Question on the boiling points of ketone compared with aldehyde

[real]

according to the boiling point graphs, i found that the boiling point of ketones when there are three and six carbon atoms, their boiling points are abnormally higher than aldehyde.
what is the reason behind it?

 

[Bystander]

Could you post some numerical values, please?

 

[real]

3-Pentanone, also known as diethyl ketone, is a colorless liquid ketone with an odor like that of acetone. Its formula is C5H10O.

Its boiling point is 215°F (101.7°C), and its freezing point is -44°F (-44.2°C).

Two other ketones which are isomers of 3-pentanone are 2-pentanone and methyl isopropyl ketone.


In chemistry, acetone (also known as propanone, dimethyl ketone, 2-propanone, propan-2-one and beta-ketopropane) is the simplest representative of the ketones.

Acetone is a colorless mobile flammable liquid with melting point at -95.4 °C and boiling point at 56.53 °C. It has a relative density of 0.819 (at 0 °C). It is readily soluble in water, ethanol, ether, etc., and itself serves as an important solvent. The most familiar household use of acetone is as the active ingredient in nail polish remover. Acetone is also used to make plastic, fibers, drugs, and other chemicals.

propanal : Boiling point 321 K (48°C)
pentanal : boiling point 103°C

 

[GCT]

Exactly how are you comparing the ketone and aldehydes? Ketones can have the higher boiling point than the corresponding aldehyde analog (in relevance to nomenclature) simply due to the higher molecular weight, and at times the symmetry of the ketone (such as particular three/six carbon ketones) may enhance the effectiveness of intermolecular interactions. Try observing the structure, carbon chain of aldehyde grows in one direction; although aldehydes of lower molecular weight may have greater dipoles associated with them, what happens as the carbon chain grows? And if you were to consider the difference between the two soley in terms of van der walls attraction, what would be the relative effect of boiling point between the two with increasing molecular weight?

 

[rachmaninoff]

In your 5-carbon example, pentanal (the aldehyde) has a slightly higher b.p. than 3-pentanone (the ketone) - you might not have noticed this. With aldehydes you have more of a dipole moment; in fact you actually get a small amount of H-bonding, which tends to increase the b.p. But (as GCT already said), their asymmetry, and the length of their nonpolar carbon chains, tends to decrease the b.p.

 

[real]

that mean the reason for these are caused by diople moment and H-bonding?

 

[GCT]

Actually no H bonding, as the molecular weight grows for ketone and aldehyde analogs both of them increase in non-polar character, however symmetric ketones will have more effective intermolecular dispersion interactions. There may be more to this subject, but this is one relevant factor.

 

[movies]

You can have H-bonding with aldehydes! It's not the classic kind of H-bonding, but it does exist. This about taking two aldehyde molecules and arranging them so that the H-C-O of each is used to form a six-membered ring with the O across the ring from the other O, the C across from the C, and the H across from the H. The C-H bond in an aldehyde is relatively weak, as far as C-H bonds go, so having it behave sort of like a proton is not unreasonable.

(props to rachmaninoff for pointing this effect out!)

 

[GCT]

Yeah, rachminoff's point was significant. Movies, are you saying that aldehydes can interact intermolecularly as both hydrogen bond acceptors and donors? If so, how different is it with ketones? Also I would appreciate some links relevant to the "six-membered ring" configuration.

 

[movies]

With ketones you would have to make an eight membered ring because there isn't an hydrogen on the carbonyl carbon.

Such hydrogen bonds (called "formyl hydrogen bonds") have been implicated in the transition states of a number of transformations. See page 11 in http://www-chem.harvard.edu/groups/myers/handouts/Aldol.pdf, and these papers by Corey:

Tetrahedron Lett 1997, 38, 33-36.
Tetrahedron Lett 1997, 38, 37-40.
Tetrahedron Lett 1997, 38, 1699-1702.

Also a review by Denmark:
Chem. Rev. 2003, 103, 2763−2793

There is no reason to suspect that the same effects are not present in neat aldehyde samples.

 

[GCT]

Thanks for the references, this is certainly an interesting subject, definitely made this post more interesting. Not to push your buttons, but how effective do you think such interactions will be with increasing molecular weight; have you heard of such ring formations with higher molecular weight aldehydes and ketones? Relatively how strong are these "hydrogen bonds?"

Also, do these "rings" exist in pure formaldehyde liquid, perhaps locally? Perhaps this "ring" configuration is relevant during the boiling process...the surface area configuration of the cavities (bubbles). The latter aspect is interesting. However I would think that such discreet intermolecular configurations would actually lower the bp, unless it would somehow be related to the increased surface tension of the cavity.

 

[movies]

I haven't seen a number for how strong these H-bonds would be, but I think they would be quite a bit weaker than the H-bonding in an alcohol or amine. That type of H-bond is worth about 3-5 kcal/mol. Increasing the MW of the aldehyde shouldn't have too big an effect on this interaction so long as the area right around the aldehyde isn't too crowded. But, as I said, it would be a weak interaction anyway.

Don't read too much into the ring idea. It's just a way to think of hydrogen bonding of two aldehydes to one another. Think of them more like the rings that water forms when it makes intermolecular hydrogen bonds (which are also 6 membered rings, in regular ice at least).

As for formaldehyde, that's a special case, I think. I think formaldehyde is more likely to just polymerize. Formaldehyde also trimerizes into a trioxane ring (6-membered ring with alternating CH₂ and O groups) which you can buy commercially. It's useful for reactions when you need dry formaldehyde. Formaldehyde itself is a gas under normal conditions, the stuff you use to preserve dead animals and such is actually a aqueous solution of formaldehyde.

More to the point, in general intermolecular forces raise the boiling point of like-sized molecules. I'm not entirely certain of the physical aspects of how these intermolecular effects manifest in observable properties, but I suppose it would have to do with changing surface tensions, as you suggested. I tend to think of it on a molecular level where you have now added more bonding interactions that must be overcome (broken) before the transition to the gas phase. Of course, there are some molecules that do distill as more complex mixtures. Azeotropes are one example, another is the mixture of formic acid and triethylamine, which distills as a 3:2 mixture, I think. I don't really know how those work on a molecular level though.

 

[rachmaninoff]

I think a paper I read recently is relevant here. It was a computational study which involved analyzing the microstructure of azeotropic liquid mixtures, including equimolar acetone-chloroform vs. pure acetone. They characterized some very weak H-bonds there.

http://pubs.acs.org/cgi-bin/abstract.cgi/jpcbfk/2005/109/i41/abs/jp0535238.html

(it's new stuff! published in 2005.)

 

[GCT]

Increasing the MW of the aldehyde shouldn't have too big an effect on this interaction so long as the area right around the aldehyde isn't too crowded. But, as I said, it would be a weak interaction anyway.

that would certainly be an interesting molecule, kind of analogous to fatty acids in soap.

So its possible that this "ring" structure defines a network for solid and liquid formaldehyde?

 

[GCT]

I think a paper I read recently is relevant here. It was a computational study which involved analyzing the microstructure of azeotropic liquid mixtures, including equimolar acetone-chloroform vs. pure acetone. They characterized some very weak H-bonds there.

http://pubs.acs.org/cgi-bin/abstract...jp0535238.html

very nice, I'll read it in full later.

 

[movies]

So its possible that this "ring" structure defines a network for solid and liquid formaldehyde?

Yes, most likely. Just like in water, where there are bunches of hydrogen bonded chains and rings and few discrete water molecules.

When you say solid formaldehyde do you mean formaldehyde crystals at very low temperatures, or the polymers I mentioned above?

 

[GCT]

both actually, this is a pretty interesting topic I'll have to research further sometime.

 

[movies]

As I said above, the bonding in the formaldehyde polymers is totally different. It's repeating -CH₂O- units. Hydrogen bonding would likely be a lot less favorable in that case than in solid formaldehyde.