Alkenes are more acidic than alkanes due to?

[cta]

Q: Alkenes are more acidic than alkanes. What is the best explanation for this trend?

I thought it was because the sigma bond present in alkenes helps to stabilize the negative charge generated when a proton is removed.

Reason why I jumped the gun on sigma bond stabilization: s orbitals are closer to the nucleus than p orbitals are. So electrons that are in s orbitals will be closer to the nucleus than electrons in p orbitals – and therefore, lower energy (“opposite charges attract”). For this reason, electrons that are in sp orbitals are lower energy than sp2, which is lower energy than sp3, since they have greater s character (33% for sp2) than sp3 (25%). This makes the anions more stable. That's why alkyne protons are more acidic than alkenes>alkanes...

But the correct answer is that the pi bond present in alkenes help to stabilize the negative charge generated when a proton is removed.

The answer key just says because alkanes and alkenes both have sigma bonds so that eliminates sigma bond stabilization


I understand a negative charge that is adjacent to one or more Pi bonds can disperse its negative charge over multiple atoms (aka resonance) So a negatively charged alkane is much less stable than a negatively charged alkene, where the negative charge can be dispersed over multiple carbons through resonance.

Can someone explain why my thought process was wrong? Does sigma bond (and therefore s character) not play as big a role to the acidity of alkenes as they do for alkynes? Is it because the difference between s character of alkenes and alkanes are not as great as the contribution of pi bonds to this stabilization to make alkenes more acidic?

I guess I thought s character was important because all the textbooks I had said alkynes s-character was the reason why it was more acidic and applied that same concept between alkenes and alkanes.

Any enlightment would be greatly appreciated.

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[Spinach Dip]

It's the pi bond, not the sigma bond, which allows electrons to move around.

As far as I recall, this is because single pi bonds are weaker than sigma bonds, and thus easier to break via resonance.

 

[Superfans]

agree... pi bonds can break much more easily.

 

[Markymark15]

I am pretty sure that the s character of the carbon has more to do with the acidity than the presence of a pi bond. There is no resonance with the pi bond because you can't arrow push so it must be the s character of the carbon that dominates. Since alkenes have more s character than the alkanes, alkenes are more acidic. This is what is in any organic chemistry textbook and what my organic chemistry professor said.

However, when there is a carbocation, pi bonds and sigma bonds can donate electrons.

GL

 

[hippiedoc14]

I believe the slight increase in acidity in Alkenes is due to resonance. The electrons from the double bond are able to delocalize an provide increased stability.

 

[gettheleadout]

It has nothing to do with breaking the pi bond; loss of an acidic vinylic H does not involve the pi bond directly.

The reasoning I'm familiar with involves the orbital hybridization of the acidic C-H bond. A vinylic carbon is sp2 hybridized, meaning the acidic C-H bond is sigma_[sp2-s]. Compared to a sigma_[sp3-s], the vinylic C-H bond exhibits greater electron density toward the carbon due to the relative increase (versus sigma_[sp3-s]) of s-character in the bond at the carbon. This "lopsidedness" of the bond electron density places the bond in an energy state more similar to the carbanion formed upon deprotonation, resulting in a lower heterolytic bond dissociation energy and more acidic character.

 

[Helixkam]

You also have to remember that the pi group is electron donating and hence destabilizes the proton when it is there, BUT it stabilizes the charge if the proton is removed because of the same reason. Just depends which way you want to look at it. Either way an alkene is more acidic.

 

[RC4L]

The double bond is electron withdrawing. Acids are electron acceptors in the lewis definition. Always look for the simple answer.

 

[RC4L]

Pi bonds are electron withdrawing specifically is what I should have said.

 

[hippiedoc14]

You also have to remember that the pi group is electron donating and hence destabilizes the proton when it is there, BUT it stabilizes the charge if the proton is removed because of the same reason. Just depends which way you want to look at it. Either way an alkene is more acidic.

Word! Lol

 

[SN2ed]

Thread closed. These types of questions belong in the Study Q&A form only.