10
$\begingroup$

enter image description here

The ketone has one oxygen atom in it and there are two lone pairs hanging out, both of which make it seemingly capable of hydrogen bonding. But in reality it is not.

Did some research and found this on chemguide:

In methoxymethane, the lone pairs on the oxygen are still there, but the hydrogens aren't sufficiently δ+ for hydrogen bonds to form. Except in some rather unusual cases, the hydrogen atom has to be attached directly to the very electronegative element for hydrogen bonding to occur.

Based on the info, I am assuming that the carbon atom in carbonyl group is not electronegative enough to give a partial charge large enough for hydrogen bonding to the hydrogen atoms, say in pentanone.

Improve this question
$\endgroup$
1
  • $\begingroup$ Technically they can, but it's very weak. $\endgroup$
    – Mithoron
    Nov 28, 2017 at 20:41

1 Answer 1

Reset to default
9
$\begingroup$

Short answer: the ketone oxygen can participate in hydrogen bonding

The compound you mention, methoxymethane, is an ether not a ketone.

enter image description here

Ethers don't hydrogen bond very well because the oxygen isn't very polarized.

On the other hand, the carbonyl in a ketone is polarized. We can draw resonance structures that show this polarization.

enter image description here

(image source)

The negative polarization on the carbonyl oxygen allows it to play the role of hydrogen bond acceptor as illustrated in the following drawing.

enter image description here

(image source)

Edit: If you are asking if the hydrogens on the carbon adjacent to the carbonyl carbon (or any other carbon in the backbone of a simple ketone) can participate as the donor in a hydrogen bond, then the answer to this question is "no". These $\ce{C-H}$ bonds are typically less acidic and less polar than the $\ce{O-H}$ bond in water.

Therefore, while the oxygen in pentanone can participate in hydrogen bonding with donor hydrogens in other molecules, the hydrogens in pentanone cannot participate in hydrogen bonding. Pentanone cannot hydrogen bond with itself.

Improve this answer
$\endgroup$
8
  • $\begingroup$ Umm, he's asking about C-H-O hydrogen bonds - ketone as donor $\endgroup$
    – Mithoron
    Apr 29, 2015 at 23:38
  • $\begingroup$ @Mithoron Thanks for pointing this out. I've edited my answer accordingly. $\endgroup$
    – ron
    Apr 29, 2015 at 23:58
  • $\begingroup$ "Isn't very polarized"? Can you see just by looking? $\endgroup$
    – user134789
    Apr 30, 2015 at 0:36
  • $\begingroup$ And one more question, so what you are saying is in order for a H atom to be a hydrogen Bon donor, donating itself, it has to be DIRECTLY attached to either a sulfur, oxygen, or fluorine atom? $\endgroup$
    – user134789
    Apr 30, 2015 at 0:39
  • $\begingroup$ first comment - for example, the electronegativity of H is 2.1, C is 2.5, so a C-H bond is not very polar. O is 3.5 so an O-H bond is polar. second comment - Yes, N, O, S, halogen are the most common. $\endgroup$
    – ron
    Apr 30, 2015 at 0:44

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge that you have read and understand our privacy policy and code of conduct.

Not the answer you're looking for? Browse other questions tagged or ask your own question.