polarity of ketone vs alcohol?

[unsung]

Why is cyclohex-2-enol more polar than cyclohex-2-enone? I thought the carbonyl C=O bond was pretty polar, resulting in a delta-+ charge on the C? So I don't understand why the alcohol cyclohex-2-enol is considered more polar?

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[161927]

intermolecular and intramolecular hydrogen bonding with the alcohol

 

[unsung]

intermolecular and intramolecular hydrogen bonding with the alcohol

H-bonding makes it more polar?

 

[dakker]

Why is cyclohex-2-enol more polar than cyclohex-2-enone? I thought the carbonyl C=O bond was pretty polar, resulting in a delta-+ charge on the C? So I don't understand why the alcohol cyclohex-2-enol is considered more polar?

Think about the bonds between the atoms. We know oxygen is very electronegative. So which is more electronegative H or C? C is. So... This will give us a bigger electronegativity difference between O and H and not O and C. You don't need to know electronegativity values, but a rough idea is good. Know the basics: N, F, O, C, S, Cl, H. Or just memorize the periodic trends. Good luck.

 

[161927]

H-bonding makes it more polar?

yes

 

[cornpops21]

H-bonding makes it more polar?

Yes. Compare the boiling point of an alcohol to an alkane of similar molecular weight. You'll see that due to H-bonding, the bp of the alcohol is significantly higher.

 

[zogoto]

I don't think it's more polar, but the H-bonding makes it have a higher bp and mp, cuz it has more intermolecular forces.

 

[niranjan162]

H bonding happens because its so polar

 

[amar21]

-OH = h-bond = more polar.

 

[Lukkie]

think about where the electrons hang out. for OH they are clearly being pulled towards the O. for C=O, they are pulled to the O a bit, but also to the C a little bit. think about the resonance structures as well. O can eat one of the bonds and have a negative charge, but so can C. Hydrogen, however, cannot, so there is a greater polarity difference in the -enol

edit - sorry for the mistake - thanks for correcting it DCODY.

 

[161927]

think about where the electrons hang out. for OH they are clearly being pulled towards the H. for C=O, they are pulled to the O a bit, but also to the C a little bit. think about the resonance structures as well. O can eat one of the bonds and have a negative charge, but so can C. Hydrogen, however, cannot, so there is a greater polarity difference in the -enol

For OH, the electrons are being pulled towards O, away from H.

The electrons of C=O are pulled towards O, generating a partial positive charge on the carbonyl carbon.

I've never heard oxygen described as eating bonds, but it sounds cool.

When was an enol formed?

 

[Lukkie]

For OH, the electrons are being pulled towards O, away from H.

The electrons of C=O are pulled towards O, generating a partial positive charge on the carbonyl carbon.

I've never heard oxygen described as eating bonds, but it sounds cool.

When was an enol formed?

whoops - yeah I meant the electrons will be pulled towards O

what i meant by eating bonds was, draw the resonance structures. in C=O, there are two different ones - one where the negative charge is on the C, and one where its on the O. this is not possible with the OH

and by enol i was referring to the original quesiton - cyclohex-2-enol more polar than cyclohex-2-enone

 

[SuperOrchestra]

An Enol may be more polar than than an ketone, but i'm pretty sure an alcohol is not more polar than a ketone. Hydrogen bonding is a result of polarity, but that doesnt' mean its greater than the polarity of a ketone, the increase in bp is because of intermolecular bonding not an insane amount of polarity. In an Alcohol you can recieve electron density from both the hydrogen and the carbon. In the Ketone it can only recieve electron density from the carbon. Leaving a large partial negative charge on the oxygen than is not as pronounced in the Alcohol. Draw the dipole and you will see its greater in a ketone.

 

[tco]

In short, there are no hydrogens that can hydrogen bond in the ketone. Who cares which is more polar?

 

[SuperOrchestra]

LOL, I think the person who asked the question cares

 

[Foghorn]

An Enol may be more polar than than an ketone, but i'm pretty sure an alcohol is not more polar than a ketone. Hydrogen bonding is a result of polarity, but that doesnt' mean its greater than the polarity of a ketone, the increase in bp is because of intermolecular bonding not an insane amount of polarity. In an Alcohol you can recieve electron density from both the hydrogen and the carbon. In the Ketone it can only recieve electron density from the carbon. Leaving a large partial negative charge on the oxygen than is not as pronounced in the Alcohol. Draw the dipole and you will see its greater in a ketone.

You're only considering the dipole moment between C & O in the carbonyl. Both the dipole moments for C & O and H & O have to be considered in the alcohol.

A better way to determine which molecule is more polar is by placing each one in water to observe which is more misible/soluble. In water, the alcohol (pKa ~15-16) will more readily form a homogenous solution compared to the ketone (pKa~20-21). The opposite will be true if each is separately placed in hexane, a non-polar solvent.

 

[SuperOrchestra]

Your mixing concepts. If only polarity determined miscibility with water, ketone would be more, its the hydrogen bonding that makes the alcohol more miscible. Polarity is a function of the dipole moment, If you drew a picture you would see one dipole moment going towards the oxygen in the ketone. In the alcohol there is an arrow pointing up towards the oxygen from the carbon and an arrow pointing down from the hydrogen, this diminshes the dipole somewhat and thus makes it less polar.

 

[unsung]

Your mixing concepts. If only polarity determined miscibility with water, ketone would be more, its the hydrogen bonding that makes the alcohol more miscible. Polarity is a function of the dipole moment, If you drew a picture you would see one dipole moment going towards the oxygen in the ketone. In the alcohol there is an arrow pointing up towards the oxygen from the carbon and an arrow pointing down from the hydrogen, this diminshes the dipole somewhat and thus makes it less polar.

My reasoning was similar to yours, but according to the answer key, the alcohol is supposed to be the more polar one. I guess I'll just keep in mind anything capable of H-bonding is likely to be more polar, since ability to H-bond is a function of polarity.

 

[SuperOrchestra]

You do understand that your problem is comparing tautomers. Cyclohexen-2-ol is different than cyclohex-2-enol. So unless the answer guide explicitly states something along the lines of hydrogen bonding, I believe its saying then the enol may be more polar than the ketone, nothing about an alcohol.

 

[phltz]

The answer given to this problem is incorrect. The AAMC doesn't often screw up, but in this case they did.

The alcohol has a lower retention time because it is a hydrogen-bond donor. However, the answer they give is false, because it actually has a lower dipole moment than the enone.

 

[Geekchick921]

You're only considering the dipole moment between C & O in the carbonyl. Both the dipole moments for C & O and H & O have to be considered in the alcohol.

A better way to determine which molecule is more polar is by placing each one in water to observe which is more misible/soluble. In water, the alcohol (pKa ~15-16) will more readily form a homogenous solution compared to the ketone (pKa~20-21). The opposite will be true if each is separately placed in hexane, a non-polar solvent.

This is going to be what I was going to point out as well.

 

[phltz]

Solubility in water is _not_ a good way of comparing polarity of these two molecules. Solubility in water is affected by polarity and also ability to participate in hydrogen bonding. The ketone is a hydrogen bond acceptor, while the alcohol is both an acceptor and a donor. Comparative solubility is thus going to give you a muddled mix of information about solubility and hydrogen bonding ability which is almost entirely worthless.

 

[RickyJamesMD]

I don't think it's more polar, but the H-bonding makes it have a higher bp and mp, cuz it has more intermolecular forces.

I agree with this statement.

On average, I believe alcohols are more polar than carbonylls. Only carboxylic acids are more polar than alcohol really.

 

[UCSultanPepper]

The answer given to this problem is incorrect. The AAMC doesn't often screw up, but in this case they did.

The alcohol has a lower retention time because it is a hydrogen-bond donor. However, the answer they give is false, because it actually has a lower dipole moment than the enone.

Sorry to dig up an old thread guys, but can anyone validate if phltz is correct here? I got this question wrong, because I thought that ketone would be more polar than the alcohol due to the fact that the oxygen would be sp2 hybridized, and that the oxygen of alcohol is sp3. 33% S character vs 25% S character means more electronegative, and thus a greater dipole moment.

I realize that this question truly boils down not to affinity of a more polar group to the silica, but rather to hydrogen bonding to the silica (SO2). It probably would have helped if I remembered silicas molecular structure during the practice test.

 

[Chunkle]

Sorry to dig up an old thread guys, but can anyone validate if phltz is correct here? I got this question wrong, because I thought that ketone would be more polar than the alcohol due to the fact that the oxygen would be sp2 hybridized, and that the oxygen of alcohol is sp3. 33% S character vs 25% S character means more electronegative, and thus a greater dipole moment.

I realize that this question truly boils down not to affinity of a more polar group to the silica, but rather to hydrogen bonding to the silica (SO2). It probably would have helped if I remembered silicas molecular structure during the practice test.

In general protic substances with h-bonds will be more polar. H-bonding can be considered a stronger dipole force. At least that's what was accepted in my year of Ochem.

EDIT: sorry, to answer the question, I think AAMC was right on this one.

So hydrogen bonding more polar than carbonyl. TPR review book screws this up.

 

[TheRealCookieMonster]

disclaimer: So I realize this thread is old but I struggled with this question and I'm one of those people who need to understand why... so I did some searching. Although I didn't get a clear cut answer, after compiling the facts, this seemed logical. It's my understanding that an alcohol will have greater polarity than a similar ketone or aldehyde for one and only one reason:

ANSWER: the alcohol has a greater net dipole moment than the carbonyl. To explain this phenomenon, as stated previously by dakker, C is more e- withdrawing than H, and O more e- withdrawing than both C and H. In a carbonyl the O must share two bonds (pulling in the same direction, meaning they add) with C. In an alcohol the O shares one with C and one with the weaker H atom at an angle that still favors a greater net dipole moment toward the oxygen (simple vector addition). All other characteristics (e.g. H bonding, Rf value, or BP) arise from this fact. SO... Even a single alcohol molecule is more polar than a single carbonyl...

As far as the AAMC and the question related to Rf and which sticks better, H bonding is superior to polarity when considering silica paper. Silica paper contains polar alcohol groups and Os for H bonding.

 

[JoeCaputo]

Carbonyl is an H-Bond acceptor. Alcohol is both an H-bond donator and H-bond acceptor. So it will be more polar. Plus I remember from running TLC on different compounds.

 

[TheRealCookieMonster]

Carbonyl is an H-Bond acceptor. Alcohol is both an H-bond donator and H-bond acceptor. So it will be more polar. Plus I remember from running TLC on different compounds.

Yes but H bonding is the result of polarity, not it's cause. Agreed? So although H bonding will be stronger on the TLC plate via the OH due to the double interaction, the question asked by OP was "Why is cyclohex-2-enol more polar than cyclohex-2-enone?"

 

[aldol16]

ANSWER: the alcohol has a greater net dipole moment than the carbonyl. To explain this phenomenon, as stated previously by dakker, C is more e- withdrawing than H, and O more e- withdrawing than both C and H. In a carbonyl the O must share two bonds (pulling in the same direction, meaning they add) with C. In an alcohol the O shares one with C and one with the weaker H atom at an angle that still favors a greater net dipole moment toward the oxygen (simple vector addition). All other characteristics (e.g. H bonding, Rf value, or BP) arise from this fact. SO... Even a single alcohol molecule is more polar than a single carbonyl...

I don't think this argument is too good for explaining the phenomenon. For most practical purposes, we take carbon and hydrogen bonds to be essentially non-polar and covalent - that is, their electronegativities are too similar to give rise to measurable differences (with exceptions, of course). In order for your argument to work, the H has to be substantially less electron-withdrawing than a carbon for the overall dipole moment to be greater in the alcohol, taking into account the C-O-H bond angle. Your argument would be better suited for ethers.

Further, the basis of carbonyl reactivity is the strong electrophilic character of the carbon atom - this is quite literally what makes life tick. Alcohol functionalities are much less susceptible to nucleophilic attack, comparatively speaking.

Carbonyl is an H-Bond acceptor. Alcohol is both an H-bond donator and H-bond acceptor. So it will be more polar. Plus I remember from running TLC on different compounds.

If you know how TLC works, then you'll know that silica gel is nothing more than SiO2, a strong hydrogen bond acceptor. The reason alcohols are retained on the column longer than carbonyls is because it can hydrogen bond, not because of its polarity. A lot of people here are using polarity in the same breath as hydrogen-bonding ability and that's not a fair assumption to make. Hydrogen bonding ability does not necessarily map with polarity because say you have something like SiO2 and you're trying to measure polarity. Imagine something insanely polar like propylene carbonate. It's one of the most polar organic compounds but it cannot donate hydrogen bonds. So something like an alcohol could still stick to the silica better than propylene carbonate, resulting in better pull-down.

 

[JoeCaputo]

Yes but H bonding is the result of polarity, not it's cause. Agreed? So although H bonding will be stronger on the TLC plate via the OH due to the double interaction, the question asked by OP was "Why is cyclohex-2-enol more polar than cyclohex-2-enone?"

My mistake, thank you guys. I thought this question was in reference to TLC and which would travel faster under the general rule for TLC, more polar moves slower. I can see you guys would probably give your lab directors a hard time with that general rule.

 

[TheRealCookieMonster]

I don't think this argument is too good for explaining the phenomenon. For most practical purposes, we take carbon and hydrogen bonds to be essentially non-polar and covalent - that is, their electronegativities are too similar to give rise to measurable differences (with exceptions, of course). In order for your argument to work, the H has to be substantially less electron-withdrawing than a carbon for the overall dipole moment to be greater in the alcohol, taking into account the C-O-H bond angle. Your argument would be better suited for ethers.

Further, the basis of carbonyl reactivity is the strong electrophilic character of the carbon atom - this is quite literally what makes life tick. Alcohol functionalities are much less susceptible to nucleophilic attack, comparatively speaking.



If you know how TLC works, then you'll know that silica gel is nothing more than SiO2, a strong hydrogen bond acceptor. The reason alcohols are retained on the column longer than carbonyls is because it can hydrogen bond, not because of its polarity. A lot of people here are using polarity in the same breath as hydrogen-bonding ability and that's not a fair assumption to make. Hydrogen bonding ability does not necessarily map with polarity because say you have something like SiO2 and you're trying to measure polarity. Imagine something insanely polar like propylene carbonate. It's one of the most polar organic compounds but it cannot donate hydrogen bonds. So something like an alcohol could still stick to the silica better than propylene carbonate, resulting in better pull-down.

How then can we explain that alcohols are more polar than carbonyl functional groups, if not by dipole moment? And how is it that the observed dipole moment of an alcohol bond is noticeably greater than any comparable carbonyl bond?

 

[aldol16]

How then can we explain that alcohols are more polar than carbonyl functional groups, if not by dipole moment? And how is it that the observed dipole moment of an alcohol bond is noticeably greater than any comparable carbonyl bond?

With regard to your second question, see: http://macro.lsu.edu/howto/solvents/Dipole Moment.htm Compare isopropyl alcohol to acetone.

With regard to your first question, it depends on how you define polarity. If you define it as dipole moment, your argument isn't really going to work. But as chemists, we usually define polarity in terms of observed phenomena. For instance, boiling point. Or solubility. And the problem with that is that something with a lower dipole moment but with the ability to hydrogen bond can still have a higher boiling point than something else that has a comparatively higher dipole moment but without the ability to hydrogen bond. If you define polarity as retention time on a silica column, well that's a problem too. Because hydrogen bonding largely determines that. So if you want to define polarity as a function of boiling point or retention time on a TLC, that's fine. But a dipole moment argument would not work.

Just consider this - let's go back to first principles. So you have cyclohexanone vs. cyclohexanol. Now, the oxygen on the cyclohexanone has two bonds to it, both to carbon. They are oriented in the same direction, 0 degrees with respect to each other. So you get the full force of the carbon pulling on the oxygen. In cyclohexanol, you have two bonds to oxygen, one with carbon and the other with hydrogen. These are oriented at 109 degrees relative to each other. So the dipole moment is diluted, so to speak, and only one component of the dipole vectors will play a role.

 

[7331poas]

Just consider this - let's go back to first principles. So you have cyclohexanone vs. cyclohexanol. Now, the oxygen on the cyclohexanone has two bonds to it, both to carbon. They are oriented in the same direction, 0 degrees with respect to each other. So you get the full force of the carbon pulling on the oxygen. In cyclohexanol, you have two bonds to oxygen, one with carbon and the other with hydrogen. These are oriented at 109 degrees relative to each other. So the dipole moment is diluted, so to speak, and only one component of the dipole vectors will play a role.

So then the conclusion is that cyclohexanone is more polar than cyclohexanol. Is the consensus here that the question was wrong?

From my look at the problem I would will answer the ketone functionality as more polar due to the directionality of the dipole moments. H bonding has nothing to do with it.

 

[aldol16]

So then the conclusion is that cyclohexanone is more polar than cyclohexanol. Is the consensus here that the question was wrong?

From my look at the problem I would will answer the ketone functionality as more polar due to the directionality of the dipole moments. H bonding has nothing to do with it.

Well, the conclusion depends on how you define polarity and that's where most people in this thread get mixed up. If you want to define it as retention time on TLC or boiling point, then cyclohexanol is more "polar." But you have to keep in mind that this definition of "polar" takes into account more than just dipole moment - it takes into account hydrogen bonding, which is the crucial factor on TLC and boiling points.

But as a chemist, I see polar as synonymous with having a large dipole moment. In which case carbonyls usually have higher dipole moments than alcohols (compare isopropyl alcohol and acetone).

So it all depends on what the question is asking. Is it asking which one comes off on the TLC last? Or which one has the higher boiling point? Or is it asking which has the higher dipole moment?

 

[7331poas]

Well, the conclusion depends on how you define polarity and that's where most people in this thread get mixed up. If you want to define it as retention time on TLC or boiling point, then cyclohexanol is more "polar." But you have to keep in mind that this definition of "polar" takes into account more than just dipole moment - it takes into account hydrogen bonding, which is the crucial factor on TLC and boiling points.

But as a chemist, I see polar as synonymous with having a large dipole moment. In which case carbonyls usually have higher dipole moments than alcohols (compare isopropyl alcohol and acetone).

So it all depends on what the question is asking. Is it asking which one comes off on the TLC last? Or which one has the higher boiling point? Or is it asking which has the higher dipole moment?

And we make that assumption in the MCAT testing center?

 

[aldol16]

And we make that assumption in the MCAT testing center?

I highly doubt the MCAT will ask you outright, is A more polar than B. Because chemists have vetted the chemistry questions, they tend to be pretty clear and not arguable. So instead, you might see something like, "A and B were run on a silica column. Here are the structures. Which one is retained longer?" Or "Which one has a higher boiling point?"

 

[7331poas]

I highly doubt the MCAT will ask you outright, is A more polar than B. Because chemists have vetted the chemistry questions, they tend to be pretty clear and not arguable. So instead, you might see something like, "A and B were run on a silica column. Here are the structures. Which one is retained longer?" Or "Which one has a higher boiling point?"

Thanks. I actually think the AAMC do a pretty great job with the MCAT. I assume it must be pretty hard to make a test for people who may have basic "learned the test" understanding, all the way to PhD level understanding. The hard part is making sure the questions are non disputable which is pretty hard when you have intelligent people taking an intro level test.

 

[TheRealCookieMonster]

With regard to your second question, see: http://macro.lsu.edu/howto/solvents/Dipole Moment.htm Compare isopropyl alcohol to acetone.

With regard to your first question, it depends on how you define polarity. If you define it as dipole moment, your argument isn't really going to work. But as chemists, we usually define polarity in terms of observed phenomena. For instance, boiling point. Or solubility. And the problem with that is that something with a lower dipole moment but with the ability to hydrogen bond can still have a higher boiling point than something else that has a comparatively higher dipole moment but without the ability to hydrogen bond. If you define polarity as retention time on a silica column, well that's a problem too. Because hydrogen bonding largely determines that. So if you want to define polarity as a function of boiling point or retention time on a TLC, that's fine. But a dipole moment argument would not work.

Just consider this - let's go back to first principles. So you have cyclohexanone vs. cyclohexanol. Now, the oxygen on the cyclohexanone has two bonds to it, both to carbon. They are oriented in the same direction, 0 degrees with respect to each other. So you get the full force of the carbon pulling on the oxygen. In cyclohexanol, you have two bonds to oxygen, one with carbon and the other with hydrogen. These are oriented at 109 degrees relative to each other. So the dipole moment is diluted, so to speak, and only one component of the dipole vectors will play a role.

My argument isn't going to work, if the original question's logic was correct and I now realize, thanks to aldol16, it wasn't. Because we cannot change the definition of polarity. Chemistry defines "Polarity", is, an uneven distribution of charge between two or more bonded atoms it's value is determined experimentally by measuring dipole moment/net dipole moments of multiple atoms and adding up the vectors, not by BP, Rf or some other thing that results from the dipole moment or other phenomenon like H-bonding. Like you said C=O is more polar than C-OH and so the MCAT got the explanation on that question wrong, although the answer is still correct because of H-bonding to the silica.

 

[aldol16]

Thanks. I actually think the AAMC do a pretty great job with the MCAT. I assume it must be pretty hard to make a test for people who may have basic "learned the test" understanding, all the way to PhD level understanding. The hard part is making sure the questions are non disputable which is pretty hard when you have intelligent people taking an intro level test.

Exactly. So they always make it so that it's reasonable and undisputable. So they won't ask a question that you can argue - they'll ask it in a way that has only one correct answer. So, for example, if you're a biology major, go back to your intro bio classes and look at their exam questions. You'll notice that while they ask the questions in a really primitive way, it'll still be worded in such a way that there's only one correct answer. Now that you're a junior/senior bio major, you should be able to discern the naivete of the question yet the beauty in its wording. A common technique for doing this is inserting qualifiers, i.e. "assuming that these energies were obtained at 298K and are not temperature-dependent..." Though by now you should know that energies are in fact temperature-dependent.

 

[7331poas]

Chemistry defines "Polarity", is, an uneven distribution of charge between two or more bonded atoms it's value is determined experimentally by measuring dipole moment/net dipole moments of multiple atoms and adding up the vectors, not by BP, Rf or some other thing that results from the dipole moment or other phenomenon like H-bonding.

Like you said C=O is more polar than C-OH and so the MCAT got the explanation on that question wrong, although the answer is still correct because of H-bonding to the silica.

Isnt that a direct contradiction? I am fairly confident that C=O is more polar due to the dipole moments. Rf values due to a TLC plate is not definitional polarity.

 

[TheRealCookieMonster]

Isnt that a direct contradiction? I am fairly confident that C=O is more polar due to the dipole moments. Rf values due to a TLC plate is not definitional polarity.

The original question asked by the AAMC was which compound would be slowest on TLC, a carbonyl compound or a similar alcohol? The correct answer given was the alcohol. This is correct. However, the explanation given in the question by the AAMC was that the alcohol would travel slowest on the silica because it was most polar. This is incorrect. H-bonding is the only reason that the alcohol travels slowest on the silica longest. I made the assumption that the AAMC was correct in there explanation. There is only one way to define polarity.

 

[7331poas]

Oh that was the question. I misread then. I thought it asked which was more polar