Effects of branching on BP/MP

[Maygyver]

I could be mistaken, but I thought Chad's video said that increased branching caused MP to increase and BP to decrease. However, Kaplan says increased branching causes decreased BP and MP. Which is correct?

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[UndergradGuy7]

I believe if you are branched then the packing is more compact. This makes the melting point increase as the molecules are closed and tighter. But since this also reduces the surface area, the boiling point is lowered.

But also remember the melting point increases with the number of carbons and increasing molecular weight.

http://books.google.com/books?id=1B6ijcTkD5EC&lpg=PA156&ots=GJ2h5sPwP0&dq=organic%20chemistry%20increased%20branching%20melting%20point&pg=PA156#v=snippet&q=branching%20melting&f=false

 

[Maygyver]

I believe if you are branched then the packing is spread out apart of the molecules. They are not as close and this makes them more "liquid" so the melting point is lowered compared to a straight chain.

But also remember the melting point increases with the number of carbons and increasing molecular weight.

Alright, that sounds correct to me. I was kind of confused when Chad's videos said the opposite. Thanks.

 

[UndergradGuy7]

Alright, that sounds correct to me. I was kind of confused when Chad's videos said the opposite. Thanks.

It sounded right to me, but after looking it up it WAS WRONG. I thought it would be the same as a plasma membrane with cholesterol being more "liquid," but I guess that is not the case with alkanes.

Here is a another link http://forums.studentdoctor.net/showthread.php?t=111078

 

[Maygyver]

It sounded right to me, but after looking it up it WAS WRONG. I thought it would be the same as a plasma membrane with cholesterol being more "liquid," but I guess that is not the case with alkanes.

Here is a another link http://forums.studentdoctor.net/showthread.php?t=111078

Ah ok. So in short:
MP inc with inc branching
BP dec with inc branching
MP inc with inc chain length
BP inc with inc chain length

Is that correct?

 

[UndergradGuy7]

I believe that is now correct.

I didn't know alkanes make a compact structure when branching, but they do. So this branching increase MP, dec BP.

Higher weight = higher MP, BP.