Melting Point of Organic Compounds

Melting Point of Organic Compounds

1. The temperature at which a solid becomes a liquid at standard atmospheric pressure.

2. Sharpness of the melting point is often used to determine whether a substance is pure or impure. A pure organic compound usually melts over a range of 1℃ or less. An impure organic compound usually has a broader and lower melting point.

3. A very small quantity (about 20 crystals) of finely pulverized sample should be loaded onto the cover slip.

4. The temperature should be 20℃ below the expected melting point.

5. If there is a rapid increase in the temperature at the melting point, the result will have an erroneous high reading and thermal equilibrium will not take place between the sample and block with the thermometer.

6. Grind and mix the unknown and the known compound, which melts closest to the unknown, separately. On a clean watch glass add approximately equal amount of each substance. Obtain a small (a spatula tip is enough), equal amount of both the unknown and the known compound on a watch glass. Using the spatula and watch glass grind the sample together so that it is thoroughly mixed. On a melt point cover slip, transfer about 20 crystals of the mixture and cover with another cover slip.

7. To determine the melting point, first heat the unknown at a rate of about 10℃ per minute and this will give a rough idea of which of the 3 compound is the unknown. This is called the preliminary or fast melting point and it will also help understand how the instrument heats based on the position of the dial. Once the fast melting point is determined, a mixed melting point can be performed with one of the 3 known compounds. To determine a mixed melting point, place the cover slip on the circular depression on the stage in the melting point apparatus. A sharp melting point can be observed only if the substance is a 1:1 ratio with itself. If the melting point range of the mixture differs substantially from that of the...