is this right....

[pistolpete007]

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branching decreases BP but increases mp

cis has a higher bp than trans but a lower mp?

 

[osimsDDS]

yea cis does have a higher bp than trans and a lower mp because cis molecules are polar and polar molecules show higher bp...buuuut because it is cis it can not form a wide array of intramolecular and intermolecular attractions that trans molecules can...

Is my explanation right anyone?? someone back me up on this..does it have to do with intramolecular attractions between the molecules

 

[bumpski20]

def right about the polar vs nonpolar stuff. im not positive about the second half of that (not that i think its wrong...im just not familiar with that). the way i look at it is bps are based on dipoles (cis = more polar = higher bp) and mp point is based on stability (trans = more stable = higher mp). the way i remember this is that a saturated carbon chain has a higher melting point then an unsaturated carbon chain, and saturated chains are more stable.

 

[nixon13]

As chain length increases, so does BP, MP. As branching increases, MP and BP decrease.

Cis has a higher BP than trans due to polarity.

Trans has a hgher MP than cis due to symmetry.

 

[Hugh Mannity]

As chain length increases, so does BP, MP. As branching increases, MP and BP decrease.

Cis has a higher BP than trans due to polarity.

Trans has a hgher MP than cis due to symmetry.

MP increases with branching because it packs into a crystal better. BP decreases with branching because the surface area is decreased, which means less dispersion forces.

 

[lphiewok]

i thought branching with same numbers of carbons (let's say isopropyl and n-propyl) would decrease both bp and mp. can someone clear this up?

i know for alkene, cis = higher bp due to dipole and trans = higher mp because it has the higher symmetry which allows them better packing in the solid state

branching would decrease mp and bp because it reduces the surface area so it is easier to melt it and boil it.

can someone tell me about branching?

btw, for the longest time i was thinking MP was FP (because I was studying gen chem before ochem). hopefully noone is making that mistake!

 

[lphiewok]

okay i found a paragraph from kaplan blue book about branching and MP.

"branched molecules have slightly lower boiling and melting points than their straight chain isomers. greater branching reduces the surface area of a molecule, decreasing the weak intermolecular attractive forces (van der Waals forces). Hence, the molecules are held together less tightly.. blah blah"

i know kaplan is very basic but im guessing if you are comparing same numbers of carbons, then branching will reduce both BP and MP.


hope this helps

 

[Hugh Mannity]

No it is too basic. It says nothing about the packing into a crystal which is the reason why branching raises MP. Do a google search or also wiki (although wiki briefly mentions it in little detail). MP for branched alkanes is tougher to determine but I've heard overall that it tends to increase MP.