http://forums.studentdoctor.net/archive/index.php/t-221544.html
i found this:
Short answer: Branching will decrease mp and bp. Condensing and freezing happen for alkanes because of dispersion forces caused by temporary, induced dipoles, which are the only intermolecular forces holding nonpolar molecules like alkanes together in the liquid and solid states. Branched alkanes have smaller dispersion forces compared to straight-chain alkanes of the same MW, so they will be harder to liquify or freeze.
Long answer: Let's imagine that we are cooling down a gaseous alkane like hexane that is straight-chained, versus another of the same MW that is branched, like 2,3-dimethylbutane. Remember that gases don't have any intermolecular interactions, at least not if they're ideal. As we lower the temperature, the molecules stop moving as much, and they begin to have intermolecular interactions that are due to induced, temporary dipoles (
http://forums.studentdoctor.net/showpost.php?p=2872471&postcount=10) called London dispersion forces.
Ok, so now we need to consider what kinds of molecules will have the strongest induced dipoles. The strength of the induced dipoles is directly proportional to the surface areas of the molecules that are coming into contact. This is intuitive, because if contact can be made over a greater area, there is a greater chance that electrons will distribute unequally at some point over that surface, causing the temporary dipole and inducing dipoles in the neighboring molecule. You may know that the shape with the smallest surface area-to-volume ratio is a sphere. So molecules that are more spherical (ie, highly branched) do not have very much surface area relative to molecules that are long and extended (straight chains). That is why branched molecules have weaker dispersion forces versus straight-chains. Since they have weaker dispersion forces, the branched molecules will tend to want to stay in the gaseous phase longer, and you'll have to cool them further to force them to condense into a liquid. This means that branched compounds have a lower bp (condense at a lower temperature) versus straight chains.
As we continue to cool, the molecules continue to move closer and closer together, and their interactions continue to increase. Eventually, we reach a point where they begin to crystallize, or at least form an amorphous solid. So we need to consider how well the molecules pack together at this point. Branched compounds are like little spheres, or like porcupines. It's hard to get them to pack well, and this means that you will have to cool them to a lower temperature to freeze them (lower mp) compared with straight chains, which can stack up nicely, more like a cord of firewood. So the mp of a branched compound will be lower than that of a straight chain, assuming that they have the same MW.
Caveat: This information is probably beyond what you'd be expected to know about isomer properties for the MCAT, but I wanted to clarify this issue since a few people have asked about it. Please note that the effect of chain branching on alkane melting points is much harder to predict compared to its effect on alkane boiling points. In general, branching lowers van der Waals overlap between the two molecules, and therefore also lowers melting points. For example, n-pentane (mp = -130 C) has a higher mp than 2-methylbutane (mp = -160 C). However, some highly symmetrical branched alkane isomers will have abnormally high melting points because of their excellent packing properties. For instance, 2,2-dimethylpropane (mp = -17 C) has a much higher mp than either n-pentane or 2-methylbutane for this reason.
