CBSE Class 11-science Answered
Their melting and boiling points of alkanes are relatively low. The higher the molecular weight (the greater the number of carbons), the higher the boiling point.
The interaction of the electrons with each other, due to what are called van der Waals forces, causes this increase in boiling point.
If two alkanes have the same formula weight (differing only in connectivity of atoms due to branching), the one that has more extensive branching has the lower boling point because the branching prevents the molecules from being packed together as tightly as occurs in a straight chain. This results in weaker intermolecular forces and therefore a lower boiling point.
Branching often has the reverse effect on the melting point. Usually the melting point increases because of an increase in symmetry, which aids in the formation of crystal lattices.
For the same number of carbon atoms, increased branching leads to more compact molecule that can pack more closely in a solid lattice.
The intermolecular attractive forces are stronger and melting point higher.
Alkenes and alkynes:
The melting points and boiling points of alkenes and alkynes increase with an increase in molecular weight. Alkenes and alkynes generally have physical properties similar to those of alkanes.
However, the presence of the double bond in alkenes creates cis/trans isomerisation, as previously discussed. This can affect the net polarity of the molecule and therefore affect intermolecular interactions.
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