Rank the following alcohols in order of descending pK...

Question

Rank the following alcohols in order of descending pK_a value. Explain your ranking.

Accepted Answer

Hi, everyone. Let's look at our next problem. It says arrange the following alcohols in decreasing order based on their PK A values explain. And we have four molecules labeled 123 and four. They each are a three carbon chain with an alcohol attached to the last carbon. And then each have different substituents. On carbon two, the middle carbon molecule one has one chlorine molecule, two has two, chlorine molecule three has one bromine and molecule four has two. Brom means and in my answer choices A through Dr I'm just going to read the order and the problem they're printed with greater than signs. So again printed in decreasing order A is 3142. And it says the fewer the number of Halide atoms and the less electron, the atom, the greater the inductive effect which stabilizes the conjugate base and makes the acid stronger choice B same order 3142. But the reason is the greater the number of hide atoms and the more electron the atom, the greater the inductive effect choice C has the order 2143. And it says the fewer the number of hide atoms and the less electron the atom, the greater the inductive effect, which stabilizes the conjugate base and makes the acid stronger or D 2143. So the same order as C and it says the greater the number of hide atoms and the more electron the atom, the greater the inductive effect which stabilizes the conjugate base and makes the acid stronger. So it's clear that the inductive effect is the key concept here. And the inductive effect is that when there are nearby electron atoms, the induction by then stabilizes a negative charge or a lone pair. And we can see why that would work. Let's look at number one, for example, we have, we're looking at the acidity of the alcohol that oxygen on the alcohol. How likely is it to give up a proton? And the oxygen is attached to a carbon and the carbon adjacent to that has a chlorine chlorine is an electron atom quite electron. So it will have a partial positive charge or partial negative charge. Excuse me, it's electron. The result of that is that it will give its attached carbon a partial positive charge. Now imagine if we lose a proton from this alcohol and now our electron has a, our oxygen, excuse me has a negative charge. Well, now we've got this negative charge close to this partial positive charge and we have nearby opposite charges. So you can see how that would stabilize the negative charge on this oxygen if alcohol lost its proton. So we want to look at which of these molecules has the greater inductive effect. Because the greater inductive effect means there is a more stable conjugate base because that negative charge is stabilized. If the conjugate base is more stable, the acid is stronger. It's happier to give up a proton if its conjugate base is more stable. So we can look at the reasoning in our answer choices and use that to eliminate some answer choices if any of them has that explanation wrong. But as we look through our answer choices, we remind ourselves that no, every single one says the greater inductive effect stabilizes the conjugate base. So the explanation is correct in that area for each one. So now we need to move on and looking to our specific molecules and what distinguishes them. So we've got two molecules that have two halogen atoms, two molecules that have one. And we've got chlorine versus bromine. Well, obviously two X atom, since we use X to symbolize a halogen will lead to a greater inductive effect. Then one and then finally, chlorine versus bromine. Well, the more electron atom will have a greater inductive effect. And bromine is below chlorine on the periodic table. So br below cl as we move down the periodic table, we get less electron negative because the electrons are getting further away from the nucleus. So that means chlorine is more electron, I'll put E minus NG sort of as the abbreviation there. So when it comes to comparing chlorine to bromine, then chlorine will have greater induction and cause the proton to be more readily lost. So now the last little piece we need to know is PK A. How does that relate to a stronger acid? Well, this is somewhat easy to remember because if we think that a low PH means an acidic solution, a low PK A equals a stronger acid. So if we want to arrange in decreasing order based on PK A values that would be increasing order of acidity, so we should be from least acidic to more acidic. So that's right. Least acidic, two more acidic, keep straight. So our least acidic again will have the least electron atom first. And that would be a single bromine. And so that is number three. So number three should be our first molecule. So we'll see least acidic nearby molecule three. So choice A has number three. First. Choice B has number 33 1st. So C and D which have number two first can be eliminated. Then after a single bromine, the next would be a single chlorine, which is number one. So we see both of those and we know they have the same order. So if one of them is correct, the other is then of course, two chlorine or sorry, it's easy to get mixed up here. The next would be two brom means which is number four and then two chlorine, which is number two. But again, no difference there between A and B. So what's the difference in A and B A says the fewer the number of highlight atoms, hall light atoms and the less electron, the atom, the greater the inductive effect. Well, this is incorrect because the truth is in choice B, the more he light out you have, the more electron negative they are the greater the inductive effect because it has to do with the electron withdrawing nature of that hide that's there. So what's the correct order decreasing based on PK A and Y choice B 3142, the greater number of hide atoms, the more electron the atom, the greater the inductive effect which stabilizes the conjugate base and makes the acid stronger. See you in the next video.

Rank the following alcohols in order of descending pK_a value. Explain your ranking.

Key Concepts

Acidity of Alcohols

Inductive Effect

Comparative Analysis of Substituents

Watch next

Rank the following alcohols in order of descending pKa value. Explain your ranking.

Key Concepts

Acidity of Alcohols

Inductive Effect

Comparative Analysis of Substituents

Watch next

Rank the following alcohols in order of descending pK_a value. Explain your ranking.