Melting Point Boiling Point - Branching

[Addallat]

Two sources state different things:

Chad's videos states:

Branching increases boiling point but decreases melting point


TPR page 73 states:

Branching decreases both boiling point point and melting point

who's right?

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[TheFuture_22]

Chad

 

[bluedevil61212]

I think TPR is right. Branching would decrease the amount of intermolecular forces and as a result decrease the boiling point. It would also make the compound pack less efficiently and as a result decrease the melting point.

 

[ClemsonFirst]

I have to say TPR, too.

 

[Temperature101]

I know branching decreases BP but I am not sure about MP.

 

[TheFuture_22]

Chad

I stand corrected. I think I learned it that way from EK, not sure though. It seems like MP can go either way though from what I have read.

 

[BerkReviewTeach]

From chapter 4 of the BR book:

. . . . . . . . . . . .n-hexane . . . . .. . . . . b.p. 69^oC . . . m.p. -95^oC
. . . . . . . . . . . .2-methylpentane . . . . b.p. 60^oC . . . m.p. -154^oC
. . . . . . . . . . . .3-methylpentane . . . . b.p. 63^oC . . . m.p. -118^oC
. . . . . . . . . . . .2,2-dimethylbutane . .. b.p. 50^oC . . . m.p. -98^oC

From this trend, it looks like Temperature has the best answer. This is because branching has two impacts on the solid: (1) it disrupts packing and (2) it increases molceular density. These two factors work in opposite ways on the melting point, as seen by the fact that 2,2-dimethylbutane and n-hexane have almost the same mp.

 

[AFLATPEG]

From chapter 4 of the BR book:

. . . . . . . . . . . .n-hexane . . . . .. . . . . b.p. 69^oC . . . m.p. -95^oC
. . . . . . . . . . . .2-methylpentane . . . . b.p. 60^oC . . . m.p. -154^oC
. . . . . . . . . . . .3-methylpentane . . . . b.p. 63^oC . . . m.p. -118^oC
. . . . . . . . . . . .2,2-dimethylbutane . .. b.p. 50^oC . . . m.p. -98^oC

From this trend, it looks like Temperature has the best answer. This is because branching has two impacts on the solid: (1) it disrupts packing and (2) it increases molceular density. These two factors work in opposite ways on the melting point, as seen by the fact that 2,2-dimethylbutane and n-hexane have almost the same mp.

So, increase in branching decreases the packing, thus decreasing BOILING POINT.

no "one-way" trend for MELTING POINT? as to which one ways more into a inc/dec direction?

 

[ClemsonFirst]

From what I remember from other sources, melting point is more ambiguous than BP. Just know the general trend and you should be fine.

 

[BerkReviewTeach]

So, increase in branching decreases the packing, thus decreasing BOILING POINT.

no "one-way" trend for MELTING POINT? as to which one ways more into a inc/dec direction?

Exactly. With bp, those molecules have the same mass so the energy needed to lift the molecule out of solution is equal. The difference comes into play with how well the molecules intertwine in solution and thus differ in terms of intermolecular forces. As R-groups branch, the chain length is reduced and the molecule is not as flexible, causing less contact between neighboring molecules. I don't want to use the word packing, as that terms generally relates to solids (and thus melting), but the idea you describe is right on. Less molecule-molecule interaction = lower bp.

As for melting point, a little branching (one methyl from a long chain) will reduce the melting point by disrupting the packing, but with more and more branching (especially symetric branching) the breakdown in packing is less significant and the center of mass for nearby molecules get closer together.

My gut feeling is that the MCAT writers would never ask anyone to describe the trend from recall but instead would give a series of mps and bps for some similar compounds (structural isomers perhaps) and ask for an explanation of the trend that is observed.

 

[Domomuffin]

Aren't we supposed to pay attention to symmetry in the branching when discussing MP? If the branching is symmetric, the molecules can be packed more easily and hence - will be harder to melt. But if the molecules are not symmetrical, they wouldnt pack evenly into that nice crystal-like structure and would be easier to melt.
I'm not sure about this, but let me know what u think please

 

[monkeyvokes]

Aren't we supposed to pay attention to symmetry in the branching when discussing MP? If the branching is symmetric, the molecules can be packed more easily and hence - will be harder to melt. But if the molecules are not symmetrical, they wouldnt pack evenly into that nice crystal-like structure and would be easier to melt.
I'm not sure about this, but let me know what u think please

Yeah I agree the type of branching is important