Organic Chemistry: Branched vs. Unbranched

[BrazilianRider]

Alright, just want to make sure what I have in my head right now is right...

Unbranched molecules have higher IMFs (due to increased surface area) and thus have higher BPs, but have higher Dispersion Forces and thus have lower MPs.

Branched molecules have lower IMFs (due to decreased surface areas) and lower BPs, but higher MPs due to the ability to "clump" more.

Symmetrical Branched molecules have the highest MPs since they can "clump" the best.


Is this right? Just wanna get some confirmation before I lock it down in my head, haha.

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[1Overdrive]

Correct.

The way I remembered it is:

Branching IMP (as in imps, like, short things), decrease BP (bird poo (more branches on tree = less bird poo falls on you)). Whatever works. This helped me, so I'm not complaining. 😀

 

[biomolecmed]

Yes. If you have branching, that will lower your Boiling point because those molecule would not be able to pack tightly.

 

[Thanhn]

Currently studying for the DAT and here's how I thought of it...

Boiling point is a measure of when a liquid turns into a gas. Less branching means more uniform stacking so it would require a lot more energy to break the individual stacks into gas phase. When there's branching, there stacks aren't as uniform and therefore, weaker, resulting in a lower BP.

This is contrary to what occurs with melting point. Since melting point is the same as freezing point, I thought of it in this manner.

Freezing point is the temperature where a liquid turns into solid (and vice versa for melting point). In solid formation, a "lattice" like network forms from the individual atoms. With branching, the overall molecule becomes much more compact and spherical. More spherical characteristics make it easier for a solid to form meaning the molecule is less inclined to go from solid --> liquid. This means that melting point increases.

Therefore, branching leads to a decrease in boiling point and an increase in melting point (freezing) point.