Boiling Point/Melting Point Organic

[Farcus]

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Ok if given organic molecules how do you tell which one have the highest boiling point and melting point? I think its by the intermolecular attractions right? But how do you tell though? if you have an alochol functional group, a carboxylic functional group, an aldehyde functional group, and an alkylhalide how do you tell?

nevermind I know melting point, the question only goes for boiling point.

 

[niranjan162]

more H bonds= higher boiling point. SO alcohol would make the most h bonds and thus have the highest boiling point. Also molecular weight plays a role in boiling but if it came down to MW and Hbonds I would choose the one that made the most Hbonds

 

[Quantum Chem]

Actually, the acid would have a higher boiling point. It would be able H bond more than an alcohol. The alcohol only has one O and the acid would have two O's that could H bond. Also, assuming that the compounds have the same amount of carbons then the acid would also have a higher bp because of the increase in weight due to the extra Oxygen. You can think about this in a real life example (maybe, I work in a lab so it makes since to me). Ethanol tends to evaporate if you leave it out, but Acetic acid doesn't evaporate as fast because of a higher BP.

 

[amar21]

yeah
carboxylic acid BP > OH > aldehyde and so on....

its always like that

C=O and the -OH can form resonance structures when you remove the H...

 

[Farcus]

but if there is a molecule with 3 O bonds and another with 3 O bonds but one of them have a double bond to O, would the double bond with O have higher boiling point?

 

[161927]

but if there is a molecule with 3 O bonds and another with 3 O bonds but one of them have a double bond to O, would the double bond with O have higher boiling point?

What kind of bonds? By "3 O bonds" do you mean 3 alcohols? If there was a double bonded oxygen, by that do you mean a ketone, and two alcohols? Or do you mean a carboxylic acid derivative?

 

[niranjan162]

oops wasnt reading very carefully

 

[Farcus]

ok bad question, a better question would be, The best way to find out melting point is to first find the oxidation state of the carbon if its attached to electronegative element, and then if there is just plain hydrocarbons, they decrease with molecular weight and branching right? Thats what I am getting out of problems I been doing and its working. But I'm not sure if it always work, comments please.

 

[LaCasta]

Remember that branching also plays a role in boiling point

 

[SuperOrchestra]

ok bad question, a better question would be, The best way to find out melting point is to first find the oxidation state of the carbon if its attached to electronegative element, and then if there is just plain hydrocarbons, they decrease with molecular weight and branching right? Thats what I am getting out of problems I been doing and its working. But I'm not sure if it always work, comments please.

That seems a little much, but if it works for you great. I view it like this.

1.)look for is hydrogen bonding, so your looking or F,O,N bonded to a hydrogen.
2.) Look for polar functionality
3.) Look for larger molecule
4.) look for straightest chain (isntead of branching)

Basically go down that checklist, if you have a tie, (two alcohols) go to the next one, if its absent or another tie, go to the next one. Once you find a difference you have found the higher boiling point.

I also don't think the MCAT would try to trip you up with a gigantic non-polar molecule vs. Methanol, as that would be difficult to gauge.

LOL, that seems more complicated than your answer now, its gotten intuitive for me I guess.

 

[Farcus]

That seems a little much, but if it works for you great. I view it like this.

1.)look for is hydrogen bonding, so your looking or F,O,N bonded to a hydrogen.
2.) Look for polar functionality
3.) Look for larger molecule
4.) look for straightest chain (isntead of branching)

Basically go down that checklist, if you have a tie, (two alcohols) go to the next one, if its absent or another tie, go to the next one. Once you find a difference you have found the higher boiling point.

I also don't think the MCAT would try to trip you up with a gigantic non-polar molecule vs. Methanol, as that would be difficult to gauge.

LOL, that seems more complicated than your answer now, its gotten intuitive for me I guess.

well if you just find the oxidation state of the atom, which is pretty easy(#electronegative atom attach to carbon - #hydrogen bond) then you get oxidation state.

 

[dodjie16]

For organic molecules, these are the rules:

The more branched the organic molecule, the lower the boiling point and the melting point. This is because branching reduces the surface area of the compound, which will decrease the london dispersion forces that the compound feels. A decrease in london dispersion will lead to a decrease in intermolecular forces, which will then lead to a lower boiling point and melting point.

The heavier the compound (greater molecular weight), the higher the boiling point and melting point. This is because a longer hydrocarbon chain increases the surface area and there are more london dispersion forces acting on the compound.

The higher the number of hydrogen bonding the compound is capable, the higher the boiling point and melting point.

 

[Danny289]

For organic molecules, these are the rules:

The more branched the organic molecule, the lower the boiling point and the melting point. This is because branching reduces the surface area of the compound, which will decrease the london dispersion forces that the compound feels. A decrease in london dispersion will lead to a decrease in intermolecular forces, which will then lead to a lower boiling point and melting point.

The heavier the compound (greater molecular weight), the higher the boiling point and melting point. This is because a longer hydrocarbon chain increases the surface area and there are more london dispersion forces acting on the compound.

The higher the number of hydrogen bonding the compound is capable, the higher the boiling point and melting point.

you are right in general but must becareful about melting point instance neo pentane has higher melting point than iso pentane because neo pentane can arrange better in solid form than isopentane. result: for melting point pay attention to the molecule shape.