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I'm trying to figure out why #2 is more acidic than #1.
I thought that the ring would stabilize the base if a proton was removed from #1, making it more acidic than the terminal proton of the alkyne.
Which proton is more acidic?
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I'm trying to figure out why #2 is more acidic than #1.
I thought that the ring would stabilize the base if a proton was removed from #1, making it more acidic than the terminal proton of the alkyne.
would say that #2 is more aciding, based on orbitals. #2 is an SP orbital, whereas #1 is an sp3 orbital i think. Electrons favor S orbitals (and theres "more" s in an sp orbital, compared to an sp3) , because this is where theyre most stable. The more stable the base, the weaker the base, and the stronger the acid. So the terminal alkyne would have the strongest/most acidic proton. those are my thoughts.
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Haha thanks! Did Chad mention that in his videos at all?
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wasn't the rule CARDIO?
and that Resonance comes before Orbitals?
so hydrogen 1 is stabilized by resonance whereas hydrogen 2 isn't making hydrogen 1 more acidic? well, this is according to chad?
and that Resonance comes before Orbitals?
so hydrogen 1 is stabilized by resonance whereas hydrogen 2 isn't making hydrogen 1 more acidic? well, this is according to chad?
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Correct me if I am wrong. Based on Chad Videos and using the term CARDIO we can see that 1 is more acidic as it can stabilized by resonance by the benzene ring as well as the pi bonds in the straight chain that is attached to the benzene ring.
Also benzylic H is more acidic than allylic correct?
H 1 is benzylic as well as allylic. Thus, making it more acidic.
Agree? disagree?
Also benzylic H is more acidic than allylic correct?
H 1 is benzylic as well as allylic. Thus, making it more acidic.
Agree? disagree?
2 is more acidic due to the sp character of the bonds once its deprotonated. IE the conjugate base is more stable than the number 1 carbon.
to the above poster, removing the 1 H doesnt give you resonance. your benzyl ring assures that youre carbon to the left always has four bonds, IE it cant resonate with y our 1 carbon because that would give it five bonds.
to the above poster, removing the 1 H doesnt give you resonance. your benzyl ring assures that youre carbon to the left always has four bonds, IE it cant resonate with y our 1 carbon because that would give it five bonds.
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I'd say 1 is more acidic than 2. The way I was taught to look at these questions was to see which H can be plucked off more easily - an H bound to a terminal alkyne is very difficult to pull off because it is extremely stable. In other words, nature doesn't like it when a terminal alkyne's carbon forms a carbocation. The first H, on the other hand, is benzylic and due to resonance, that carbon doesn't have quite as good of a hold on the H as the terminal alkyne carbone does, meaning the H can be pulled off easier.
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Agree, 1 is more acidic due to benzylic position carbocation as well as you can form many resonance structures.
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Where did you take this question from? Do you have the right answer?
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Alkynes have a smaller pKa than benzylic Hydrogens, so 2 is more acidic.
I guess this is one of those exceptions where the CARDiO rule won't work.
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Thanks for the explanation. I went back to my orgo textbook. Alkyne indeed more acidic, therefore, #2 is more acidic.
guys, to about three of you up above. the carbon that becomes a carbocation when deprotonated (#1) is one of the most unstable structures in orgo chem. this is def number 2 due to sp character of the conjugate base
Yes that is true. Follow CARDIO to see which is the weaker conjugate base, which means the stronger acid.
Charge
Atom
Resonance
Dipole Induction
Orbital
As you can see that the charge are the same, atom is the same but resonance is different. Carbon 1 can make more resonance then carbon 2, therefore it is the more stable conjugate base which means stronger acid. For anyone who thinks otherwise, they are wrong. It is true that the alkyne has a more stable conjugate base compared to the alkene, due to the fact that it is sp compared to sp2 and the electrons are closer to the center of the atom so it's more stable, but since there is resonance, the electrons from the benzene will stabilize carbon 1 therefore it is more likely to give away it's hydrogen atom.
DQLEUCD, I am kinda confused now. I see what your reasoning is and that is how I was thinking but other people have said otherwise and their logic make sense as well. Now, if anyone else can elaborate on this issue please, it would be helpful.
There logic makes sense if you are comparing just those 2 things, sp and sp3, but you also have to take into account the resonance stability of the benzene which will stabilize the conjugate base of the sp3 carbon. The more stable it is the weaker the base. The weaker the base, the stronger the acid. You should look over chad's video's, it explains it very well and will clear up the confusion you are having with everyone else saying this or that. If you follow the CARDIO rule you are good to go, but there are some exceptions. This, however, is not one of the exceptions so follow CARDIO.
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Sorry to be a pain but this is certainly grounds for an exception here. For instance, look at toluene (methylbenzene) versus acetylene. By the cardio rule, toluene should be more acidic (due to greater resonance stabilization) but in fact acetylene is 10^18 more acidic (pKa of 25 vs. 43 for toluene).
In this instance it helps that there are two neighbouring pi systems to destabilize the negative charge into, but it is far from clear that this is enough to surmount the 18 orders of magnitude necessary to eclipse the acidity of the sp-hybridized acetylene hydrogen. For instance in allylbenzene (which is similar to the molecule we're talking about) the pKa is 34, which is still 10^9 less acidic.
Mneumonics are useful and handy but they have their limitations. It's one thing to argue from first principles (as Mol3 did, first post) and another to argue from a mneumonic.
I believe Mol3 got it right - proton #2 is more acidic due to the fact that it is sp hybridized carbon and the negative charge is closer to the nucleus.
On a side note I am surprised that this is even a question. Usually each question clearly illustrates some underlying principle, and in this case I can't tell what their logic is. It seems too ambiguous to be useful.
My $0.02 - James
