Organic Chem

[Temperature101]

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Which of the foloowing explanations accounts for the pKa of 1,3 cyclopentadiene beong only 15, while the pKa for hydrogen on other sp3-carbon is around 49?

A. The strain of the five-member ring forces the proton off.
B. The proton is involved in resonance.
C. The conjugate base is aromatic.
D.The streric hingrance of the sp3-carbon weakens the C-H bond on that carbon.

Answer: C

I got the right answer by process of elimination but I am having a hard time understanding TBR explanation.

 

[Jepstein30]

Strength of acid is directly related to the stability of conjugate base.

So since the conj. base would be aromatic, it would be much more stable than the conj. base formed when donating the proton on the other sp3 carbon.. therefore it is more acidic and has a lower pka.

 

[cloak25]

Strength of acid is directly related to strength of conjugate base.

Isn't the stronger the acid, the weaker its conj. base? or am I missing something...

 

[Jepstein30]

Isn't the stronger the acid, the weaker its conj. base? or am I missing something...

Two different (but related) things!

A stronger acid's conjugate base is MORE STABLE.
A stronger acid's conjugate base is LESS BASIC (i.e weaker).

They kind of interplay.. the conjugate base is less basic because it is very stable.. and hence has 'no reason' to accept a proton (act like a base) when its good as is.

 

[CodeRedDew]

Isn't the stronger the acid, the weaker its conj. base? or am I missing something...

Maybe he shouldn't have said strength but rather stability. The strength of an acid is directly related to the stability of its conjugate base. His explanation is right, but I can see how the words would get confusing.


Sent from my iPhone

 

[Jepstein30]

Maybe he shouldn't have said strength but rather stability. The strength of an acid is directly related to the stability of its conjugate base. His explanation is right, but I can see how the words would get confusing.


Sent from my iPhone

Yea, probably would have been a better word choice.

I'm used to "strength" in o-chem to mean stability and "strength" in gen chem to mean "strong acid/base"

 

[Temperature101]

Isn't that anything related to aromatic compounds are not gonna be tested in the mcat? The stability of this conjugate base has to do with aromaticity? Am I wrong? I have trouble with the term stability in organic. Can someone give a quick overview about it with regards to acid/base chemistry.

 

[indianjatt]

The stability of the conjugate base here does depend on aromaticity as you've said. Since the deprotonated version of 1,3-cyclopentadiene experiences much greater stability due to full conjugation and aromaticity, it will be VERY stable (relatively speaking). This is why 1,3-cyclopentadine has such a low pKa. By losing that hydrogen, it goes to a rather stable state.

Regarding stability and acid/base chemistry, if the conjugate base is very stable (due to polarization, electronegativity, or resonance), then the conjugate base will "not do much." It won't act as a very good base because it is already PRETTY stable. That's why if you have acids whose conjugate bases are very stable, they will easily lose that proton because they will go to a lower and more favorable energy state. I don't know if I'm making any sense lol, but the more stable the conjugate base is (see if the molecule can resonate and delocalize the charge, whether the molecule is EN enough to handle the extra - charge, or whether it is polarizable), the stronger the acid.

 

[Temperature101]

The stability of the conjugate base here does depend on aromaticity as you've said. Since the deprotonated version of 1,3-cyclopentadiene experiences much greater stability due to full conjugation and aromaticity, it will be VERY stable (relatively speaking). This is why 1,3-cyclopentadine has such a low pKa. By losing that hydrogen, it goes to a rather stable state.

Regarding stability and acid/base chemistry, if the conjugate base is very stable (due to polarization, electronegativity, or resonance), then the conjugate base will "not do much." It won't act as a very good base because it is already PRETTY stable. That's why if you have acids whose conjugate bases are very stable, they will easily lose that proton because they will go to a lower and more favorable energy state. I don't know if I'm making any sense lol, but the more stable the conjugate base is (see if the molecule can resonate and delocalize the charge, whether the molecule is EN enough to handle the extra - charge, or whether it is polarizable), the stronger the acid.

You make perfect sense...Thanks.

 

[gtbROX]

Strength of acid is directly related to the stability of conjugate base.

So since the conj. base would be aromatic, it would be much more stable than the conj. base formed when donating the proton on the other sp3 carbon.. therefore it is more acidic and has a lower pka.

True, but I like to say that a strong acid has a weaker conjugate base (equivalent to saying that the conjugate base is more stable).

Aromatic compounds are very stable so forming them tend to be favored. Since acid/base reactions are actually equilibrium reactions (with the pKa describing the nature of that equilibrium - the ratio of protonated vs deprotonated forms in solution), it makes sense that the pKa of cyclopentadiene would be so low compared to normal alkanes and alkenes. Hope that helps too.