Why do aldehydes and ketones have lower boiling points than al...

Question

Why do aldehydes and ketones have lower boiling points than alcohols with similar molecular weights? Why are their boiling points higher than those of alkanes with similar molecular weights?

Accepted Answer

Hi, everybody. Let's look at the next question. It says, consider an Alcaine alcohol aldehyde and a ketone with similar molecular weights, the aldehyde and the ketone will have a lower boiling point than the alcohol, but have a higher boiling point than the alkane explain why. So before I read all of these rather long answers, let's just think about what affects boiling point and then we can evaluate our answer choices. So the temperature of the boiling point is dependent on the strength of the intermolecular forces. We recall that the stronger the intermolecular forces, the higher the boiling point since it takes more energy to spread those molecules apart from each other and overcoming that attraction. So let's think about which intermolecular forces are the strongest and therefore which molecules are the higher boiling point. So of course, our strongest IMF s are ion dipole interactions and those are stronger than or greater than hydrogen bonds. We'll just put h bonds which are stronger than dipole dipole interactions, which in turn are stronger than dispersion forces. Note that dispersion forces are the only types of IMF in nonpolar molecules. The others all involve polar or ionic compounds So right away, when we look at our different compounds, we know that al canes have only C and H they're hydrocarbons and therefore have, they are non polar. So we would expect al cans of course, to have the lowest boiling point of all because they have this weakest IMF then when we look at, let's look at our other functional groups. Well, we have aldehyde ketones and alcohols. So aldehydes of course, are in our group attached to a carbon that has a double bond to an oxygen. And the Garbin is and bonded to a hydrogen. So there's our aldehyde, I'll put a L period. The ketone we recall is an R group attached to a carbon who has a double bond to an oxygen and then attached to another R group. And then finally, our ketone and then finally, our alcohol is in our group attached to an oh hydroxy group. So let's think about what kind of IMF forces each of these has. Well, our alcohol we see right away has a hydrogen bound to an electron oxygen. So this gives us hydrogen bonds just as in water, we don't have any ions. So this hydrogen bond will be the strongest dile dile interactions or excuse me, the strongest IMF that we see then in aldehyde and ketone very similarly, we have a carbon with this double bond in oxygen, the oxygen is more electron than the carbon. So you have this dipole moment where the electrons are more attracted to the oxygen. Now, in neither case, well, OK, in a ketone, perhaps depending on your R groups. But given that we're talking about something with similar molecular weights, so we assume the rest of the molecule is similar if as long as you don't have any canceling out of that dipole moment with the rest of the molecule, the aldehyde and ketone will both be similar. They are therefore polar molecules and have dipole dipole interactions. So these are stronger than the dispersion forces but weaker than hydrogen bonds. So with that in mind, let's take a look at our answer. Choices. Choice A says alcohols have the highest boiling point because they exhibit hydrogen bonding. That's correct. Then it says aldehydes and ketones have higher boiling points than alkanes because aldehydes and ketones exhibit dipole dipole interactions. And that's correct. And then it says, while al Cannes only exhibit London dispersion forces weakest IMF so everything in choice A is correct. So we're going to assume that this is our answer since we don't have any option of all or correct or anything like that. But to be thorough, we'll just work our way through here. Choice B says alcohols have the highest boiling point because they exhibit covalent bonding. Well, this would not just having a covalent bond does not tell you anything about the strength of the IMF alco alcohols, aldehydes, ketones alkanes all have covalent bonds, binding them together. The question is, do they have these um dipole dipole interactions? Do they have a polar nature to them? So this would not be a factor in the strength of the IMF then it says aldehydes and ketones have higher boiling points than alkenes because al aldehydes and ketones exhibit London dispersion forces while cannes only exhibit hydrogen bonding weakest IMF well, this is of course, very wrong alkanes only have the dispersion forces which are the weakest IMF and aldehydes and ketones of course exhibit the dipole dipole interactions. So that's why choice B is incorrect. Choice C says alcohol is of the high its boiling point because they exhibit hydrogen bonding. While that part is correct aldehydes and ketones have higher boiling points than alkenes because aldehydes and ketones exhibit London dispersion forces while ALCs only exhibit dile dial interactions. And of course, this is reversed aldehydes and ketones have a higher boiling point because they exhibit dile dial interactions while all cannes only have the dispersion forces. Now, it's important to note that alcohols aldehydes, ketones also have London dispersion forces. They have these as well just as alcohols also have dile diaper interactions, but they also have those stronger hydrogen bonds. Aldehydes and ketones also have those stronger dile dile interactions. But that's why choice C is incorrect. Choice. D says that alcohols are the highest boiling point because they exhibit covalent bonding. We've already discussed why that isn't a correct reason aldehydes and ketones have higher boiling points than alk because aldehydes and ketones exhibit dippo diaper forces. While al canes only exhibit hydrogen bonding weakest IMF and of course, that's incorrect. Hydrogen bonding is the strongest and alk only have dispersion forces. So choice d also incorrect. So once again, our explanation of the different boiling points here in these similar molecular weight compounds is choice. A alcohols have the highest boiling point because they exhibit hydrogen bonding aldehydes and ketones have higher boiling points in ALCs because they exhibit diy Diapy interactions. While al canes only exhibit London dispersion forces the weakest IMF see you in the next video.

Why do aldehydes and ketones have lower boiling points than alcohols with similar molecular weights? Why are their boiling points higher than those of alkanes with similar molecular weights?

Key Concepts

Hydrogen Bonding

Molecular Polarity

Van der Waals Forces

Watch next