PHARMACEUTICAL ORGANIC CHEMISTRY LABORATORY # 2
Melting Point
Objective:
The purpose of this experiment is to determine the melting points of organic
compounds and check the purity of compound.
Theory:
The melting point (m.p) 0C of a compound is the temperature at which it
changes from a solid to a liquid. Since this requires that the intermolecular forces
that hold the solid together have to be overcome. The melting point is a physical
property (melting point, boiling point, density, solubilities.) often used to identify
compounds . Usually, chemists can only obtain a melting range of a 2 - 3 oC
accuracy. A sharp melting point is generally accepted to have a range of 1-2 oC . A
mixture of very small amounts of miscible impurities will produce a depression of
the melting point and an increase in the melting point range.
Melting point range:
The interval between the temperature at which a solid sample just begins to turn to
liquid and the temperature at which the entire sample becomes liquid. or range of
temperatures in which the first crystal starts to melt until the temperature at which
the last crystal just disappears.
What are the factors that affect the melting point?
1- Molecular weight
Melting points are higher for higher molecular weight compounds. The reason
the melting point increases with the weight is that it takes more energy to
separate larger molecules from a crystalline structure than it takes to
separate smaller ones.
2- The nature of the organic compounds
Ionic compounds, often form crystal lattices in which each ion is surrounded
by ions of the opposite charge, because of the strength of this electrostatic
attraction, Breaking up an ionic crystal lattice requires considerable energy as
a result, ionic compounds have high melting points, While the Covalent
compounds consist of molecules rather than ions. Since the attraction
between molecules is weaker than is the attraction between ions, covalent
compounds usually have a lower melting point than ionic compounds.
Moreover, some covalent compounds have polar molecules in which one end
is more electronegative than the other, such polar compounds have a higher
melting point than nonpolar molecules .
3-Impurities
Impurities decrease the melting point. ice-salt mixtures. contaminants
normally lower the melting point and broaden the melting point range Pure
samples usually have sharp melting points, for example( 149.5-150) 0C,
impure samples of the same compounds melt at lower temperatures and over
a wider range, for example 145-1480C .
4-Shape of Molecules
Molecular shape influences the melting point of a substance. If the shape of
the molecules allows them to be packed together in a compact fashion, For
example, symmetrical molecules give compound ( 4- n –propyl benzoic acid )
a higher melting point than that of ( 4- iso –propyl benzoic acid ) , the
molecules of which do not pack well,
Types of measuring apparatus of melting point:
Figure 1: Melting point apparatus (old fashion model)
Figure 2: Melting point digital apparatus (New model)
Properties of oil bath
1) It has a high Boiling point 3000C rather than water (1000C)
2) It does not emit toxic gases
3) It is transparence and you can see the M.P tube through it. It stays pretty
clear on heating.
4) It has high density rather than water
5) It does not decompose
apparatus / Materials:
oil bath(beaker contain oil )
capillary tubes
Thermometer
solid organic compounds
Bunsen burner
Rubber bands(ring)
Procedure:
1- Take a capillary tube and sealed an open end of capillary tube by inserting
the tip into a Bunsen flame near the base of the flame and turning the
tube in your fingers using benzen burner
2- Place a small amount of the compound (Benzoic acid) in a clean surface.
Push the open end of the capillary tube into the compound as shown in
Fig: 3a .
3- Move the powder to the closed end of the capillary tube by tapping it on
the tables. Repeat until the compound occupies 1-2 mm of the capillary
tube end Fig:3a.
4- Attach the capillary tube to a thermometer using a thread. And align the
bulb of the thermometer with the closed end of the capillary tube as
illustrated in Fig: 3b.
5- put an oil in a beaker and place it over a piece of wire gauze placed over a
tripod stand.
6- Heat the beaker slowly while constantly stirring the contents using a
stirrer to insure a uniform temperature throughout. Fig: 3c .
7- Note the temperature at which the compound melts and Record the
temperature at which the solid in the capillary tube melts.
The amount of samples that you use and the heating rate through the
melting range are critical. If you use too much sample or heat too rapidly you will
not get accurate results. Be patient and do it right the first time. Once the sample in
the capillary melts, don't use it again. If you have to repeat a measurement for any
reason, you must start with a fresh sample in a new capillary.
PHARMACEUTICAL ORGANIC CHEMISTRY LABORATORY # 2
Name: ______________________________________________ Date: ______________
______________________________________________
Year and Section: __________________________
Observation: (Record the melting point of the substance)
Questions:
1. Define the "melting point" of a substance.
2. What is the purpose of determining melting points?
3. Why is this method not used for finding the melting points of inorganic
compounds?
4. Why could the rate of heating influence the melting point?
