Melting Point of an Organic Compound.

Our Objective

Our objective is to determine the melting point of an organic compound.

The Theory

What is the importance of knowing the melting point of organic compounds?

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. 

What are the factors that affect the melting point? 

 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

Learning outcomes

  •  Students understand the term ‘melting point’ through this experiment.
  •  Students will able to do the experiment in the real lab quicker and more accurately after they understand the steps.
  •  Students understand that the melting point of a solid is a constant value.