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120
Viva Voce in
Pharmaceutical Chemistry
Q 618 What is the inference if the substance is soluble in dilute
sodium hydroxide solution but not in dilute sodium
bicarbonate solution?
Ans: The substance is weakly acidic. Weakly acidic substances can
dissolve in strong bases like sodium hydroxide but not in weak
bases such as sodium bicarbonate, e.g. phenols.
Q 619 If the substance is soluble in dilute hydrochloric acid, what
is the indication?
Ans: If the substance is soluble in dilute hydrochloric acid, it may
be basic in nature, e.g. primary aliphatic amines form salts
with HCl. Primary aromatic amine, aniline also dissolves but
the basicity of the nitrogen in secondary and tertiary amines
is very less and thus they cannot dissolve in HCl.
Q 620 If the substance is soluble in concentrated H
2
SO
4
, what is
the indication?
Ans: Sulphuric acid is a very effective proton donor and can
protonate very weak bases. The compounds containing
oxygen (have weakly basic character) are soluble in
concentrated sulphuric acid. Neutral compounds like
alkenes, alkynes, alcohols, ketones, esters, ethers, amides,
aromatic nitro compounds and some hydrocarbons are
soluble in H
2
SO
4
.
Q 621 If the compound is soluble in sulphuric acid but not in
hydrochloric acid, what is the inference?
Ans: The compound is a weak base.
Q 622 What is the inference if the compound is soluble in both
H
2
SO
4
and HCl?
Ans: The compound may be a strong base.
Q 623 If the compound is insoluble in ether, what does it indicate?
Ans: Vast majority of compounds are more or less soluble in ether.
Ionic compounds (salts) and very polar compounds like
carbohydrates are insoluble in ether.
Q 624 What is the inference if the compound is insoluble in HCl
as well as H
2
SO
4
?
Ans: The compound may be inert, e.g. alkanes, alkyl halides, simple
aromatic hydrocarbons and aryl halides.
Q 625 With reference to IP, state the different terms and their
meaning, to describe the solubility.
Fundamentals of Preparative Organic Chemistry 121
Ans: The statements of solubilities are applicable in, between 20°C
and 30°C.
Descriptive term Approximate volume of
solvent in ml/g of solute
• Very soluble Less than 1
• Freely soluble From 1 to 10
• Soluble From 10 to 30
• Sparingly soluble From 30 to 100
• Slightly soluble From 100 to 1000
• Very slightly soluble From 1000 to 10,000
• Insoluble or practically insoluble More than 10,000
STORAGE
Q 626 What is the importance of proper storage of chemicals/drugs?
Ans: The chemicals/drugs should be properly stored under
conditions that prevent contamination and as far as possible,
deterioration.
Q 627 When specific storage conditions are not given, how would
you store the chemical/drug?
Ans: We should store it in well-closed container to protect from
moisture, light, avoiding excessive heat as well as freezing.
Q 628 What are ‘cold’ storage conditions?
Ans: Any temperature not exceeding 8°C and usually between
2°C and 8°C indicates ‘cold’ storage. The refrigerator is
maintained thermostatically between 2°C and 8°C.
Q 629 What are ‘cool’ storage conditions?
Ans: Any temperature between 8°C and 25°C indicates ‘cool’
storage conditions.
Q 630 What is room temperature?
Ans: The temperature prevailing in working area is room
temperature.
Q 631 What do you understand by ‘warm’ place?
Ans: Any temperature between 30°C to 40°C is warm temperature.
Q 632 What is excessive heat?
Ans: Any temperature above 40°C is excessive heat.
122
Viva Voce in
Pharmaceutical Chemistry
MELTING POINT
Q 633 Define melting point.
Ans: The melting point of the compound is a constant temperature
at which the solid begins to change into liquid state under a
pressure of 1 atmosphere.
Q 634 What do you understand by the term sharp or definite
melting point?
Ans: Sharp or definite melting point means the substance melts
entirely within a range of about 1°C.
Q 635 How is melting point useful in identification of an organic
compound?
Ans: Melting point is a valuable criterion of purity of an organic
compound. A definite and sharp melting point indicates a
pure compound. Melting point is characteristic for an organic
compound. Thus, it helps in identification of an organic
compound.
Q 636 State the importance of melting point determination.
Ans:
1. It is the criterion for determination of purity of a substance.
2. It helps in identification of an organic compound.
Q 637 Which bath liquids can be used in the determination of
melting point and why?
Ans: 1. Water: When melting point is up to 60°C.
2. Glycerin: When melting point is up to 140°C.
3. Medicinal paraffin:
• Liquid paraffin has low specific heat and therefore
temperature can be easily increased using only a small
flame.
• Even when hot, it is non-inflammable.
• The oil is non-corrosive and thus causes remarkably
slight burns even if split at a high temperature, on the
hands.
• The oil can be safely heated up to 220°C.
4. Sulphuric acid: It is used when the substance is melting
beyond 220°C.
• Fresh sulphuric acid can be heated safely up to 280°C.
Fundamentals of Preparative Organic Chemistry 123
• A crystal of potassium nitrate is added to prevent
charring and consequent darkening in colour at high
temperatures.
• Its use should be avoided in elementary classes.
5. Silicone: It is used for compounds of high melting point.
• It can be safely heated up to 300°C without decompo-
sition or ignition.
Q 638 What is the effect of impurity on the melting point? Whether
impurity increases, decreases or has no effect on melting
point.
Ans: Impurity decreases the melting point of the substance under
consideration. The presence of impurities will usually produce
a marked increase in the melting point range.
Q 639 Why is slow heating recommended to get accurate melting
point?
Ans: Slow heating is recommended when the temperature of the
bath is within 15°C of the expected melting point, i.e. 2°C
per minute because this will help in noting down the correct
temperature at which the collapse of the crystals commences
and the temperature at which the sample becomes liquid.
The difference between these two temperatures is about 0.5
to 1°C for the pure substance so we should heat the bath
slowly.
Q 640 What is melting point range of an organic compound?
Ans: Melting point range of an organic compound is the difference
between the temperature at which collapse of the crystals is
first observed and the temperature at which the sample
becomes completely liquid.
Q 641 Why should the thermometer be calibrated before melting
point determination?
Ans: The thermometer should be calibrated to ensure that the
temperatures recorded by the thermometer are correct. This
would also ensure that the melting point recorded is accurate.
Q 642 What is revealed by a sharp and definite melting point?
Ans: Sharp and definite melting point indicates:
• The substance is chemically pure, or
• The substance is a eutectic mixture of two or more
compounds.
124
Viva Voce in
Pharmaceutical Chemistry
Q 643 What is revealed by indefinite melting point?
Ans: Indefinite melting point indicates:
• The substance is impure.
• The substance is pure but it, on warmirig, undergoes slight
thermal decomposition before melting point is reached and
the decomposition products then act as impurities and
depress the melting point.
Q 644 Why do carbohydrates and amino acids not have fixed
melting points?
Ans: Carbohydrates and amino acids are decomposed due to heat
and thus melt over wider range of temperatures.
Q 645 Which tube is used for melting point determination?
Ans: Thiele’s tube is used to determine melting point. The
arrangement of Thiele’s tube helps in uniform heating.
BOILING POINT
Q 646 Define boiling point.
Ans: When the saturated vapour pressure of the liquid becomes
equal to the atomospheric pressure, liquid starts boiling.
Boiling point is the temperature at which liquid starts boiling.
Q 647 What is vapour pressure?
Ans: The pressure exerted by the vapour existing in the viscinity
of the liquid surface is called vapour pressure.
Q 648 What are the applications of boiling point determination?
Ans: A sharp boiling point at constant pressure may serve as a
criterion of purity for liquids. However, azeotropic mixtures
(i.e. constant boiling point mixtures) also have sharp boiling
point but they are rare.
For purposes of qualitative organic analysis constant boiling
point would be useful:
• To establish purity.
• For identification.
Q 649 Whether boiling point varies from place to place. Is boiling
point constant for a liquid?
Ans: The boiling point may remain sharp but it may vary in value
over a range of several degrees, owing to fluctuations in the
barometric pressure from place to place. The change in boiling
Fundamentals of Preparative Organic Chemistry 125
point for every 10 mm variation in mercury pressure (at 760
± 20 mm) may be assumed to be 0.5°C. The change in
atmospheric pressure is related to height of the place from
the sea level. Another way of change in the atmospheric
pressure would be due to application of vacuum (for reducing
pressure) or external application of pressure (as in pressure
cooker).
Q 650 What will be the effect on boiling point if atmospheric
pressure is reduced?
Ans: The boiling point would be reduced if the atmospheric
pressure is reduced.
Q 651 How is boiling point of liquid serve as criterion of purity?
Ans: At constant atmospheric pressure boiling point of a pure liquid
has a definite value and thus it can serve as criterion of purity.
Q 652 What will be the effect on boiling point if two liquids are
mixed?
Ans: If two liquids are mixed then the boiling point of the liquid
may
a. Remain constant (if azeotropic mixture is formed).
b. Rise steadily as the liquid boils.
c. Rise in a series of definite steps according to the nature
and quantity of the liquid added.
Q 653 What are the effects of impurities on the boiling point of a
liquid?
Ans: The boiling point of an impure liquid will depend upon the
physical nature of the impurities. If all the impurities are non-
volatile, the liquid will have sharp ‘boiling point and the solid
impurities will remain behind when the liquid has evaporated.
If the impurities themselves are volatile then the effect is as
given in previous question.
Q 654 How can one differentiate between a pure liquid and a
constant boiling mixture?
Ans: The difference between a pure liquid and a constant boiling
mixture can be detected by redistilling at a different pressure.
Under these conditions pure liquid will change its boiling
point but the composition of distillate remains unchanged. A
constant boiling mixture will change both boiling point and
composition of the distillate.
126
Viva Voce in
Pharmaceutical Chemistry
Q 655 What do you understand by the term synthesis?
Ans: Synthesis is a process in which a chemical compound with
specific structural formula, molecular weight and melting/
boiling point is produced via a chemical reaction.
Q 656 What are the processes involved in the synthesis?
Ans: The processes involved in the synthesis of an organic
compound are chemical reaction, filtration/separation of
crude product, activated charcoal treatment if required,
purification/recrystallization, melting/boiling point
determination.
Q 657 Name the basic organic reactions involved in synthesis of
organic compounds or drugs.
Ans: Some of the basic organic reactions involved in synthesis of
organic compounds are acetylation, bromination, nitration,
hydrolysis, esterification, benzoylation, diazotization,
oxidation, reduction, etc.
ACETYLATION
Q 658 What is acetylation?
Ans: Acetylation is a process in which the reactive hydrogen atom
from an amino group or hydroxyl group is replaced by acetyl
(–COCH
3
) group. The reagents used for acetylation are either
acetic anhydride or acetyl chloride along with pyridine. Acetic
anhydride is preferred over acetyl chloride as acetylation is
faster with acetic anhydride.
Q 659 Why is pyridine needed while acetylating, using acetyl
chloride?
Ans: During acetylation with acetyl chloride , hydrogen chloride
is formed. Pyridine absorbs the hydrogen chloride as fast
Preparation of
Organic
Compounds
6
126
Preparation of Organic Compounds 127
as it is formed, keeping forward balance of the reaction,
e.g.
Q 660 What is the possible disadvantage of using acetic anhydride?
Ans: The disadvantage of using acetic anhydride (especially excess
amount) is that with amines a diacetyl compound, i.e.
RN(COCH
3
)
2
may be formed. But during recrystallisation
using aqueous solvent, diacetyl derivative will rapidly
hydrolyse to the monoacetyl derivative.
Q 661 Give two products formed by acetylation process.
Ans: Acetanilide can be prepared from aniline by acetylation.
Acetyl salicylic acid can be formed from salicylic acid by
acetylation.
BROMINATION
Q 662 What is bromination?
Ans: Bromination is the process in which the replaceable hydrogen
atom is substituted by bromine atom. Bromine with glacial
acetic acid or bromine with iron or bromine water is used for
bromination.
If bromine water is added to unsaturated compounds,
addition of bromine occurs giving addition products.
Bromination of alkanes produce isomers but one isomer
predominates.
Q 663 How does bromine appear?
Ans: Bromine is highly reactive chemical and is available as gas
converted into liquid under pressure. It appears as brown
liquid and it gives out brown fumes.
128
Viva Voce in
Pharmaceutical Chemistry
Q 664 What precautions one must take while handling bromine?
Ans: Bromine must be handled with utmost care in fume cupboard.
The liquid produces vapours which are unpleasant. Bromine
produces painful burns and they should be treated imme-
diately with liberal quantity of glycerine. If the vapours are
inhaled, relief can be obtained by inhaling the vapours of
alcohol by holding handkerchief wetted by alcohol near the
nose.
Q 665 Give two products formed by bromination.
Ans:
• 2,4,6-tribromophenol can be formed from phenol using
bromine.
• p-bromoacetanilide can be prepared from acetanilide by
bromination using bromine and glacial acetic acid.
Q 666 Why bromine solution is added gradually?
Ans: Bromine solution is added gradually to ensure thorough
mixing and completion of reaction. The bromine vapours are
corrosive and to avoid sudden vapours, it is added slowly.
NITRATION
Q 667 What is nitration?
Ans: Nitration is the process in which the replaceable/reactive
hydrogen atom is replaced by a nitro group.
Preparation of Organic Compounds 129
Q 668 Which reagents are used for nitration?
Ans: Nitration can be carried out using concentrated nitric acid
alone but the yield is very small. High yield can be obtained
if nitric acid is first mixed with concentrated sulphuric acid.
Q 669 Why is sulphuric acid mixed with nitric acid during
nitration?
Ans: Nitration is caused by the nitronium ion (N
+
O
2
). The highly
reactive, electrophilic nitronium ion is produced in the mixture
of nitric and sulphuric acid as under.
Q 670 Why is nitration usually carried out at lower temperatures?
Ans: Nitration is carried out at comparatively low temperature
(about 50°C) because high temperatures may lead to poly-
nitration and oxidative breakdown of aromatic ring system.
Q 671 What is the effect of presence of electron withdrawing
groups (in aromatic hydrocarbon) on nitration?
Ans: The nitration of aromatic hydrocarbons containing electron
withdrawing groups (e.g. –NO
2
, –SO
3
H, –CHO, –CO.R,
–COOH, –COOR) requires forcing conditions, i.e. use of
fuming nitric acid and conc. sulphuric acid. The reason is, the
electron withdrawing group does not activate the nucleus
towards attack by electrophillic species, i.e. nitronium ion,
rather it deactivates the ring by reducing electron density.
Q 672 What is the effect of presence of electron releasing groups
(in aromatic hydrocarbon) on nitration.
Ans: The electron releasing groups (e.g. –R, –OH, –NH
2
,
–NHCOCH
3
) activate the aromatic nuclei towards attack by
the electrophillic species and thus milder conditions, i.e. nitric
acid and conc. sulphuric acid are sufficient for nitration.
Q 673 Which type of hydrocarbons easily undergo nitration?
Ans: Aromatic hydrocarbons when treated with nitrating mixture
(i.e. HNO
3
and H
2
SO
4
) easily undergo nitration.