Branching and melting point...

[Sublimation]

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Ok sorry to bother everyone with this but ive been getting mixed answers on this and im starting to get frustrated. Now, increased branching cause a increase or decrease in melting point. According to Destroyer it increases cuz sym. compounds will pack into crystals better and according to kaplan the increase in branchin will result in less surface area, and subsequently less intermolecular forces and thus a lower melting point????which one do i go with both seem tru but i just did number 172 in destroyers org. and i got it wrong??? thanks in advanced.

 

[jd399]

i think it will increase melting point since they close together like u said... they same reason i think why trans has a higher melting point... i could be wrong anyone agree or disagree??

 

[Seenary]

i think it will increase melting point since they close together like u said... they same reason i think why trans has a higher melting point... i could be wrong anyone agree or disagree??

Yes you are right, trans allows for better packing and thus explains the higher melting point....symmetry is important for melting points

 

[Shinpe]

I'm pretty positive that branching in alkanes lowers the MP because you can have less stacking and intermolecular forces. In alkenes, trans alkenes have better stacking geometry than cis alkenes and therefore have higher MP. This second might be what destroyer is talking about. Don't know about the actual question.

 

[Sublimation]

its Q 172 in destroyer.

 

[Shinpe]

its Q 172 in destroyer.

Are you making me look it up now??? GRrrrrr lol

 

[avian777]

this is what i think: increasing branching of alkane does decrease bp and mp because of less surface area to be contacted with, less dispersion force, weak intermolecular interaction, however, when consider alkene isomers, with bp you consider polarity so cis alkene has more dipole moment so it has stronger dipole-dipole interaction so it has higher bp. For melting point, symetry of the molecule has more influence plus trans alkene becomes nonpolar as it was canceled out by symetry. Also, trans alkene is more symetric which allows tighter packing so it has higher mp and lower bp than cis alkene. Am I confusing you more????

 

[Shinpe]

Hmmm I would have definitely thought A (pentane) but destroyer is right that it's D (neopentane) when I looked at the BPs on wiki.

I just took physical chemistry and know that in polymers (which can simply be longer chaing alkanes), branching lowers MP because it impairs intermolecular forces. I'm guessing here it's somewhat an exception since neopentane can form crystaline structures easier. The branching -> lower MP rule seems to still apply (Isopentane has lower MP than pentane) but seems to be overwrited by crystallinity ability if the molecule is very symmetric.

I wonder if the same applies to longer chains though. For example would 2,2-diethylpentane have a higher MP than nonane or this doesnt apply there anymore since that molecule might be too big to allow easy crystalization even with it's symmetry. I doubt they'd give us anything that tricky though, so I guess we should know this case but I think the general trend is still more branching -> lower MP.

 

[Sublimation]

I pretty much get it now THANKS