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Melting Point of Organic Compound [ Theory and Procedure ]

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 (C4H10O), 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 

CompoundMelting Point (oC)CompoundMelting Point (oC)
Phenol42α-Naphthylamine50
α-Naphthol96Acetamide82
β-Naphthol123Benzamide128
Oxalic Acid101Urea132
Benzoic Acid122Fructose103
Cinnamic Acid133Glucose146
p-Toludine43Sucrose186
Naththalene80Acetanilide114.3

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. 

Materials Required

The Procedure

Real lab procedure

  • First powder the crystalline substance.
  • Take a capillary tube and seal one end by heating it.
  • Fill the capillary tube with the substance. To fill the tube, make a heap of the powdered substance on the porous plate. Push one end of the capillary tube into the heap. Some of the substance will enter the capillary tube.
  • Now tap the sealed end of the capillary tube on the porous plate gently. Fill the capillary tube upto 2-3 mm.
  • Attach the capillary tube to a thermometer using a thread.
  • Take liquid paraffin in a beaker and place it over a piece of wire gauze placed over a tripod stand.
  • Clamp the thermometer carrying the test tube to an iron stand and immerse them in the bath of liquid paraffin. The surface tension of the bath liquid is sufficient to hold the capillary tube in position.
  • Heat the beaker slowly while constantly stirring the contents using a stirrer to maintain a uniform temperature throughout.
  • When the temperature is within 15o of the melting point of the pure substance, the flame is reduced. Then the temperature rises slowly.
  • Note the temperature (t1) when the substance starts melting.
  • Again note the temperature (t2) when the substance has completely melted.
  • The average of the two readings gives the correct melting point of the substance.

Observations

Record your observations in the table given below.
Note the temperature when the substance;Melting point of the given substance (t1 + t2 / 2 (oC)
Starts melting t1(0C)Has completely melted t2 (0C)
   

 Inference

Melting point of the given substance = ... oC

Precautions

  • Use dry and powdered sample for the determination of melting point.
  • Keep the lower end of the capillary tube and the thermometer at the same level.
  • Packing of the powder should be uniform without any large air gaps in between the solid particles. 
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