IIT JEE , AIEEE Forum - Chemistry , Organic Chemistry

prakhar_galaxy

On what parameters does melting point of alkanes depend?

&.why there is a more increase in melting point when carbon chain changes from odd no. to even no ?

gopi_mith

Melting point of alkane does depend on size, surface area, branching etc. but the most inp.thing which decides its pattern is the OSCILLATION EFFECT.
IN odd numbered alkanes, when they are grouped in a crystal lattice, the end carbons are attached to each other. so the fitness of the alkane in the crystal lattice is very very poor.
But in even numbered alkanes the end carbons ar anti co-planarly attached in space in the crystal lattice, leading to its better fitness and high melting point.
Now let us see the pattern of increase in melting point as the size of alkane increases:
methane is odd numbered and also has small size. so it has the lowest m.p in alkanes. as the next homologue ethane is even numbered, m.p. increases drastically due to both increase in size and the above mentioned oscillation effect.
but in propane which is an odd numbered alkane , m.p. does increase (due to increase in the size)but it does not increase drastically. it incraeses only by a small amount.
but m.p. again increases drastically from propane to butane.
Hope u understans it.

deepak_agarwal

melting point depends on size and most importanatly on packing in the crystal lattice.

increase in melting point is relatively more in moving from an alkane having odd number of carbon atoms to the higher alkane while it is relatively less in moving from an alkane with even number of c-atoms to higher alkane.

reason:

internmolecular forces in a crysatal depend not only on the size of the molecule but also on how well the molecules fit into the crystal latticethe molecules which fit into crystal lattice more easily due to symmetrical structures usually have high lattice energy and therefore high melting points.

c-atoms of carbon chain make a zigzag pattern (c chains are not straight)

in even numbered: terminal c atoms are on same side

in odd numbered : terminal c atoms are on opposite sides

so,even is symmetrical than odd numbered.

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