What are the rules again for ranking BP/MP?

[Pigglyjuff]

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Considering there are molecular weight, branching, intermolecular interactions, etc.

 

[Pigglyjuff]

Oh, got it:

Increasing branching lowers MP and BP
Increasing molecular weight raises MP and BP.

 

[LoLCareerGoals]

Ah no.
Increased branching leads to better packing in a solid -> greater MP.
Increased branching leads to poorer LDF forces due to reduced surface area -> lower BP.
Increased molecular weight increases LDF -> higher BP.
Increased molecular weight can have all kinds of effect on packing -> who know what it will do to MP.

 

[sciencebooks]

Ah no.
Increased branching leads to better packing in a solid -> greater MP.
Increased branching leads to poorer LDF forces due to reduced surface area -> lower BP.
Increased molecular weight increases LDF -> higher BP.
Increased molecular weight can have all kinds of effect on packing -> who know what it will do to MP.

Just because I've seen a bunch of confusion around on this, here's a good/helpful explanation: http://forums.studentdoctor.net/archive/index.php/t-111078.html The most relevant bit is copied in below.

This trend (speaking of increased branching = increased melting point) works within branched alkanes, but not if you include the straight chain (or normal) alkane..

i.e.: MP C BP C
hexane -95 69
2-methylpentane -154 60
2,4-dimethylbutane -135 58
2,2-dimethylbutane -98 50

So the trend seems to only work when you compare branched with other branched. The more compact the structure, the higher the mp, and the lower the BP reflecting packing/intermolecular forces. The mp difference fails when you include hexane in the mix."

So there is the answer. Straight chain alkanes have the highest melting point, but once there is some branching, melting point will increase with any further branching. So I guess in a sense both sources are right, but neither explains the phenomenon fully.

 

[LoLCareerGoals]

Very useful info! Thanks!