Melting Point vs. Boiling Point, quick question

[dentista123]

How does branching influence the BP and MP of organic molecules?

I have two different answers, one from Kaplan and one from Destroyer.

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[bbwreckerz04]

Yea I'll go with Destroyer, Kaplan has lots of mistakes.

 

[dentista123]

so it's the more branched=more tightly compact= higher MP and lower BP

Can someone please check this? Because it does not agree with Kaplan?

 

[bbwreckerz04]

so it's the more branched=more tightly compact= higher MP and lower BP

Can someone please check this? Because it does not agree with Kaplan?

See this post

http://forums.studentdoctor.net/showthread.php?t=542327

 

[dentista123]

perfect! thank you!

 

[UCSD6]

Yo, just to help you out
melting point is based on crystal lattice of compound, which is based on intermolecular forces (between molecules, not within) like van der wall foces, etc. So, if you increase branching, this doesn't allow molecules to pack together as tightly so it has a weaker crystal lattice because less surface area which results in smaller london attractions. This also results in lower boiling points.

so, more branched = less tightly packed = less london forces = weaker crystal lattice = lower melting and boiling point

Of course there maybe some exceptions!

 

[nartnad]

Increasing carbon #s increases MP and BP. Thats easy.

Branching effect on BP: Branching reduces the surface area, so there are less london forces, which lowers the BP.

Branching effect on MP: unbranched is has the highest MP because the molecules can stack and be stable. But as soon as you have branching, the MP lowers a lot. The amount the MP lowers depends on the amount/type of branching:

hexane mp=-95 bp=69
2-methylpentane mp=-154 bp=60
2,4-dimethylbutane mp=-135 bp=58
2,2-dimethylbutane mp=-98 bp=50

2,2-dimethylbutane is more compact and can create a crystal lattice easier than 2-methylpentane, so its MP is closer to the hexane value, but still not quite as high.

 

[dentista123]

Increasing carbon #s increases MP and BP. Thats easy.

Branching effect on BP: Branching reduces the surface area, so there are less london forces, which lowers the BP.

Branching effect on MP: unbranched is has the highest MP because the molecules can stack and be stable. But as soon as you have branching, the MP lowers a lot. The amount the MP lowers depends on the amount/type of branching:

hexane mp=-95 bp=69
2-methylpentane mp=-154 bp=60
2,4-dimethylbutane mp=-135 bp=58
2,2-dimethylbutane mp=-98 bp=50

2,2-dimethylbutane is more compact and can create a crystal lattice easier than 2-methylpentane, so its MP is closer to the hexane value, but still not quite as high.

Thank you, but... 2,2-dimethylpropane will have a higher melting point than n-pentane because although it has branching, it possess symmetry. Branching without symmetry results in a lower melting point.


2,2-dimethylpropane melting point:
−18 °C (255 K)



pentane meting point:−129.8 °C (143 K)


Thank you again for your help!!