Organic Chemistry question about polarity

[DoctorWannaBe]

I was wondering if anyone knows whether an amide or a carboxylilc acid is more polar, and why? Thanks!

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[LUBDUBB]

HMMM....good question

Amides are more polar than Carboxylic acid. I say this becuase primary and secondary amides have much higher boiling points than Carboxylic acids - so we can assume that amides have the strongest intermolecular forces. Amides can form hydrogen bonds at the Oxygen and the Nitrogen.

The higher boiling point of the amides is due to resonance between the unshared pairs of electron on the carbonyl oxygen and the amino nitrogen.The resonance causes the single bond between Nitrogen-Carbon to act like a double bond, therefore the bond is shorter and rotation is restricted. The hydrogen bonds that results are therefore stronger in the amides than in the carboxylic acids.



Hope that was somewhat clear 🙂

~Lubdubb


P.S. I also think that amides are the most polar of all the organic compounds.

 

[Nutmeg]

I think that the carboxylic acid is more polar. The amide is less likely to carry a formal chage than the carboxylic acid, due to the resonance stablization, no? Besides which, in general, oxygen is more likely to hydrogen bond due to the electronegavity of oxygen being far greater than that of nitrogen. And the localization of two oxygens in close proximity would have stronger electron withdrawing effects on the rest of the molecule.

My .02

 

[babinski bob]

I agree w/ the OP. However, the N in the amide will NEVER participate in H-bonding. It is conjugated to the carbonyl and thus the lone pair is not free to serve as a H-bond donor. In the case of the simplest amide, foramide (HCONH2), the intermolecular H-bonding that occurs is between the carbonyl O of one molecule and the H attached to the N of the other molecule. Hope these two posts clear it up for you.

 

[PianoGirl04]

Originally posted by Nutmeg
Besides which, in general, oxygen is more likely to hydrogen bond due to the electronegavity of oxygen being far greater than that of nitrogen.

That's it. IIRC, the difference in electronegativity is the definition of polarity. Therefore, since O has a greater electronegativity than N, the carboxylic acid is more polar. The other thing that molecular polarity depends on is shape (since molecular polarity is the vector sum of all the bond polarities), but amids and acids have roughly the same shape.

 

[LUBDUBB]

Resonance stabl of amides gives a partial negative charge on Oxygen and a partial postive charge on the Nitrogen. The nitrogen polarizes the N-H bond. These N-H hydrogens can from strong H-bonds with the oxygen's lone pair. Okay we all agree on this.

So how do you explain that Amides (compared to carboxylic acids) have higher boiling boints?

It has to be because amides have stronger secondary forces. Any Orgo textbook will tell you that Amides have the strongest of all secondary forces and are the most polar.

 

[ASDIC]

carboxylic acid is more polar because:
resonance causes a dipole moment. a slight delta negative on the acyl oxygen and a slight delta positive on the acidic oxygen (provided the solution is in a low pH environment).

in an amide there is no resonance because the nitrogen cannot accept any electrons from the acyl oxygen (since nitrogen itself has an unbonded pair of electrons). So no dipole moment is created. However, amides are soluble cuz the nitrogen can get ionized.

Hope this helps.

 

[stoic]

The answer is definately carboxylic acids. Think about it, a carboxylic acid has two electronegative oxygens that work together to increase the total dipole moment of the molecule in the direction of the CO2H functional group.

I'd stake my B's in orgo on this. lol.

 

[DoctorWannaBe]

Well, this is a very interesting discussion, and I appreciate everyone's help. I turned in my assignment this morning, so hopefully I got it right. My textbook had no guidance on this issue, so who knows what is right. Thanks again!

 

[swankydude]

I know that I am a little late for this discussion, but I couldn?t help myself to a good OCHEM question...

I would go with the COOH. Here's my reasoning:

The OH attached to the carbonyl has ONLY ONE hydrogen attached to the oxygen atom. Thus, the oxygen atom has 2 "free" lone pairs of electrons to participate in hydrogen bonding. If one of the lone pair of e- on the oxygen resonates with the C=O, you still have one pair of e- available to participate in OH bonding. On the other hand, if the one pair of e- on the N of the NH2 group resonates with the C=O, you will then be left with NO pairs of electrons to participate in intermolecular hydrogen bonding THUS resulting in a lower boiling point.

😎

 

[LUBDUBB]

Alright I'm gettin all frustrated, so I did a google search on this BS.

http://www.elmhurst.edu/~chm/vchembook/213organicfcgp.html

 

[jlee9531]

haha i love a good ochem debate........

 

[swankydude]

Originally posted by LUBDUBB
Alright I'm gettin all frustrated, so I did a google search on this BS.

http://www.elmhurst.edu/~chm/vchembook/213organicfcgp.html

I guess I was wrong.... I still don't fully understand the rationale on the website... sorry for the bad info.

 

[PianoGirl04]

So why is an amide more polar? The acid should have a greater dipole moment...😳

 

[Nutmeg]

Originally posted by LUBDUBB
Alright I'm gettin all frustrated, so I did a google search on this BS.

http://www.elmhurst.edu/~chm/vchembook/213organicfcgp.html

PianoGirl04! swankydude! daveswafford! Quick, we have to crash this website to save face!!!!

Well Lub, I guess I was wrong. I think that there's a difference between the polarity of the functional group and the polarity of the entire molecule. I maintain that a molecule w/ an acid group would have a greater overall dipole moment (most likely), but since the dipole moment depends on the other functional groups, this is a conjectural answer to a ridiculous question--like "how long is a piece of string?" It makes sense that the group itself would be more polar.

"I'll get you next time, Gadget! Next time!!!"

--Dr. Claw 😡 😡 😉

 

[DoctorWannaBe]

I actually found that website LUBDUBB mentioned a few weeks ago and used that information to figure out the problem. It makes since that there is a difference in the polarity of a molecule vs. the functional group. I did have specific molecules I was comparing: acetamide, oxalic acid, and 3-chlorobutanoic acid. I found their boiling points which were consistent with the amide>COOH hypothesis, so I put them in that order.

 

[swankydude]

Originally posted by DoctorWannaBe
I actually found that website LUBDUBB mentioned a few weeks ago and used that information to figure out the problem. It makes since that there is a difference in the polarity of a molecule vs. the functional group. I did have specific molecules I was comparing: acetamide, oxalic acid, and 3-chlorobutanoic acid. I found their boiling points which were consistent with the amide>COOH hypothesis, so I put them in that order.

Of course, I agree there is a difference in the polarity of a molecule vs. the functional group... I still don't understand why CH3COOH has a lower boiling point that CH3CONH2 according to the explanation on that website...

 

[Nutmeg]

Originally posted by swankydude
Of course, I agree there is a difference in the polarity of a molecule vs. the functional group... I still don't understand why CH3COOH has a lower boiling point that CH3CONH2 according to the explanation on that website...

Boiling points are for the pure species, not an aqueous mixture. Pure acetic acid has proton acceptors but no good donors--hence, there's little H-bonding. Acetamide has a good acceptor and a good donor, esp since the oxygen is electron withdrawing--thus, more H-bonding than in acetic acid.

Oxalic acid gets disqualified from polarity argutments due to symmetry anyways. There's more to the question than just functional groups.

 

[LUBDUBB]

Sorry to bring up an old topic, but for anyone who is interested:

"The amide should be less polar at the N-H bond than the O-H bond (because the electronegativity of N is more similar to H than that of O is to H), but there is also polarity in the amides due to the difference between the double-bonded O and the N. It is not immediately clear which should win. By one definition, dipole moment, tables show that amides generally win hands down. The boiling point evidence also supports this (although there are other factors which can be at play, such as whether dimers form). And dielectric constant (another measure of polarity) also weighs in on the side of the amides. "
--Dr Calvin (scott) of
examkrackers fame

 

[DoctorWannaBe]

Well, here is the answer according to my TA.

1. Oxalic acid (most polar)
2. 3-chlorobutanoic acid
3. Acetamide
4. Methyl ethyl ether
5. chloroform (least polar)

So I guess that website was wrong, since that is what I followed and got the answer wrong. Maybe it's just me, but I think this question was too difficult for first semester O-Chem students to figure out on their own after 5 weeks. Most of them barely know the functional groups.

 

[gary5]

COOH is more polar due to higher electroneg of O versus N.

 

[Brickhouse]

WELL - if anyone ever wondered why O-chem has such a reputation for being so impossible....

SEE ABOVE!

I woulda said -COOH too, and I teach ochem at Kaplan !

Go Chiefs!

 

[csx]

Sooo...does anyone know which is more polar...an amide or a carboxy acid?

tldr from above debate: both sides saying amide is def polar or carboxy is def polar.

 

[immolard]

Sooo...does anyone know which is more polar...an amide or a carboxy acid?

tldr from above debate: both sides saying amide is def polar or carboxy is def polar.

Amides more polar. Both oxygens on the carboxylic acid compete for charge which does not allow for good polarization. An amide is similar to a carboxyl in structure, but nitrogen has a different electronegativity than the oxygen so the charge is drawn more towards that end, polarizing the functional group slightly more so.