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130
Viva Voce in
Pharmaceutical Chemistry
Example:
Aliphatic hydrocarbons rarely undergo this reaction.
Q 674 Give two examples of nitration.
Ans:
• Nitrobenzene can be prepared from benzene.
• Picric acid can be prepared from phenol.
HYDROLYSIS
Q 675 What is hydrolysis?
Ans: Hydrolysis involves breaking of bond by addition of water
molecule.
Derivatives of carboxylic acids like amides and esters can
be easily hydrolysed in acidic or alkaline medium, producing
corresponding acid in the form of salt and amine or alcohol/
phenol, respectively.
Q 676 Which reagents are used for hydrolysis?
Ans: An acid like sulphuric acid or an alkali like sodium hydroxide
can be used for hydrolysis.
Q 677 Give mechanism of hydrolysis under acidic as well as
alkaline conditions.
Acidic Conditions
Preparation of Organic Compounds 131
Under acidic conditions attack by water is on protonated
amide.
Alkaline Conditions
Under alkaline conditions, hydrolysis involves attack by the
strongly nucleophillic hydroxide ion on the amide itself.
Q 678 Which products can be synthesized using hydrolysis
process?
Ans: Using hydrolysis, we can synthesize acids, amines and alcohols.
Q 679 How can you prevent the decomposition of the chemicals
by hydrolysis?
Ans: The decomposition, of the chemicals can be avoided/reduced
by proper storage, i.e. in a tightly closed container free from
moisture.
ESTERIFICATION
Q 680 What is esterification?
Ans: Esterification is the process of formation of an ester by
chemical reaction between an alcohol/phenol and acid/acid
chloride. The acid or acid chloride may be aromatic and when
the acid chloride is benzoyl chloride, it is called benzoylation.
Q 681 Why is sulphuric acid required in esterification process?
Ans:
Esterification reaction is reversible in nature. To keep the
forward balance of the reaction, the water molecule formed
needs to be removed. Sulphuric acid acts as dehydrating agent
and absorbs the water. The acid acts as catalyst and increases
the rate of reaction.
Q 682 Which dehydrating agent other than sulphuric acid can be
employed in esterification?
Ans: Dry hydrogen chloride can be employed as dehydrating agent
in place of H
2
SO
4
.
132
Viva Voce in
Pharmaceutical Chemistry
Q 683 How much sulphuric acid should be added in esterification?
Ans: H
2
SO
4
equivalent to about 3% of the weight of alcohol, is
added to alcohol during esterification.
Q 684 What is the mechanism of esterification when H
2
SO
4
is
employed?
Ans:
1. The proton from acid attacks negatively polarized carbonyl
oxygen to produce oxonium ion (I). The carbonyl carbon
becomes more positive and thus susceptible to attack by the
nucleophilic alcohol molecule.
2. The alcohol molecule attacks the carbonyl carbon and
produces an addition product (II).
3. Proton transfer occurs within the addition product producing
intermediate (III) by proton transfer.
4. Loss of water from (III), followed by transfer of a proton gives
the ester IV.
Q 685 Which reagent can be used in esterification in place of an
acid?
Ans: Acid chloride can be used in the esterification in place of an
acid.
Preparation of Organic Compounds 133
Q 686 What is the disadvantage of performing esterification using
an acid and an alcohol?
Ans: Reversibility of the reaction is the major disadvantage in
preparation of an ester directly from an acid.
Q 687 What is the advantage of performing esterification using an
acid and an alcohol?
Ans: The process is single step synthesis.
Q 688 What is the advantage of using acid chloride in place of acid
in esterification?
Ans: The preference of use of acid chloride is due to the fact that
following two steps go to completion.
1. preparation of acid chloride from acid
2. preparation of ester from acid chloride is irreversible.
Q 689 Which alcohols react faster during esterification?
Ans: Reactivity of alcohols during esterification is as under
1°> 2°> 3°.
Q 690 What is the effect of bulky groups on esterification?
Ans: Presence of bulky groups near the site of reaction, whether in
alcohol or in acid, slows down the esterification.
Reactivity HCOOH > CH
3
COOH > RCH
2
COOH
> R
2
CHCOOH > R
3
CCOOH
Q 691 Name two products formed by esterification process.
Ans: Ethyl acetate, n-butyl acetate can be prepared by esterification
process.
BENZOYLATION
Q 692 What is benzoylation?
Ans: Benzoylation is the process in which benzoyl group
(C
6
H
5
CO–) replaces the reactive hydrogen group from
hydroxyl or amino group.
Q 693 Which reagent is used for benzoylation?
Ans: Benzoyl chloride is used for benzoylation. Students should
be careful and should not use benzyl chloride in place of
benzoyl chloride.
134
Viva Voce in
Pharmaceutical Chemistry
Q 694 What are the advantages of benzoylation over acetylation.
Ans:
l. Acetylation needs to be performed in absence of water as
acetylating agents like acetic anhydride and acetyl chloride
are readily hydrolysed.
2. Benzoyl derivatives are insoluble in water and can be easily
isolated from the crude reaction products while acetyl
compounds are soluble and thus difficult to isolate.
3. Benzoyl derivatives have higher melting points than acetyl
derivatives.
4. Benzoyl derivatives are much less soluble in most solvents
and thus can be readily recrystallized.
Q 695 Benzoylation requires the presence of sodium hydroxide.
Why?
Ans: The excess of benzoyl chloride left after benzoylation is
neutralized by sodium hydroxide.
Q 696 Name two products formed by benzoylation.
Ans: The two products formed by benzoylation are phenyl benzoate
and benzanilide, i.e. benzoyl aniline or phenyl benzamide.
DIAZOTIZATION
Q 697 What is diazotization?
Ans: Preparation of diazonium salt by action of nitrous acid (HNO
2
)
on primary aromatic amine is called diazotization. The nitrous
acid is generated in situ by reaction between mineral acid (HCl,
H
2
SO
4
) and sodium nitrite.
Q 698 Give the process of diazotization.
Ans: The primary aromatic amine is dissolved in mineral acid like
hydrochloric acid and the solution is cooled to 5°C. To this,
cooled (5°C) sodium nitrite solution (aqueous) is added.
Preparation of Organic Compounds 135
Q 699 Why does the temperature of the reactants need to be
maintained between 0–5°C?
Ans: The solutions of amine and sodium nitrite should be cooled to
around 5°C and maintained at same temperature throughout
the addition of sodium nitrite solution because the heat of
reaction would cause rise in temperature with consequent
decomposition of the diazonium chloride giving phenol.
If the temperature is reduced to 0°C or below, diazotization
becomes very slow.
Q 700 How much sodium nitrite should be used during diazoti-
zation?
Ans: Sufficient quantity of sodium nitrite must be added to
diazotize all the primary aromatic amine present. We must
ensure that slight excess of nitrous acid is produced in situ.
About 10% excess of the theoretically required amount of AR
grade sodium nitrite is recommended.
Q 701 What is the importance of diazotization?
Ans:
l. The diazonium nitrogen can be replaced by –Ar, –NO
2
,–OR,
–SH, –SR, –NCS, –NCO, etc. to give wide variety of chemicals
and thus it is useful in synthesis.
2. The coupling of diazonium salts with aromatic phenols and
amines yields azo compounds, which are of tremendous
importance to the dye industry.
Q 702 What is Sandmeyer reaction?
Ans: Replacement of the diazonium group by –Cl or –Br with the
help of cuprous chloride or cuprous bromide is called
Sandmeyer reaction.
Q 703 Name two compounds prepared using diazotization
reaction.
Ans:
1. Preparation of iodobenzene. First diazotization of aniline is
done, followed by replacement of diazonium group by iodine
using potassium iodide.
2. Azo dye can be formed by coupling the diazonium salt of
aniline with 2-naphthol. Benzene azo-2-naphthol, a scarlet dye
is formed.
136
Viva Voce in
Pharmaceutical Chemistry
OXIDATION
Q 704 What is oxidation?
Ans: Oxidation is the process in which there is loss of electrons/
loss of hydrogen/gain of oxygen.
Q 705 Which products can be formed by oxidation of alcohols?
Ans: Aldehydes, ketones and carboxylic acids are formed after
oxidation of alcohols.
• Primary alcohols on restricted oxidation produce corres-
ponding aldehyde which can be further oxidized to corre
sponding carboxylic acid.
• Secondary alcohols are oxidized to corresponding ketones.
• Tertiary alcohols are not oxidized as they do not contain
-hydrogen.
Q 706 Name oxidizing agents used for oxidation.
Ans: Heptavalent Mn(VII) is used in the form of potassium
permanganate, KMnO
4
. Hexavalent chromium in the form
of chromic acid (in glacial acetic acid or pyridine) or acidic
aqueous potassium dichromate, K
2
Cr
2
O
7
is used for oxidation.
The other agents for oxidation of alcohols are hypohalite
like sodium hypoiodite (NaIO) or periodic acid.
Q 707 Which oxidizing agent is used for controlled oxidation of
alcohol to aldehyde?
Ans: Pyridinium chlorochromate (C
5
H
5
NH
+
CrO
3
Cl
–
) is used.
Q 708 Name two products formed by oxidation.
Ans: Acetaldehyde and acetic acid can be formed by oxidation of
ethanol using oxidising agent sodium dichromate in sulphuric
acid.
REDUCTION
Q 709 What is reduction?
Ans: Reduction is the process of gain of electrons/gain of
hydrogen/loss of oxygen.
Q 710 Which product can be formed by reduction of nitro group?
Ans: Corresponding primary amine is formed by reduction of nitro
group.
Preparation of Organic Compounds 137
Q 711 Which product is formed by reduction of an acid?
Ans: Reduction of an acid leads to formation of an aldehyde/
ketone and further reduction gives primary/secondary
alcohol.
Q 712 Which product can be formed by reduction of aromatic
compounds?
Ans: The reduction of aromatic compounds would give cyclo-
alkanes.
Q 713 What is the effect of electron withdrawing group on
reduction of aromatic compounds?
Ans: Electron withdrawing groups would decrease the electron
density in the aromatic ring and promote the acceptance of
the electron, i.e. reduction.
Q 714 What is the effect of electron donating group on reduction
of aromatic compounds?
Ans: The electron donating group would increase the electron
density in the aromatic ring and thus hinders the acceptance
of an electron.
Q 715 Name three reducing agents.
Ans: The reducing agents may be Zn/Hg in presence of HCl or Na
in liquid ammonia, in the presence of a proton donor like
methanol, ethanol or Sn/HCl pair.
Tollen’s reagent, Fehling’s solution are the reducing agents
used in qualitative organic analysis.
Q 716 Name three products prepared by reduction process.
Ans:
1. Heptan-2-ol can be prepared from heptan-2-one using
reducing pair Na/C
2
H
5
OH
2. Aniline can be prepared from nitrobenzene using reducing
agent Sn/HCl
3. 2-methyl butane can be prepared from 2-methyl but-2-ene by
reduction with H
2
/Pd or H
2
/Pt.
GENERAL QUESTIONS RELATED TO SYNTHESIS
Q 717 What is crude product?
Ans: The unpurified product formed by reaction between the
reactants is called crude product.
138
Viva Voce in
Pharmaceutical Chemistry
Q 718 What is purification?
Ans: Purification is the process of removing impurities from the
crude product.
Q 719 How is purification done?
Ans: Purification is done by either sublimation or crystallization.
Sublimation is applicable only to those compounds which can
be directly converted from solid state to vapour state.
Crystallization is most widely used method.
Q 720 What is recrystallization?
Ans: Recrystallization is the process in which the compound/crude
product is dissolved in selected solvent with heating, then
filtered while hot and cooled slowly to a saturated solution
from which pure compound crystallises out.
Q 721 What are the criteria for selecting a solvent for recrys-
tallization?
Ans: The solvent should be such that it dissolves the crude material
readily when hot, but only to a small extent when cold. The
solvent should not be inflammable.
Q 722 Why are solvent pairs used for recrystallization?
Ans: When the crude product is highly soluble in one solvent (A)
and insoluble in other (B), it is dissolved in hot solvent A and
then hot solvent B is added till slight turbidity is produced.
Then the solution is cooled to get crystals, e.g. methanol-water,
ethanol-water.
Q 723 Name the solvents commonly used for crystallization.
Ans: The solvents commonly used for crystallization are water,
ethanol, methanol, acetic acid, chloroform, carbon tetra-
chloride and acetone. Inflammable solvents like benzene, ether
should normally be avoided.
Q 724 How would you remove the coloured impurities?
Ans: Coloured impurities are removed by adsorption on activated
charcoal, when the solid is refluxed in the solvent during
recrystallization. The solution is filtered while hot to get the
clear, clean solution and cooled to get crystals.
Q 725 What is the importance of condensers in the synthesis?
Ans: Condensers are used for following purposes:
1. Purification by distillation
2. Extraction of the constituents
3. Solvent recovery.
Preparation of Organic Compounds 139
Q 726 Give two types of condensers.
Ans: Water condensers and air condensers.
Q 727 What is the use of reflux condenser?
Ans: Reflux condenser prevents loss of solvent and sends back the
evaporated solvent (after condensing) in the form of liquid to
round bottom flask.
Q 728 Why are porcelain pieces added during heating of reaction
mixture?
Ans: Unglazed porcelain pieces are added to provide the nucleus
for formation of continuous stream of bubbles. If the pieces
are not added, then great amount of heat would be absorbed
by the liquid and then suddenly bumping/explosion can
occur. To avoid bumping porcelain pieces are added.
Q 729 Define theoretical yield.
Ans: The theoretical yield is the quantity/weight of the product
one should get based on the stoichiometric quantities of the
reagents, assuming 100% completion of reaction.
Q 730 What is practical yield?
Ans: Practical yield is the weight of the product actually obtained
after purification of the product.
Q 731 Define percentage practical yield.
Ans: Percentage practical yield can be defined as
Q 732 Is it possible to get 100% percentage practical yield? Why?
Ans: It is not possible to get 100% percentage practical yield,
because losses are inevitable during filtration, drying,
purification/recrystallisation.
Q 733 What are the possible impurities in the product formed after
synthesis?
Ans: The possible impurities are:
1. The unused reactants
2. The solvent
3. The by-products of the reactions
4. Insoluble foreign substances
% Practical yield =
× 100
Practical yield
Theoretical yield