Like nitrogen and carbon, oxygen also shows this same hybridiz...

Question

Like nitrogen and carbon, oxygen also shows this same hybridization effect on acidity. Both of the following compounds can lose a proton from a positively charged oxygen with three bonds to give a conjugate base containing a neutral oxygen with two bonds. One of these structures has pK_a= −2.4, while the other has pK_a= −8.0.
a. Show the reaction of each compound with water.
b. Match each structure with its pK_a, and explain your choice.

a. Show the reaction of each compound with water.

b. Match each structure with its pK_a, and explain your choice.

Two chemical structures showing proton loss from positively charged oxygen, with highlighted protons indicating acidity differences.

Accepted Answer

All right. Hello everyone. So this question says that oxygen shows the same hybridization effect on acidity as carbon and nitrogen. Each structure below loses an H positive from its po positively charged oxygen resulting in a neutral oxygen. One of the structures has a pka of negative 3.2. While the other has a pka of negative 7.3. Part one is asking to show how each reacts with water by writing the chemical reaction. And part two is asking to match each structure with its PK and to justify. All right. So let's go ahead and start with the chemical reactions for both of these ions. Both of these ions. First of all are going to lose a proton during the reaction with water, meaning that both of them are going to react as acids in their respective reactions. So lets start off with the first one. Here we have propane, two iodine Oxonian or rather the propane two iodine Oxon ion. Here is our acid and now our base is going to be water. Now recall that during an acid base reaction, we're going to see the transfer of a proton or H positive from the acid towards the base. So when drawing out the products, first of all, water is going to have an additional proton, meaning that the conjugate acid is going to be H 30. But recall also that the charge is going to change as a result because an additional proton has been gained, the charge is going to increase by one. So if water is neutral, then the conjugate acid of water is going to have a positive charge since it's gained a new proton. And so here we can use that same principle to draw the conjugate base of our acid, which is the species that results after the acid loses the proton. So when drawing out the structure of the conjugate base, it's going to look very similar to the structure of the acid except now that acidic proton is no longer attached to oxygen. And because that proton has been lost, the charge is going to decrease by one. So if the acid had a positive charge, then the conjugate base is going to be neutral and that concludes our first reaction. So now let's repeat this with the second. So now our second acid is the isopropyl Oxon ion. So in this particular case, our base is still going to be the same because we're still using water. This means that the conjugate acid is still going to be HVO positive. And so our conjugate base is once again going to look very similar to the structure of the starting acid. The difference. However, is that now oxygen has one less proton, meaning that the charge is going to decrease from positive 1 to 0. So now this concludes part one of the question. So now let's scroll up to talk about the concept of PK A and how that relates to acidity recall that the PK values follow the same logic as the PH scale. What that means is that the smaller number is going to indicate the more acidic species. So the structure that has a PK A of negative 7.3 is going to be more acidic than the species with a PK A of negative 3.2. Recall that in the beginning of this video, in the starting text of the question, the hybridization effect on acidity was mentioned for oxygen. So that's what we're going to use to match each of our structures with their respective PK values. Recall that the acidity of a given substance is dependent to a certain extent on the stability of the conjugate base. Once again, the conjugate, the conjugate base is the species that results after the acid loses the proton during the reaction during the course of the acid base reaction. After that proton was lost, a lone pair of electrons was gained. So here the stability of that new pair of electrons on the conjugate base is going to help us determine the stability of that species. So here we have to talk about the hybridization of the orbital where that lone pair of electrons resides. Now, when it comes to that stability, it's worth mentioning that an electron in the two S orbital has a lower energy than an electron in the two P orbital. This means that the higher the s character of a given orbital, the more stable that pair of electrons is going to be inside of that orbital. So for example, sp three orbitals have an S character of 25% sp two orbitals, 33% and P orbitals 50%. So because of this, the stability of a lone pair in a given orbital increases when we go from SP three two P two. And finally, sp being the most stable increasing, the S character also has the added consequence of having those electrons be held closer to the nucleus. If it's held closer to the nucleus, it's going to be held more tight. So as stability increases, then reactivity decreases. So now let's compare the hybridization of both of our conjugate bases here. Now, our first conjugate base is propane to own. And our second conjugate base is propane two, the oxygen atom of propan two own is SP two hybridized. While the oxygen atom of propan 20 is sp to be hybridized. So because of this, we can compare their respective s characters and come to the conclusion that the SP two hybridized oxygen atom is going to be more stable or rather to be a bit more precise, the new lone pair of electrons on the SP two hybridized oxygen atom is going to be more stable. And so thats why our first acid should have a PK of negative 7.3 which means that our second acid should have a pka of negative 3.2. All right. So from here, we have arrived at our final answer. So I'm going to briefly pause this recording so that I can write out the final answer in a sentence for the justification part. All right. So now I resumed the recording and our final answer is as follows. For part one, we previously outlined both acid base reactions using the two given Oxonian ions and water behaving as a base in both cases. So now for part two, the first acid provided should have the PK of negative 7.3. Whereas the second should have the PK of negative 3.2. The justification for that reads as follows, the more negative PK A belongs to the more acidic structure with a weaker and more stable conjugate base due to the SP two hybridization of the oxygen atom given similar acids, the acidity of the O bond increases with the hybridization of the conjugate base in the order from SP three to SP two to SP and there you have it. So if you watch this video all the way through, thank you. So very much. For doing so. And I hope you found this helpful.

Key Concepts

Hybridization and Acidity

pKa and Acidity

Resonance Stabilization

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