ORGANIC cHEMISTRY - III

Experiment 7

Aim of the Experiment

Determination of melting and boiling points of given organic compounds.

Principle

The melting point is an important physical property of a compound. The melting point can be used to identify a substance and as an indication of its purity. The melting point of solid is defined as the temperature at which the solid exists in equilibrium with its liquid under an external pressure of one atmosphere.

A pure crystalline compound usually possesses a sharp melting point and it melts completely over a narrow range of temperature of not more that 0.5-1^oC. The presence of even small amount of impurities usually produces a depression in the freezing points and shows a marked increase in the width of the melting point range. The melting point range of > 5^oC indicates that the substance is impure. For a material whose identity is known, an estimate of degree of purity can be made by comparing its melting point with that of a pure sample.

Melting point is also used for the identification and characterisation of a compound. If the melting point of two pure samples shows a clear difference in melting points, it indicates that the two compounds must have different structural arrangements. or they must have different arrangements of atoms or configurations. If two materials have the same melting point, then they may (not necessarily) have the same structure.

The force of attraction between the molecules affects the melting point of a compound. Stronger intermolecular interactions result in higher melting points. Ionic compounds usually have high melting points because the electrostatic forces holding the ions (ion-ion interaction) are much stronger. In organic compounds the presence of polarity, or especially hydrogen bonding, generally leads to higher melting point.

The boiling point of organic compounds can give important information about their physical properties and structural characteristics. Boiling point helps identify and characterize a compound. A liquid boils when its vapour pressure is equal to the atmospheric pressure. Vapour pressure is determined by the kinetic energy of a molecule.

The boiling point of a liquid varies with the surrounding atmospheric pressure. A liquid at a higher pressure has a higher boiling point than when that liquid is at lower atmospheric pressure.

The normal boiling point of a compound is an indicator of the volatility of that compound. The higher the boiling point, the less volatile is the compound. Conversely, the lower the boiling point, the more highly volatile is the compound. At a given temperature, if a compound’s normal boiling point is lower, then that compound will generally exist as a gas at atmospheric pressure. If the boiling point of the compound is higher, it then exists as a liquid or a solid.

Materials required

Thermometer , Thiel's tube ,Burner

Procedure

DETERMINATION OF MELTING POINT

The melting point of an organic solid can be determined by introducing a tiny amount into a small capillary tube, attaching this to the stem of a thermometer centred in a heating bath, heating the bath slowly, and observing the temperatures at which melting begins and is complete.

Pure samples usually have sharp melting points, for example 149.5-150C or 189-190C; impure samples of the same compounds melt at lower temperatures and over a wider range, for example 145-148C or 186-189C.

DETERMINATION OF BOILING POINT

The organic compound whose boiling point is to be determined is taken in fusion tube .

Now repeat the above procedure .

Questions

1.Why do some compounds have high melting points and some compounds have low melting point ?

2.What does high melting point of the compound indicate ?

3.If a compound has high boiling point then what does it indicates about the inter molecular forces of attraction present in the compound ?

4.Suppose there are two compounds A and B . Compound A has dipole dipole interactions as inter molecular forces of attraction . Compound B has hydrogen bonding as inter molecular force of attraction. Which of the compound has high boiling point ?