Our objective is to determine the melting point of an organic compound.
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 0.5-1oC. 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 > 5oC 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.
Consider the isomers n-butanol and t-butanol. Both have the same molecular formula (C4 H10O), but differ in their 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.
Consider the following examples.
The only force of attraction between butane molecules is weak Van der Waals force of attraction, so it has very low melting point. But in the case of methyl propionate, because of the presence of polar C – O group, the molecules are held together by dipole-dipole interaction. Therefore, its melting point is greater than that of butane. In the case of butyric acid, the molecules are held together by hydrogen bonding, so it has a higher melting point. The melting point of sodium butanoate is higher than that of butyric acid because the attractive force in sodium butanoate is strong ionic interation.
|Compound||Melting Point (oC)||Compound||Melting Point (oC)|