Why does branching increase melting point of alkanes

Initially, melting points decrease with increasing branching but then become larger again as the symmetry increases.

Clicking on the structures will open up a ball-and-stick model of the molecule. More Information. Alkanes: Branched Alkanes Branching of the chain in alkanes dramatically increases the diversity of conceivable structures. Mouse Zoom.

When compounds freeze, the process is a lot like stacking bricks. The more symmetrical the molecules are, the easier it will be and the fewer spaces there will be between the molecules.

Hence, when you compare hexane to its structural isomer, 2-methylpentane, hexane has a much higher melting point due to the regular arrangement of its structure. But it gets more complicated. Treat the n-hydrocarbon as a special case, and ignore it for the time being. Starting with the simplest branched compound, as you increase branching, you will increase the melting point, but decrease the boiling point.

This explains the melting point phenomenon. Boiling point is related to the forces between molecules, which in the case of hydrocarbons is Van Der Waals interactions.

As we decrease surface area, we are going to decrease intermolecular Van Der Waals interaction and therefore decrease boiling point. If this all seems rather ambiguous, contradictory, and imprecise, well, you have a point. As a final example, I give you 2,2,3,3,-tetramethylbutane.

Hi I have one question , between a lineal saturated hydrocarbon and unsaturated hydrocarbon double bond or triple bond which one has higher boiling point? Well, the first answer must be strictly based on molecular weights. I am an ohio state student getting ready to take the MCAT. Thank you so much. Because I expected alkene to have higher melting point due to the double bond strength.

Thank you! Jen — the double bond strength has NOTHING to do with boiling or melting point because the covalent bonds holding the molecule together internally do not break during a phase change. It is only the intermolecular attractions that are broken — a common mistake many students make. The intermolecular van der Waals forces of attraction are stronger and have higher melting poins. But the point decreases with the increase in branching for the same number of C atom due to more compact molecules with less surface area and, thus, having weaker intermolecular van dar Waals forces of attraction.

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The general explanation given is that the chains are further apart, so the intermolecular forces are less, and as they are further apart, they occupy more volume, and the density is lesser. But the length of each chain and thus its mass also increases, so how do we know that the density overall decreases? As a side note, whenever we talk about melting points, structural symmetry comes into play.

More symmetrical molecules pack into the crystal lattice better thus allowing attractive tendencies of molecules to dominate facilitated due to a closer approach between molecules 1. As a rule of thumb, as packing efficiency increases, melting point increases. A similar reasoning is given whenever we're to compare the melting points of trans and cisbutene.

For instance, let's say you're planning to go to a nice long vacation ask me, I'd say to Hawaii. But you've decided to carry just one suitcase for the trip. Being a chemist, your love for chemicals is intense, and you've decided that you'll either carry butane or iso-butane for the trip. But which one? More chemicals means more fun, you say. Thus, a molecule with higher packing efficiency would be a clear winner in this scenario.

Butane wins this game because it has a symetrical structure and you would be able to "stack" those molecules one over the other which accounts for its higher packing efficiency.