how do you know R-OH is more polar than R=O?

[dedicate]

How do you know R-OH is more polar than R=O? Or perhaps I should rephrase it to why is R-OH is more polar than R=O?

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[SaltyTheKracker]

You can think of it as the C=O bond having an additional C-O bond, so some of the O atom's electron density is being shared with the C.

In an R-OH bond, the oxygen retains more electron density to itself, making it more polar.

 

[dapmp91]

How do you know R-OH is more polar than R=O? Or perhaps I should rephrase it to why is R-OH is more polar than R=O?

R-OH is typically more polar than R=0 (carbonyl) because O is electronegative and it tends to pull more electrons towards it. The R=0 you must remember there are two lone pairs of electrons on the carbonyl indicating their could be resonance. And if there is resonance, the electrons are spread out and not concentrated. With that being said, the resonance basically spreads out the electrons and there is no concerted pull of electrons from an adjacent atom. Understanding resonance is key to understanding why R-OH (alcohol) is more polar than the carbonyl-hope this helps

 

[sv3]

R-OH is typically more polar than R=0 (carbonyl) because O is electronegative and it tends to pull more electrons towards it. The R=0 you must remember there are two lone pairs of electrons on the carbonyl indicating their could be resonance. And if there is resonance, the electrons are spread out and not concentrated. With that being said, the resonance basically spreads out the electrons and there is no concerted pull of electrons from an adjacent atom. Understanding resonance is key to understanding why R-OH (alcohol) is more polar than the carbonyl-hope this helps

Perhaps this is going off on a tangent but COOH is a better acid than ROH. I know this is b/c of resonance stabilization. I previously had also thought that the more polar a molecule, the better it is as an acid as electron density would be polarized (i figured C=0 was more polar than R-OH). Is that wrong then?Just making sure i am not following a false relationship. thanks

Also for the above quote, aren't there two lone pairs also on the OH oxygen?
sv3

 

[loveoforganic]

I'm under the impression that a carbonyl bond is more polarized than an alcohol as well.... A carbonyl has a resonance structure where the oxygen is negatively charged and the carbon is positively charged... All an alcohol bond has is an inductive effect...

As evidence, here's a link to a list of dipole moments http://macro.lsu.edu/howto/solvents/Dipole%20Moment.htm

Acetone v. Isopropanol is the fairest comparison I can think of, and acetone has a very significantly higher dipole moment than isopropanol.

 

[sv3]

I'm under the impression that a carbonyl bond is more polarized than an alcohol as well.... A carbonyl has a resonance structure where the oxygen is negatively charged and the carbon is positively charged... All an alcohol bond has is an inductive effect...

Yup...would be good if the OP told us if this was an actual question or something they are curious about.

I thought perhaps the fact that the double bond is shorter, and thus less polarized....but i dont think this trumps all the electron density around the carbonyl oxygen in the C=0 bond

 

[dedicate]

The resonance helped a lot, thank you!

BTW, the question stems from either kaplan or EK. I cant recall which, but they clearly said ROH IS more polar than a carbonyl. I simply couldn't figure out why exactly.

 

[Charles_Carmichael]

Wouldn't the carbonyl bond be less polar than an alcohol since the pi bond is shorter and because the electronegativity difference between C and O is smaller? And doesn't a carbonyl only have a resonance structure if electron-donating groups are nearby (or conjugated bonds)? For example, I don't think acetone would have resonance; neither would an aldehyde attached to a carbon backbone.

 

[loveoforganic]

Ketones and aldehydes absolutely exhibit resonance. Anything with pi bonds does for that matter. It's just the degree to which it happens. Although the doubly bonded form of a ketone/aldehyde is almost certainly the more favored form, the +/- form is still a contributor, and, in my opinion, a significant enough one to outweigh the shorter bond length of the carbonyl bond as compared to the alcohol bond.

The electronegativity keeps getting mentioned. Yes, the electronegativity of hydrogen is less than that of carbon. However, a carbon has substituents from which the oxygen can inductively draw electrons, while the hydrogen only has its single electron involved in that bond.

Also, you have to consider bond angles because when figuring out a dipole moment you're considering vectors. An alcohol is sp3 while a carbonyl is sp2. The vectors have more constructive addition in a carbonyl than an alcohol.

 

[sv3]

perhaps Ek or Kaplan was only looking at the elements involved and didn't concern themselves with resonance or anything more. So then you'd have carbon and oxygen, and oxygen and hydrogen. If these were single bonds it would be easy to say its the OH pairing. However, again, i am not sure how the double bond changes everything, or if it does.

We must be making more out of this, i wish there were a simple answer!
I can't stand it when there are questions like these which are supposed "easy stand alones" that I end up wracking my brain over. shoot. this sucks

 

[boaz]

Is this about "polarity" as in electron distribution, or "polarity" as in hydrophilicity? The terms are interchanged very often, especially in bio. For example chloromethane is more polar than methanol, but whereas the former is insoluble in water, the latter is completely soluble. Same applies to RC=O and ROH

 

[starbaduk]

Some questions regarding replies in this thread:

1. Someone (thid post) mentions that resonance structure of carbonyl makes C=O less polar. In fact, isn't it the opposite? Resonance structure contributes to O having full negative charge and C having full positive charge.

No offense to the third poster, but I think his reasoning is incorrect.

2. Loveoforganic: I think OP is concerned about polarity of the bond, not dipole moment of the molecule.

But considering dipole moment of the molecule, sp2 will add to the dipole moment more constructively, wouldn't it subtract more too?

My guess at why C - OH bond is more polar than C=O is similar to Kaushik's explanation. It seems that with shorter bond in C=O, electron distribution should be less uneven.

 

[loveoforganic]

1) Yeah, incorrect.

2) Your vector arrows have the same magnitude regardless of the direction they point. The closer they are to aligned with the carbonyl bond, the larger the dipole moment. How exactly can you consider the polarity of a bond by itself? C-O-H, and you must assume there are no substituents on the carbon? The only way I see to comopare the polarity is through dipole moment of two structurally similar molecules.

ishchayill - Good catch; if that's the case the writers should be shot >.<

 

[sv3]

i have Ek but don't use it as I rely more on TPR. I'll see if I can find similar questions and see what they say. Times like these I honestly think its better to know less......but alas I've already studied too much

 

[FuturaDocta]

I was going to say the same thing.... the more resonance, the more stability and less polarity. Remember why benzene is so stable? It has several resonance structures.

As for the question, the way I look at it is the length of the bond and the electronegativity of both the carbon and the oxygen.

One has a single strand and one has a double bond. If we look at the inductive effects of a double bond, we know that the polarity may be reduced because they shared more electron density. This may be due to the closer and stronger bond with the double bond.

Now, the single bond would be much weaker and longer. Therefore, the inductive effects are much weaker and creates a much more unstable molecule. There is less charge distribution and the overall polarity increases.

Analogy: Imagine that the oxygen and the carbon hate each other. One oxygen wants to get away, but the carbon grabs it's arm. Would it be harder to get away if the carbon grabbed both arms? If carbon grabs both arms, then oxygen would not be able to get away as fast, because two arms are much more stable than one. (lame I know )

*I think I am getting inductive effects mixed with hyper conjugation... Can someone help me with this?

 

[loveoforganic]

I don't really agree with your reasoning, and I think I've mostly argued against it in my previous posts. However, I don't see anything really hyperconjugative related in it. If you need a refresher on hyperconjugation I could prob help though. Would think there's no way that shows up on the MCAT though.

 

[FuturaDocta]

I don't really agree with your reasoning, and I think I've mostly argued against it in my previous posts. However, I don't see anything really hyperconjugative related in it. If you need a refresher on hyperconjugation I could prob help though. Would think there's no way that shows up on the MCAT though.

Yah, well, I can certainly look it up. But, if you could put an analogy to it, that would be great. Thanks

 

[starbaduk]

1) Yeah, incorrect.

2) Your vector arrows have the same magnitude regardless of the direction they point. The closer they are to aligned with the carbonyl bond, the larger the dipole moment. How exactly can you consider the polarity of a bond by itself? C-O-H, and you must assume there are no substituents on the carbon? The only way I see to comopare the polarity is through dipole moment of two structurally similar molecules.

ishchayill - Good catch; if that's the case the writers should be shot >.<

I thought polarity of a bond could be predicted from relative electronegativity of the atoms involved. Other substituents certainly can also affect the polarity of a bond between neighboring atoms, but since we don't know what they are, I guess we ignore them.

 

[sv3]

.

 

[loveoforganic]

Yah, well, I can certainly look it up. But, if you could put an analogy to it, that would be great. Thanks

I'm not sure I can come up with an analogy 😛 The sp2 orbital of the pi bond aligns in phase with the anti-bonding p-orbital to provide a conjugation-like stabilization effect? 😛

I thought polarity of a bond could be predicted from relative electronegativity of the atoms involved. Other substituents certainly can also affect the polarity of a bond between neighboring atoms, but since we don't know what they are, I guess we ignore them.

Relative electronegativities is definitely part of it, but you can't ignore that a carbon has substituents while a hydrogen does not; it's part of the equation.

 

[FuturaDocta]

I'm not sure I can come up with an analogy 😛 The sp2 orbital of the pi bond aligns in phase with the anti-bonding p-orbital to provide a conjugation-like stabilization effect? 😛

lol, It works 😀

 

[Crash88]

This was my reasoning during the test.

Dipole moment = qr

q = charge while r= distance
for both, the partial charges are the same, however for a double bond, r is smaller (because double bonds are shorter), therefore it has a smaller dipole moment. On the other hand, a single bond w/ OH is longer, therefore having a greater dipole moment.

greater dipole= more polar.

 

[loveoforganic]

Why do you assume the partial charges are the same?

 

[Crash88]

Why do you assume the partial charges are the same?

Good point. Not sure if this is true but here's my reasoning. OH has 2 bonds, one to carbon and one to hydrogen. I compared the O-H bond with the carbonyl C=O bond. Hydrogen is less electronegative than carbon, so if anything the Oxygen in the alcohol has more electron density than with the carbonyl, which is only bound to the more electronegative carbon. Even though in an alcohol, oxygen is also bound to carbon (presumably in the opposite direction), electron density around oxygen is still greater than with 2 bonds with carbon as in a carbonyl, since hydrogen is less electronegative than carbon. Greater electron density around the oxygen creates a greater dipole moment, particularly with H, which has a low electronegativity. This coupled with analysis of bond length, directed my answer choice.

basically for me, a bond with H and a bond with C meant more electron density around O than, essentially, 2 bonds with C. So q, if anything, is higher and r is also higher. so bigger dipole in the end.

Let me know what you think of this reasoning. Maybe it's a bit shaky but that's what went through my head.

 

[loveoforganic]

Your reasoning is accurate, but incomplete.

One thing to take into account is bond angle. Attached is an explanation of that.

Another thing to take into account is the number of electrons associated with a substituent. Carbon has several electrons while hydrogen has only one that can be inductively drawn toward the oxygen.

The last thing to take into account is resonance. One of the resonance structures of a carbonyl is a full negative charge on oxygen with a full positive charge on carbon. Alcohols have no resonance structures.

 

[sleepy425]

I'm under the impression that a carbonyl bond is more polarized than an alcohol as well.... A carbonyl has a resonance structure where the oxygen is negatively charged and the carbon is positively charged... All an alcohol bond has is an inductive effect...

As evidence, here's a link to a list of dipole moments http://macro.lsu.edu/howto/solvents/Dipole%20Moment.htm

Acetone v. Isopropanol is the fairest comparison I can think of, and acetone has a very significantly higher dipole moment than isopropanol.

whoa whoa whoa whoa hold up! that there's an LSU link! Geaux tigers!


oh, and for the record, I'm pretty sure loveoforganic is right about everything he(/she) has said in this thread (the isopropanol/acetone comparison is the most obvious comparison). I do think that I've seen some cases where a ketone was less polar than the corresponding alcohol, but for the most part, ketones are more polar, but only slightly more polar (so don't kill yourself over the point).

 

[loveoforganic]

The LSU link was unintentional 😛 I am however from the area and a tiger fan (and bama fan!), so geaux tigers and roll tide roll!