MELTING POINT OF ORGANIC COMPOUNDS

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NAME : THAMARAI A/P RAJENDRAN

ID NUMBER : 09ALB07214

LABORATORY 1A : ATOMIC STRUCTURE, BONDING    AND PERIODICITY

COURSE : BIOTECHNOLOGY (YEAR 1 SEM 1)

EXPERIMENT 7:

MELTING POINT OF ORGANIC COMPOUNDS.

TITLE:

Melting Point of Organic Compounds.

OBJECTIVES OF EXPERIMENT:

The objective of this experiment is to identify unknown organic compound by melting point depression method. Practice use of melting point apparatus by measuring melting point of some pure organic compound.

THEORY AND BACKGROUND:

The melting point of organic solid can be determined by introducing a tiny amount of the substance into a small capillary tube, attaching this to the stem of a thermometer centerd in heating bath, heating the bath slowly, and observing the temperatures at which melting begins and is complete. Pure samples usually have sharp melting points, for example 149.5°C-150°C or 189°C-190°C; impure samples of the same compounds melt at lower temperatures and over a wide range, for example; 145°C-148°C or 187°C-189°C. The contaminant that depresses the melting point and extends the melting range may be an indefinitely characterized resinous material or it may be a trace of a second chemical entity of melting point either higher or lower than that of the major component. Under equilibrium conditions (no super cooling) the temperature at which a pure solid melts is identical with that at which the molten substance solidifies or freezes. Just as salt lowers the freezing point of water, so one compound (A) depresses the melting point of another (B) with which it is mixed. If pure A melts at 150°C-150°C and pure B at 120°C-121°C, mixture of A with small amounts of B will melt unsharply at temperatures below 150°C and mixtures of B containing A will melt below 120°C. both the temperature and sharpness of melting are useful criteria of purity.

        A third substance C might have exactly the same melting point as A, namely 150°C-151°C, but if mixed melting point determination is made, that is if A and C are mixed and the melting point of the mixture is observed, the one substance will be found to depress the melting point of the other. Depression of melting point on non-depression is invaluable in the identification of unknowns.

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        An unknown D found to melt at 150°C-151°C can be suspected of being identical with one or the other substance A or C; observation that the mixture A and D shows a melting point depression would exclude identity with A, and failure of C to depression the melting point of D would prove C and D identical. If a substance melts at 150°C or higher, the thread of mercury at the upper part of the thermometer is cooler than that in the bulb and hence the temperature recorded is a little lower than the actual bath temperature. The extent of ...

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