 849 Which groups give positive azo dye test?
Ans: Aromatic primary amines and phenols give positive azo dye
test.
Q 850 What is the effect of concentrated solution of phenol on
bromine water?
Ans: The bromine water is decolourised with formation of o- or p-
bromophenol/2, 4, 6-tribromophenol, which separates out as
solid.
Q 851 What happens when excess of bromine water is added to
phenol?
Ans: White coloured precipitate of 2,4,6-tribromophenol separates.
Q 852 Name the most common derivatives which can be prepared
of phenols.
Ans: Toluene-p-sulphonates, p-nitro and 3,5-dinitrobenzoates and
phenylurethanes can be formed from phenol and these are
crystalline derivatives.
Systematic Qualitative Analysis 171
Q 853 How will you differentiate phenol,
-naphthol and
resorcinol from organoleptic tests?
Ans: Phenol appears as pinkish deliquescent crystals, -naphthol
appears as free flowing buff coloured flakes and resorcinol
appears as pink to dark violet coloured free flowing flakes.
Q 854 How will you differentiate phenol,
-naphthol and
resorcinol using chemical tests?
Ans: We can differentiate phenol, -naphthol and resorcinol using
phthalein test. Phenol gives red colouration, (due to formation
of phenolphthalein), resorcinol gives green fluorescent
solution (due to formation of fluorescein) and -naphthol
gives very faint green colour with slight fluorescence after
addition of sodium hydroxide into the product formed by
reaction between the phenol/resorcinol/-naphthol and
phthalic anhydride.
Q 855 Name five phenolic compounds and give their structures.
Ans:
ALDEHYDES
Q 856 What are aldehydes?
Ans: Aldehydes are organic compounds of the general formula
R–CHO, where R is an alkyl or substituted alkyl or aromatic
group.
Q 857 State whether aldehydes are acidic, basic or neutral.
Ans: Aldehydes are neutral in nature.
172
Viva Voce in
Pharmaceutical Chemistry
Q 858 Name the functional group present in the aldehydes.
Ans: Aldehydes contain carbonyl group to which a hydrogen atom
is attached.
Q 859 Name five organic compounds containing aldehyde group.
Ans: Formaldehyde (HCHO), acetaldehyde (CH
3
CHO)
Q 860 Describe the utility of 2,4-dinitrophenylhydrazine in the
detection of an aldehyde group.
Ans: When 2,4-dinitrophenylhydrazine is made to react with
carbonyl group present either in aldehyde or ketone,
corresponding phenyl hydrazone is formed which separates
out as orange precipitate.
Q 861 Which property of aldehyde is used in performing the
qualitative tests for detection of aldehydes?
Ans: Aldehydes undergo oxidation and can easily reduce the
oxidising agents like Fehling’s solution, Tollen’s reagent,
Schiff’s reagent, etc. producing the colour change, precipitation
or silver mirror formation.
Q 862 How will you differentiate aldehydes from ketones?
Ans: Aldehydes as well as ketones have carbonyl group but only
aldehydes are easily oxidized, while ketones do not oxidize easily.
Thus, aldehydes give positive response in Fehling’s test, Tollen’s
test, Schiff s test, etc. while ketones do not give these tests.
Q 863 What is formalin?
Ans: A 40% solution of formaldehyde in water containing about
15% of methanol is known as formalin. Methanol prevents
polymerization of formaldehyde to solid paraformaldehyde.
Q 864 Why do aldehydes easily undergo oxidation while ketones
do not?
Ans: An aldehyde has a hydrogen atom attached to the carbonyl
carbon and while ketone has two aryl or, alkyl groups attached
to carbonyl carbon. The hydrogen of the aldehyde group is
abstracted in oxidation, either as a proton or as an atom and
Systematic Qualitative Analysis 173
the analogous reaction for a ketone, i.e. abstraction of the alkyl
or aryl group cannot take place.
Q 865 What is 2, 4-DNP test for aldehydes and ketones?
Ans: 2, 4-dinitro phenyl hydrazine reacts with aldehyde or ketone
to form corresponding phenyl hydrazone which precipitates
as red or orange solid.
Q 866 Where are 2,4-DNP reagent A and B used? What is the
difference between the two?
Ans: 2,4-DNP reagent A is used in detection of aldehydes and it is
prepared by dissolving 2,4-DNP in water and hydrochloric acid.
2,4-DNP reagent B is used in detection of ketones and it is
prepared by dissolving 2,4-DNP in methanol and sulphuric
acid.
Q 867 Which chemical tests are used for detection of aldehydes?
Ans: 2,4-DNP test, Fehling’s test, Tollen’s test, Schiffs test,
Benedict’s test are used for detection of aldehydes.
Q 868 Describe Fehling’s test and give its chemistry.
Ans: Mix Fehling’s solution A and Fehling’s solution B in a test
tube and add a small quantity of organic compound in
presence of sodium carbonate solution. Boil the solution for 5
minutes on a water bath. The solution initially turns from blue
to green and on keeping, red coloured precipitate of cuprous
oxide is formed.
174
Viva Voce in
Pharmaceutical Chemistry
Q 869 Does aldehyde give positive Tollen’s test? Give chemistry
of Tollen’s test.
Ans: Aldehydes give positive Tollen’s test. When a small amount
of aldehyde is added in the test tube containing Tollen’s
reagent and heated on water bath without disturbing the test
tube, silver mirror is formed due to the reduction of silver
ions to metallic silver.
Q 870 Give the chemistry of Schiff’s test.
Ans: When an aldehyde is added to Schiffs reagent, a deep violet-
red/red/magenta colour is produced.
Systematic Qualitative Analysis 175
For formaldehyde and acetaldehyde, a magenta colour
rapidly developes in the cold. Benzaldehyde restores the
colour very slowly.
Schiff’s reagent is a dilute solution of Fuchsin hydrochloride
that has been decolourised by sulphur dioxide. This
decolourization is the result of a reaction between p-rosaniline
and sulphurous acid that destroys the quinonoid structure of
the dye and produces a colourless dye. The colourless
compound is unstable and loses sulphurous acid when treated
with an aldehyde to yield a violet-purple dye.
Precaution should be taken that free alkali or the alkali salts
of weak acids are not brought into contact with Schiff’s reagent
as it would redden the dye. The dye is also reddened by heat
or when exposed in small quantities to the air for sometime.
Mineral acids greatly reduce the sensitivity of the test.
Q 871 What is the effect of concentrated sodium hydroxide on
aliphatic and aromatic aldehydes?
Ans:
a. Aliphatic aldehydes (except formaldehyde) give resins.
b. Most aromatic aldehydes give Cannizaro’s reaction.
Benzaldehyde reacts with concentrated sodium hydroxide to
form sodium salt of benzoic acid and benzyl alcohol. When
hydrochloric acid is added, benzoic acid precipitates as white
crystals.
Q 872 What is the effect of alkaline KMnO
4
on aldehydes?
Ans: Alkaline KMnO
4
oxidises the aldehydes to corresponding
acids.
Q 873 What happens when aldehydes react with saturated sodium
bisulphite solution?
Ans: When aldehydes react with saturated sodium bisulphite
solution, a white addition product separates in case of
benzaldehyde while formaldehyde and acetaldehyde form
addition products which are soluble. The addition products
are also formed by ketones.
Q 874 Would acetaldehyde give positive iodoform test? Why?
Ans: Compounds having the -methyl group in a ketone or
aldehyde give the iodoform test. As acetaldehyde has
176
Viva Voce in
Pharmaceutical Chemistry
CH
3
—C— group it would give the iodoform test. Formal-
dehyde, benzaldehyde would not give this test.
Acetone, methyl ethyl ketone would also give positive
iodoform test.
Q 875 Sugars give positive Fehling’s test, Tollen’s test, etc. Why?
Ans: Sugars contain aldehydic group which undergoes oxidation
and in process reduces these reagents giving +ve test.
Q 876 Reducing property of sugars is due to which group?
Ans: It is due to the presence of aldehyde group.
Q 877 What product is formed when acetaldehyde and benzal-
dehyde are oxidized? Write the equation.
Ans: Acetic acid/ethanoic acid is formed when acetaldehyde is
oxidised using saturated solution of potassium permanganate
and pinch of sodium carbonate. Benzaldehyde is oxidized to
benzoic acid.
Q 878 Which organic compounds give silver mirror test with
Tollen’s reagent?
Ans: Organic compounds which are easily oxidisable and are of
reducing nature give silver mirror test. Such compounds are
aldehydes, reducing sugars such as glucose and fructose,
hydroxylamine containing compounds. Nitro group also gives
this test. Nitro group is first reduced to hydroxylamine and
this hydroxylamine is oxidised to nitroso group by the Tollen’s
reagent and in process Tollen’s reagent is reduced to metallic
silver ions.
Q 879 Name the crystalline derivatives of aldehydes.
Ans: 2,4-Dinitrophenylhydrazones, semicarbazones, acids can be
prepared from aldehydes.
O
Systematic Qualitative Analysis 177
KETONES
Q 880 What are ketones?
Ans: Ketones are the organic compounds of the general
R
1
formula C== O, where R
1
and R
2
are alkyl,
R
2
substituted alkyl or aryl groups.
Q 881 What is the nature of ketones–acidic, basic or neutral?
Ans: Ketones are neutral in nature.
Q 882 Name the functional group present in ketones.
Ans: Ketones have carbonyl group to which two alkyl or aryl
groups are attached.
Q 883 Name five ketones.
Ans: Acetone (CH
3
COCH
3
), ethyl methyl ketone (C
2
H
5
COCH
3
),
diethyl ketone (C
2
H
5
COC
2
H
5
), acetophenone (CH
3
COC
6
H
5
),
benzophenone (C
6
H
5
COC
6
H
5
), cyclohexanone .
Q 884 Which chemical tests are used for detection of ketones?
Ans: 2,4-DNP test is given positive by all ketones and is
characteristic test. This test is also positive for aldehydes.
Ketones containing CH
3
CO group give Schiff’s test, iodoform
test, sodium nitroprusside test and m-dinitrobenzene test.
Certain ketones give bisulphite addition product. Ketones
do not give Tollen’s test and Fehling’s test positive.
Q 885 Describe the Schiff’s test for acetone.
Ans: When acetone is added to Schiff’s reagent it slowly developes
light pink colour but it is not a positive test. Benzophenone or
acetophenone does not react in this way. Chemistry is same
as described in Schiff s test for aldehydes.
Q 886 Describe the 2,4-dinitrophenyl hydrazine test for ketones.
Ans: Same as described under aldehydes.
Q 887 Describe the iodoform test for ketones. What type of ketones
will give positive iodoform test?
Ans: Only ketones containing CH
3
CO group give iodoform test.
O
178
Viva Voce in
Pharmaceutical Chemistry
Take sample and dissolve in water (or methyl alcohol). Add
sodium hydroxide solution followed by a solution of iodine
in potassium iodide drop by drop until the dark colour of
iodine persists in the solution. Allow it to stand for 15 minutes,
yellow precipitate of iodoform forms.
Q 888 Describe sodium nitroprusside test for ketones?
Ans: When sodium nitroprusside is added to ketones containing
CH
3
CO-group, in alkaline medium, red colouration is
produced. The colouration is caused by conversion of acetone
by alkali to the CH
3
COCH
–
2
ion, which then reacts with the
nitroprusside ion [Fe(CN)
5
NO]
2–
giving the highly coloured
ion [Fe(CN)
5
NO
.
CH
2
COCH
3
J
2–
. This test is not given by
benzophenone.
Q 889 Describe m-dinitrobenzene test.
Ans: When 0.1 g of m-dinitrobenzene is added to acetone or
acetophenone followed by excess of dil. NaOH solution, violet
colouration is produced which slowly fades. The coloured
compound formed between m-dinitro compound and
acetone is
Q 890 What is bisulphite test for ketone?
Ans: When saturated solution of NaHSO
3
is shaken with acetone,
a white precipitate of addition product is formed.
Q 891 Which ketones do not give bisulphite addition compound?
Ans: Acetophenone and benzophenone having carbonyl group
directly joined to benzene ring, do not respond to the test.
Q 892 Would ketone give positive Tollen’s test and Fehling’s test?
Ans: Ketone will not give Tollen’s and Fehling’s test positive as it
cannot reduce these reagents.
Systematic Qualitative Analysis 179
Q 893 Which are the crystalline derivatives of ketones?
Ans: Ketones can form 2,4-dinitrophenyl hydrazones and semi-
carbazones which are crystalline solids.
Q 894 Benzophenone does not give most of the tests of ketones.
How would you identify it?
Ans: To benzophenone add naphthalene as a solvent and heat until
completely molten. Add a small piece of sodium and heat
again. The surface of sodium becomes green due to the
formation of free radical, (C
6
H
5
)
2
CONa and it spreads
throughout the molten mass on strong heating.
Q 895 What is the IUPAC name of acetone and methyl n-propyl
ketone?
Ans: Acetone is named as propanone. Methyl-n-propyl ketone is
named as 2-pentanone.
ESTERS
Q 896 What are esters?
Ans: Esters are the organic compounds of the general formula
R
1
—C—O—R
2
, where R
1
and R
2
are the alkyl, substituted alkyl
or aryl groups.
Q 897 What is esterification?
Ans: Esterification is the process in which an alcohol reacts with
carboxylic acid in presence of sulphuric acid to form an ester.
Q 898 What happens when esters are hydrolysed?
Ans: When esters are hydrolysed, they decompose to form
corresponding alcohol and carboxylic acid.
Q 899 Mention the functional groups involved in the formation of
ester group.
Ans: Hydroxy group and carboxylic groups are involved in the
formation of ester group.
Q 900 What are the general properties of esters?
Ans: Esters are colourless compounds. Most are liquids but few are
solids at room temperature, e.g. methyl tartrate or succinate.
Most are sparingly soluble in water with few exceptions.
Q 901 Name five esters.
Ans: Ethyl acetate, methyl salicylate, ethyl oxalate, methyl tartrate,
phenyl benzoate.
O