Organic Question -- Acidity

[Avery07]

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I can't figure these problems out.. well I can but I can't explain why. There just doesn't seem to be any consistency.

My question is in regards to 3.22, 3.23, and 3.25 (N containing compounds). The task is to find the most acidic proton.

Most Acidic Proton:
3.22: Proton on the right
3.23: Proton on the left
3.25: Proton on the left

I can't explain 3.22, and it is the root of my problems. I would have thought it to be the proton on the left as the N= will be the most electronegative atom and also stabilize that lone pair over an sp2 carbon??

3.23 makes sense-- Pick N as it is the most electronegative and thus offers the most stabilization.

3.25 again violates the rule in picking the carbon atom. I suppose the reasoning here is that in depronating N, you are making a strong base-- bad idea, so we pick the C which is actually a wise choice in its own due to the stabilization offered by an sp carbon.

Can someone help make sense of this please?? 😕

 

[CANgnome]

if you abstract the right hydrogen in 3.22, the electrons can be stabilized by resonance

3.25 left hydrogen is better stabilized than having a nitrogen with 2 bonds?

acidic proton = most likely to leave right? it will get extracted most easily by a base

 

[Avery07]

if you abstract the right hydrogen in 3.22, the electrons can be stabilized by resonance

3.25 left hydrogen is better stabilized than having a nitrogen with 2 bonds?

acidic proton = most likely to leave right? it will get extracted most easily by a base

I'm not seeing the resonance in 3.22, can you point it out? If that is the case, it makes sense of all the other problems.

 

[CANgnome]

the right hydrogen is extracted, leaving 2 electrons. the electrons displace to the left, knocking the electrons of the double bond onto nitrogen?

I am very untechnical yes 😛 never been good with terminology

 

[Avery07]

Ok.. I got it now. Thanks.

I didn't think of it before as that double bond seemed too far away to be affected but I suppose when the electrons make a double bond immediate to the left of that bottom carbon, it then violates the next carbons octet, displacing the electrons on to the N.

Or something like that??

 

[CANgnome]

the lone pair left is stabilized by resonance yes, making it the most acidic to donate proton o_o

you are right. well, according to my train of thought 😛

 

[UCB05]

On 22, loss of the right proton results in a lone pair of electrons that can shift to form a C-C double bond, displacing the current imine pi bond electrons onto the nitrogen. Loss of the left proton results in a lone pair of electrons on the imine carbon with no possible resonance.

On 23, loss of the left proton leaves a lone pair on the nitrogen which can form a resonance structure by shifting that pair onto the adjacent imine carbon, bumping the current double bond pair onto the other nitrogen. Loss of the right proton leaves a lone pair on that carbon with no resonance. If the pair were to try to resonate by forming an imine double bond with the nitrogen above it, that would require the current imine double bond to bounce left, resulting in a nitrogen with 4 bonds and a lone pair (not viable).

don't know about 25. I thought terminal alkynes had a higher pKa and were less acidic than amines.

 

[LaFleur]

I know that alkynes are more acidic than alkenes which are more acidic than alkanes, but I was also under the impression that amines were more acidic than those three.

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[orgohacks]

Ok.. I got it now. Thanks.

I didn't think of it before as that double bond seemed too far away to be affected but I suppose when the electrons make a double bond immediate to the left of that bottom carbon, it then violates the next carbons octet, displacing the electrons on to the N.

Or something like that??

Yep - It's exactly like forming an enolate from a ketone or aldehyde. It's stablized by putting the negative charge on a more electronegative element (nitrogen in this case).

3.23 - yes - the anion on nitrogen is stabilized by resonance.

3.25 - if you look at your pKa table, pKa of alkynes is about 25, pKa of amines is around 35-36. Reason is that hybridization on alkynes is sp, there is more s character and therefore the electrons are more closely held to the nucleus, which provides stabilization.

hope this helps - James

 

[Ibraiz]

So is 3.23 the most acidic as the lone pair electron can be stabilized by resonance by N ?

 

[orgohacks]

So is 3.23 the most acidic as the lone pair electron can be stabilized by resonance by N ?

yes - the N-H is the most acidic proton since the resulting anion can be stabilized through resonance.


It's the same reason that carboxylic acids (pKa ~4) are more acidic than alcohols (pKa ~16) - the anion is resonance-stabilized.