EK big error? Alkyl branching and MP and BP

[EECStoMed]

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So exam krackers says that with increased branching the BP will decrease which I completely agree with, but it says that MP actually increases which I do not agree with. Princeton review agrees with me. Can anyone shed some light?

 

[DiverDoc]

Its been awhile now but with branching the MP will increase because in this 'arrangement' the molecules can pack better so to speak, resulting in an increased MP.

 

[QofQuimica]

So exam krackers says that with increased branching the BP will increase which I completely agree with, but it says that MP actually increases which I do not agree with. Princeton review agrees with me. Can anyone shed some light?

As a matter of fact, I can.

http://forums.studentdoctor.net/showpost.php?p=2877597&postcount=3

 

[jochi1543]

My o chem textbook explicitly states: "increased branching LOWERS an alkane's BP. Branched-chain alkanes are lower-boiling because they are more nearly spherical than straight-chain alkanes, thus have smaller surface areas, and consequently have smaller dispersion forces."

So this contradicts the info you just mentioned about it increasing BP.

Nothing in my book about MP, but I think the same logic could be used to describe why MP would increase.

 

[DiverDoc]

LOL at myself. Thanks Q for saving the day. I went out on a whim on this one. Guess thats why I got an 8 on PS.

 

[rcd]

I don't understand, can't unbranched alkanes "pack tighter" raising their mp?

 

[DiverDoc]

.

 

[rcd]

*racks brain* ok can do. I never gained an intuitive understanding of mp. I mean bp is easy because I always think liquid->gas. But mp I can never decide which way to think (m->s or s->m).

Up intermolecular forces = up mp.
Up packing = up mp.

 

[EECStoMed]

Thank you so much. Looks like EK was wrong! but TPR was right. increased branching decreases MP and BP. Yay!

As a matter of fact, I can.

http://forums.studentdoctor.net/showpost.php?p=2877597&postcount=3

 

[christian15213]

is ek wrong with this or perhaps are you reading this wrong? I have to go check.

 

[vicinihil]

So here's my 2 Lincolns (which costs more to manufacture than they are worth...ironic? stupid us government)

BP DECREASES WITH INCREASED BRANCHING DUE TO LOWER DISPERSION (INTERMOLECULAR) FORCES IN BRANCHING MOLECULES

I found this on a variety of university websites.

however, the MP has some ambiguity...according to wikipedia

isopentane MP is -166C while n-pentane is -130C however NEOPENTANE MP is -18.

Though isopentane branched more than n-pentane, it has lower MP but Neopentane branches more than both and has the highest MP.


Shrug.

 

[vicinihil]

to clarify packing...

I think that has more to do with saturated/unsaturated fatty acids than anything...

ever wonder why trans fat kills homies? think of the structure and how easy it is to pack...therefore it has a higher MP making it solid very easily (maybe even RT? not sure). However, unsaturated fat is good because its cis-shaped and causes kinks in packing structure leading to lower MP. ditto.