How to determine melting point of organic compounds?
Answers
1. Size of the molecule
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.
2. Force of attraction between the molecules
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.
Melting point of some common organic compounds
Compound Melting Point (oC) Compound Melting Point (oC)
Phenol 42 α-Naphthylamine 50
α-Naphthol 96 Acetamide 82
β-Naphthol 123 Benzamide 128
Oxalic Acid 101 Urea 132
Benzoic Acid 122 Fructose 103
Cinnamic Acid 133 Glucose 146
p-Toludine 43 Sucrose 186
Naththalene 80 Acetanilide 114.3
Answer:
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.