BP and MP trends

[phattestlewt]

I just want to verify this reasoning with ya'll.

AN increase in BP/BP is due to an increase in the IMF.

However, BP/MP can also be increased by increased chain length, and branching.

Cyclic compounds generally have higher BP/MP due to their higher density. They pack well and as a result need more E to pry apart when boiling or melting.

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[1TB4RKSB4CK]

Branching decreases BP and MP, provided that there are the same amount of carbons in the backbone. Also, I think it also depends on ring strain for the cyclic compounds.

 

[BTheM]

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[phattestlewt]

But inreasing the number of carbons in a straight chain alkane, increases bp?

 

[sps27]

But inreasing the number of carbons in a straight chain alkane, increases bp?

Yes I would say so.

Also, BP does decrease with branching but it is not always the case with MP. Here is a table from TBR. It is clear that symmetry of molecule plays a role in MP. If branching leads to more symmetry, the melting point increases.
BP MP
Methane -162 -183
Ethane -89 -183
Propane -42 -187
Butane 0 -138
Pentane 36 -130
2methylpentane 60 -154
3methylpentane 63 -118
2,2 dimethylbutane 50 -98
2,3 dimethylbutane 58 -129

 

[Blueprint MCAT Tutor]

They pack well and as a result need more E to pry apart when boiling or melting.

Focus on packing when thinking about MP. The molecules pack down into a solid.

When it's already a liquid, the molecules are all slidy and moving around past each other. They're not "packing" together so that's a wrong place for your intuition to go for BP.