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Q.

How to find melting point and boiling point among hydrocarbons

Asked by Waseem YalGar

Updated 6 months ago

Solution

There is no theoretical way of finding out the melting and boiling points of hydrocarbons. The values obtained are always experimental. But we can definitely compare the boiling and melting points of different hydrocarbons.

Boiling Points: The boiling points of straight chain alkanes increase fairly regularly with increase in their molecular mass. On the average, the boiling point generally increases by 20 – 30K for the addition of each carbon atom or a CH\(_2\) group to the chain. This regular increase in the boiling points of straight chain alkanes with increase in the carbon content is due to a corresponding increase in the molecular size and hence the surface area of the molecules. As a result, the magnitude of the van der Waals forces of attraction goes up and hence the boiling point increases accordingly. For example,

CH\(_3\)CH\(_2\)CH\(_2\)CH > CH\(_3\)CH\(_2\)CH\(_3\)        

          (B.P)                 (B.P)

Amongst Isomeric alkanes, the branched chain isomer has invariably the lower boiling point than the corresponding n – alkane. This is due to the reason that with branching the shape of the molecule tends to approach that of a sphere. As a result, the surface area of the branched isomer decreases. Due to lesser surface area of these molecules, the van der Waals force of attraction operating between their molecules become comparatively weaker and hence lesser amount of energy is required to overcome them. As a result, the boiling points of branched chain isomers are lower than those of the corresponding n-alkanes.   

Further, it has been observed that greater the branching, lower is the boiling point of the branched isomer. For example, the boiling point of 2, 2- dimethylpropane (neopentane, 282.5 K) with two branches is lower than those of 2-methylbutane (isopentane, 301 K) with one branch chain and n-pentane (309.1 K) with no branch chain.

Melting Points: Like boiling points, the melting points of alkanes also increase with increase in carbon content but the variation is not regular.

When the melting points of n-alkanes are plotted against the number of carbon atoms present in them, a sawtooth pattern is obtained.

This property is commonly known as alternation effect and can be explained as follows :

The carbon atoms are arranged in a zig-zag chain rather than in a straight chain as commonly written. Therefore, in n-alkanes, containing an even number of carbon atoms, the two terminal methyl groups lie on the opposite sides of the zig-zag chain. On the other hand, in case of n-alkanes having odd number of carbon atoms, the two terminal methyl groups will lie on the same side of the zig-zag chain as shown below:

Updated 6 months ago

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