Melting Point

[Hemichordate]

Advertisement - Members don't see this ad

With increased branching on an alkane, does melting point increase or decrease? In my Examkrackers, it says it increases, but in my Princeton review, it says it decreases. Which one is it?

 

[muhali3]

Branching decreases the intermolecular london dispersion forces. Since there are less interactions between branched alkanes, the boiling point is lower.

 

[MLT2MT2DO]

Branching decreases the intermolecular london dispersion forces. Since there are less interactions between branched alkanes, the boiling point is lower.

OP asked about MP...the more branching the higher the mp. EK is correct on this one, if you read TPRs description of their answer you get a feel that they meant "mp should increase".

 

[Hemichordate]

Is it become the more branching you have, the more the molecules are able to stack neatly together, and the more energy has to be put in to melt it -> higher melting point?

I found a thread with the exact same question, it's weird how many conflicting answers there are.

http://forums.studentdoctor.net/showthread.php?t=111078

 

[loveoforganic]

Melting point is not one of those things that just works that well with trends. It's hard to predict how a molecule will pack into its crystalline form.

For example, hexane melts higher than 3-methylpentane, but melts pretty much the same as 2,2-dimethylbutane.

Edit: I should emphasize - these were literally the first examples I checked. Not cherry picking an exception.

 

[phltz]

Melting points are complicated. Molecules that pack together well into solid crystals make better solids, and so have higher MPs. A straight chain can pack pretty well. Think of pasta in a box, all lined up. However, and nice, round molecule like neopentane can pack pretty well too. Irregular shapes tend to pack less well.

This is a messy, complicated business, and my feeling is that you shouldn't devote a lot of time and energy worrying about it.