this presentation contain the physicochemical properties of drug

1. Physicochemical Properties of Drug
Molecules
Ms. Jyoti Jawale
Assistant professor
Department of Pharmaceutics
Institute of Pharmaceutical Science and Research
(For Girls)

2. Physicochemical Properties of Drug
1. Refractive Index
2. Optical Rotation
3. Dielectric Constant
4. Dipole Moment
5. Dissociation Constant

3. 1. REFRACTIVE INDEX
The refractive index (n) of a substance is defined as
the ratio of the velocity of light in vacuum or air, to
that in the substance
n = Velocity of light in substance
Velocity of light in air
When a ray of light passes from air into a liquid, its
direction is changed. This change of direction is called
refraction. The refractive index of the liquid with
respect to air is given by Snelle’s Law. According to
it,
n = Sin i/Sin r
where i is the angle of incidence and r the angle of
refraction.

4. Determination of Refractive Index:

5. Factors Affecting on Refractive
Index
1. Temperature:
2. Wavelength of light: Refractive index decreases with
increasing wavelength.
3. Pressure: Refractive Index increases with pressure
due to accompyaing rise of density

6. Application of Refractive Index
1. Identify and confirm purity of particular substances.
2. It is used to calculate the focal power of the
lenses and dispersive power of prism.
3. Used to estimate the thermo physical properties of
hydrocarbon and petroleum mixture.
4. It is used to examination of organic compound (oils,
solvent etc), solution, food product, serum protein
concentration.
5. In veterinary medicine, it is used to measure the
total plasma protein in blood.

7. 2. DIPOLE MOMENT
 In a molecule such as HCl, the bonding electron pair is not
shared equally between the hydrogen atom and the chlorine
atom. The chlorine atom with its greater electronegativity,
pulls the electron pair closer to it. This gives a slight positive
charge (+q) to the hydrogen atom and a slight negative charge
(– q) to the chlorine atom.
 Such a molecule with a positive charge at one end and a negative
charge at the other end is referred to as an electric dipole or simply
dipole. The degree of polarity of a polar molecule is measured
by its dipole moment, μ

8.  The dipole moment of a polar molecule is given by the
product of the charge at one end and the distance between
the opposite charges. Thus,
μ = q × r
 The dipole moment (μ) is a vector quantity. It is
represented by an arrow with a crossed tail. The arrow
points to the negative charge and its length indicates the
magnitude of the dipole moment. Thus a molecule of HCl
may be represented as
The CGS unit for dipole moment is the Debye, symbolized by
D, named after the physical chemist Peter Debye (1884-1966).
In SI system, the charge is stated in Coulombs (C) and
distance in meters (m). Thus is expressed in Coulomb meters
(Cm).

9.  Electric condenser.
 The dipole moment of a substance can be experimentally
determined with the help of an electric condenser .
 The parallel plates of the condenser can be charged by
connecting them to a storage battery.
 When the condenser is charged, an electric field is set up
with field strength equal to the applied voltage (V) divided
by the distance (d) between the plates.
 Polar molecules are electric dipoles. The net charge of a
dipole is zero.
 When placed between the charged plates, it will neither
move toward the positive plate nor the negative plate.
 On the other hand, it will rotate and align with its negative
end toward the positive plate and positive end toward the
negative plate.
 Thus all the polar molecules align themselves in the electric
field.
 This orientation of dipoles affects the electric field between
the two plates as the field due to the dipoles is opposed to
that due to the charge on the plates.

10. •The plates are charged to a voltage, say V, prior to the
introduction of the polar substance.
•These are then disconnected from the battery. On introducing
the polar substance between the plates, the voltage will change
to a lower value, V'.
•Just how much the voltage changes depends on the nature of
the substance.
•The ratio ε = V/V' is a characteristic property of a substance
called the dielectric constant.

11. 3. OPTCAL ROTATION
 Optical Rotation is an angle
at which the plane
polarization is rotated when
polarized light passes
through a layer of one liquid.
 A beam of ordinary light
consists of electromagnetic
waves oscillating in many
planes. When passed through
a polarizer (e.g., a Polaroid
lens), only waves oscillating
in a single plane pass
through.
 The emerging beam of light
having oscillations in a single
plane is said to be plane
polarized.

12.  When plane-polarized light
is passed through certain
organic compounds, the
plane of polarized light is
rotated. A compound that
can rotate the plane of
polarized light is called
optically active. This
property of a compound is
called optical activity
 Optical Activity of
substances are described as
1. Dextrorotatory: A
compound that rotates the
plane-polarized light to the
right (clockwise),
dextrorotatory.
2. Levorotatory: A
compound which rotates
the plane-polarized light to
the left
(anticlockwise),levorotator
y.

13. Measurement of Optical Rotation
A Polarimeter consist of
1. Polarizer Light source
2. Polarizer
3. Analyzer
4. Sample tube
5. Detector

14. Procedure:
1. When light is passed through polarizer only light
oscillating in one plane will leave the polarizer.
2. The plane polarized light is introduced to tube
containing solution of the substance is to be
measured.
3. If the substance is optically inactive, the plane
polarized light will not change in orientation and
observer will read an angle of α = 0 .
4. If the compound in the polarimetry cell was optical
active , the plan of the light would be rotated on its
way through the tube either clockwise or
counterclockwise direction depending on the nature
of the compound.
5. For clockwise direction, the rotation in degree is
defined as positive (+) and called dextrorotatory.
6. For anticlock wise direction, the rotation in degree is
defined as negative (-) and called levorotatory.

15. Application of Optical Rotation
1. Identification and determination of optical
active compound.
2. It is used in sugar industry to determine quality
of juice from sugarcane and refine sucrose.
3. Many chemicals exhibits specific rotation as a
unique property that can be used to
distinguish it.
4. Polarimer used in remote sensing application.
5. In Pharmaceutical industry it is used to
measure the concentration and purity of
sample such as steroid, diuretics, analgesic,
vitamins, narcotics and antibiotics.

16. 4. DIELECTRIC CONSTANT

17. Application of Dielectric Constant
1. Used to manufactures capacitors which
storing energy in electric field between
the plates.
2. It is used insulation of wire, cable etc.

18. 5. DISSOCIATION CONSTANT
 Dissociation Constant: It is defined as tendency of
particular substance in solution is dissociated into ions.
 It is equal to the product respective ion concentration
divided by the concentration of nonionic molecules.
Dissociation constant is tendency of a substance Ax Ay to
reversible dissociate in a solution into smaller component.