Boiling Point of Alkyl Halides? Urgent?

Question

Boiling Point of Isomeris Alkyl Halides having same number of carbon atoms and same halogen atom decreases from Primary to Secondary and Secondary to Tertiary. Why?

But Melting Point has reverse action.It increases from Primary to Secondary and Secondary to tertiary.Why?

Give me reason in detail. It's my homework.

Thank you

Answer 1

Alkyl haldes are polar compounds. They have an electronegative moiety (halogen). They have an electro positive moiety (that alkane chain). At the terminal end of a chain the halogen is highly exposed, and can interact (repel) the halogens of near-by molecules. This addition repulsion increases the kinetic energy of the system, lowering the boiling point.

As secondary and tertiary substitutions are introduced (and also very long alkane chains) the carbon atoms of the molecule begin to shield their halogen. Wrapping around the halogen, preventing it from interacting with adjacent molecules. As result the repulsion from other molecules is minimized and the boiling point approaches that of the base alkane.

Now think about the freezing aspect of your question. The same phenomenon is at work. I’ll give you a hint. The more possible collisions you have in a system, the greater its efficiency to transfer heat.