Next we will take a look at how the structure of the molecule affect the acidity of the acids. So previously we have talked about this rule. If the an is more stable the acid is stronger. So we're talking about if the conjugated base is more stable then the acid will be stronger. So if you are asked to compare the acidity of acids you need to take a look at the stability of the conjugate base. Okay. So the first effect is the electro negativity of the atom with the negative charge that may affect the stability of the conjugate base. So let's take a look at these two molecules. The first one is an alcohol RO. Second one is an amine R and H2. So if I ask you to compare the acidity of these two molecules, what you need to do is to get the structure of the conjugated base. The conjugated base will be uh will have one hydrogen less than what's present in the acid. The hydrogen is the most acidic hydrogen in the molecule that's being lost. Okay. So, here is the structure of the conjugate base. For alcohol, it's R minus when the proton is lost. For amine, the conjugated base is R NH minus. So now we have the negative charge on the oxygen in a oxide anni and the negative charge on the nitrogen in the amid ani. We need to compare which of these two an is more stable. So the electro negativity of the atoms that has the negative charge will have a big effect on it. So we say because oxygen is more electro negative than the nitrogen. Oxygen can stabilize more negative charges. So the RO minus is more stabilized and because the conjugated base is more stabilized the acid R O the alcohol will be a stronger acid. Okay. So as a conclusion alcohol will be a stronger acid than amines. So let's check the pKa values to verify this. The pKa for alcohol is 16. The pKa for amine 38. Okay. Alcohol has lower pKa value. Alcohols are stronger acids than amines. Okay. The second effect is the size of the atom with the negative charge. So let's take a look at these two molecules. RSH the thio and R O the alcohol. These are the acids we are looking at. Which one is a stronger acid? We need to take a look at the structure of the conjugate base and see which anion is more stabilized. So now we have the conjugate bases R S minus and R O minus. The sulfur and oxygen these two atoms are in the same column of the periodical table. So the size of these two atoms will be much different from each other. Right? Sulfur is in the third column. It's much bigger than oxygen. So we say because sulfur is bigger, it has the empty d orbitals in the third period. So the negative charge has more space to go. and that negative charge will be more stabilized. So RS minus is more stable. The conjugated base is more stable and the acid RSH will be more acidic. Okay, so thio is more acidic than alcohol. Let's check the pKa value to verify it. pKa for thio is 10. pKa for alcohol is around 16. The has lower pKa. So thio is more acidic. The third effect is the deloization of the negative charge. So let's take a look at a caroxyic acid and an alcohol. Which one is more acidic? Again, we need to take a look at the structure of the conjugate base and see which anine is more stabilized. So here are the two structures for the conjugated bases for caroxilate anal group there the CO double bond right next to the oxygen with the negative charge. We can actually draw resonance structures for that ani. The double bonded oxygen will help to share the negative charge. Okay. Actually these two resonance structures are the same. So the two oxygens will share that negative charge. each will have 1/2 of the negative charge in a real structure. Okay? So the negative charge is deloized. When the negative charge has more space to go, it's more stabilized as compared to the charge in R minus. It's localized on one oxygen. Okay. So as a result the caroxilate will be more stable and the acid the caroxilic acid will be more acidic. Again let's check the pKa value to verify this. pKa for caroxilic acid is five. pKa for alcohol is 16. The lower the pKa, the stronger the acid. Caroxilic acid is a stronger acid. The fourth effect we are going to look at is called inductive effect. Inductive effect is the electronic effect that is transmitted through sigma bonds. The electronic effect here could be the electron withdrawing or electron donating effect but they are transmitted through sigma bonds as opposed to resonance effect. Okay. So let's take a look at acetic acid and the sum of the substituted acidic acid. So we have four molecules here. Acidic acid, chloro acidic acid, dchloro acidic acid and the triricchloro acidic acid. Now we are going to compare their acidities. Again we need to take a look at the stability of the conjugate bases. So let's suppose here are our structures of the conjugated bases. We want to see which an is more stabilized. Okay. So as we go from the first annion to the second annion, you can see the second ani has a chlorine atom that is three bonds away from the negative charge. Chlorine is more electro negative than carbon or the hydrogen. So chlorine will be pulling electron density away from the carbon toward itself. Okay? So the chlorine will make that CH2 will make that carbon positive. That carbon will bear some positive charge, partial positive charge. And the positively charged carbon will affect the atoms attached to it. It will become electron withdrawing as well. It's going to pull electron density away from the atom next to it. So this will make the carbonial positive charge. Okay? and that carbon will pull more electron density away from the atom next to it which will be the oxygen with the negative charge. So the electron density is being pulled away from the oxygen toward chlorine and this effect is through the sigma bonds. Okay, chlorine carbon bond carbon carbon bond and a carbon oxygen bond is being put away through the sigma bonds. So this is called the inductive effect because chlorine is electron withdrawing. So it will help to stabilize the negative charge through inductive effect. Okay. As we go to the third molecule, we have two chlorine atoms. So we have stronger electron withdrawing effect from those two chlorines which means the negative charge will be more deoized. It will be more stabilized. Now, as we go to the last molecule with three chlorine atoms, the electron withdrawing effect will be even stronger and the negative charge will be more stabilized. Okay, so this is the inductive effect of the chlorine atom. So from what we just talked about, we know the order of these four anjugated base their stability is going to increase as we go from left to right. Okay, that means the acidity of the acids on the first line is going to increase from left to right due to the inductive effect of chlorine. Okay. So now let's check the pKa values to verify this. As you can see, the pKa values decreases going from left to right. That means the acids get stronger as we go from left to right. All right. Now I want you to take a look at another two molecules regarding the inductive effect. The first molecule is chloro acidic acid. The second molecule is three chloro propinoic acid. The difference between these two structure is there is one more CH2 between chlorine and the caroxile group in the second molecule. Okay. So if you are asked to compare the acidity of these two structures again you need to take a look at the structure of the conjugate base to see which ani is more stabilized. Now, as I just explained, this deals with the inductive effect of chlorine. But the inductive effect will decrease dramatically as we have more sigma bonds between the electron withdrawing atom and the negative charge. So the second annion we are adding one more atoms between the chlorine and the negative charge that will affect the inductive effect that will decrease the inductive effect. So as a result the negative charge in the first molecule will be more stabilized as compared to the second one. So the first acid will be more acidic. Okay. Now let's verify the acidity the pKa values. First the molecule has lower pKa value. It's the more acidic acid. The last effect regarding acidity is the hybridization of the carbon atom. So let's take a look at these three molecules. So if you are asked to compare the acidity of these three molecules what would you do? So we are talking about the proton that is attached to carbon carbon atoms that are acidic. Again, we need to take a look at the conjugated base and see which one has a more stable annion. All right. So now we have several carbanines. The difference among them is the carbon has different hybridization. The first carbon is sp hybridized triple bond. The second carbon is sp2 hybridized double bond. The last carbon is sp3 hybridized with only single bonds. When we talked about the hybridization, I've given one column with the percentage of s. I hope you still remember that for SP there are 50% S. SP2 there are 33.3% S and sp3 only 25% of S and as you know S orbital is close to the nucleus which means S will be more electron withdrawing because it's more um close to the positive positive charged side of the atom. So the more s we have the the atom the hybridized atom will be more electron withdrawing. So when the atom is more electron withdrawing it can stabilize more negative charge. Okay. So as a result the sp carbon will stabilize the negative charge more than the other two which means the aine the triple bond will be the most acidic molecule among the three. Okay. So let's verify the pKa value 25 44 51. So as we go from left to right the acidity decreases. All right. So I hope from what we just talked about from these five types of um molecular structure effect you will be able to predict the acidity of different types of acids. Always keep in mind if you are asked to compare the acidity of an acid of of two acids, you need to take a look at the stability of the conjugated base. If the annion is more stabilized, the acid will be stronger. All right. Now, moving on to the last topic for this chapter. I'm going to briefly mention the Louis acid base theory. So previously what we have talked about is the Bronstead acid base theory. We are talking about protons. We are focusing on protons. We say acids are proton donors. Bases are proton acceptors. For the Louis acids base theory, we are going to focus more on electrons not protons. Okay. So this will make Louis acid base theory more general. It covers the Bronstead acid base theory but it will cover a wider range of acids and bases. So in Louis acid base theory an acid is an electron pair acceptor and a base is an electron pair donor. Okay. So we are focusing on electrons. Now the previous reaction that we have talked about when we have a base water grabs a proton from an acid HBr to give the product. We know HBr is the acid because it give away the proton. We know water is the base because it grabs the proton. But from another point of view, you can say water is the base because it donates the electron to a proton. It's an electron donor. It gives away its electron to a proton. And acids, they are proton donors. Yes. But at the same time they are electron acceptors. Okay, HBr accepts electron from water in order to form the product. Okay, so from this point of view the bronze acids is the same as a Louis acids and the Bronstead bases are also Louis bases. Okay. But as I said, Louis acids and bases covers a wider range of acids and bases and we will see a mo we will see an example in just a minute. But one more things that I want to mention here is that electron pair acptors are things that like electrons. So we will call them electrofiles. Electrofile means electron uh means something that loves electrons. And on the other hand, electron pair donors, we will call them nucleophiles. That's something that loves nucleophile. Loves nucleus. Okay. So if we change to the to the uh adjective we can see Louis acids are electrofphilic and the Louis bases will be nucleophilic. All right let's see one example here. Suppose we have a carboation. There's a positive charge on a carbon and we have bromide. One reaction that can take place between these two species is a formation of a bond between the carbon and the bromine. And in this reaction bromine will donate the electron to the carboation to make a bond. Okay. And this reaction involves no proton at all. So from Louis sorry from Bronstead acid base theory this is not an acidbased reaction. No proton is involved. However, from the Louis acid base theory, you can tell that bromine is donating the electron to the carboat. Bromine is the electron pair donor. So as a result, it's a Louis base and the carboation is the electron pair acceptor. So it's a Louis acid from Lis acid base theory. This is also an acid base reaction. Okay. So just want to give you this idea. The majority of the organic reactions will be involving electron donors and electron acceptors. So the majority of the organic reactions can be classified as Louis acidb reactions. Okay, acids and bases. All right, so this will conclude chapter 4.