[Iloveorganic]

Can some one please confirm the affect of branching on melting point and boiling point... I understand that branching decreases boiling point because of van der waals. But I have been reading different sources particularly on the affect of branching and melting point. I have gotten mixed answers on whether branching increases or decreases melting point.

Does melting point increase with branching because they are closer together so more compact so easier to form solid thus harder to melt.. so --> Inc Melting Point

OR

Does melting point decrese with branching because they are closer together so less surface area and thus easier to melt..so --> Dec Melting Point.

I need some clarification. Please!!

[fledarmus]

The n-alkanes have by far the highest melting points of the C_nH_2n+2 isomers. It is difficult to make a general statement once you go from no branching at all to branched chains over what more extensive branching will do. For example, n-octane has a melting point of -57C, 2-methylheptane has a melting point of -109C, and 2,2,4-trimethylpentane has a melting point of -107C. The n-octane is much higher, but from one branch to three branches is almost the same.

[Iloveorganic]

So since the n-octane (straight chain) has a higher melting point than the branched (alkanes- 2-methylheptane). Can I conclude that branching decreases melting point? When looking at it from straight chain to branched ?
 

[fledarmus]

Yes - the addition of a single branch sharply decreases the melting point. Any other change in the amount of branching is harder to generalize.