![]() Is this the more acidic proton? Or is this the more acidic proton? So let's say a base comesĪlong and takes this top proton here. Here we have an organicĬompound, and our goal is to determine which is the more acidic proton. Out a more acidic proton, draw the conjugate baseĪnd look for resonance. Not as likely to donate its proton, and that's why On this one oxygen here, and that means this conjugateīase is not as stable, it's not stabilized by resonance. If we compare that to the conjugate base for ethanol, alright this isĬalled the ethoxide anion, alright we can't drawĪ resonance structure, we can't delocalize that negative charge. Stabilized by resonance, and since our conjugate base is stabilized by resonance, that meansĪcetic acid is more likely to donate to this proton,Īnd that's why we see a lower pKa value. Spread out or delocaliazed over two oxygens, and when you spread out a negative charge, that In reality, it's reallyĪ hybrid of our two resonance structures. So the negative charge on this oxygen is not localized to this one oxygen, it's spread out, it's delocalized. Oxygen to give the top oxygen a negative one formal charge. Let me make them blue, moved off onto the top This oxygen would now have only two lone pairs ofĮlectrons around it, and the electrons in Giving that top oxygen a negative one formal charge. Lone pairs of electrons around the top oxygen, So we would have for our top oxygen we would now have three Off these electrons on to the top oxygen. ![]() So for the conjugate base on the right, we can take those electrons in magenta, we can take these electrons here and we can move them in, and then we can kick Other factor to stabilize a conjugate base, and Negative charge an oxygen, so there must be some Pairs, one of those lone pairs would be the electrons in magenta. Would have another oxygen, this one with three lone ![]() Magenta are left behind on the oxygen, so theĬonjugate base would have a carbon double bonded to an oxygen here with two lone pairs of electrons. If acetic acid donates this proton, then the electrons in Let's draw the conjugateīase for acetic acid. Lone pairs of electrons, and one of those lone pairs The electrons in this bond, the electrons in magenta are We can explain why by lookingĪt the conjugate bases. ![]() So with the lower pKa value, acetic acid is more acid than ethanol. The value for your pKa, the more acidic the proton. This proton on ethanol has a pKa value of approximately 16, while this proton onĪcetic acid has a pKa value of approximately five. ![]()
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