 The goal of this video is to understand why ortho-substituted benzoic acids are more acidic than para and meta-substituted benzoic acid. As a reminder, relative to this carboxylic group, whenever there is a group attached to the adjacent carbon, we call this and this position ortho. When there is a group attached to the third position to the carboxylic carbon, we say it's the meta position and when the group is attached at the fourth position relative to the carboxylic carbon, we call that para-substituted benzoic acid. To understand why ortho-substituted benzoic acid is more acidic than para and meta, we should start comparing their acidic strength. And we have learnt in our previous videos that to compare the acidic strength, you have to write the conjugate base of that molecule. And after writing the conjugate base, you compare the stability of the conjugate bases. Till now, we have been using electronic effects to compare the stability, but today, let's study something more. To understand its acidic strength, we need to find the stability of its conjugate base. And the conjugate base is when we remove the most acidic hydrogen. We see one hydrogen here and four more hydrogens attached to this benzene ring. Let's assume for a second that this G group, which is any generic group, has no hydrogens in it. We know that the electronegative difference between carbon and hydrogen is not too much that it polarizes hydrogen atom. On the other hand, we have this super electronegative atom, which is oxygen, which possesses a partial negative and hydrogen possesses a partial positive charge, which attracts a neighboring water molecule and is taken away as hydronium ion. What we are left behind is our conjugate base, which means this oxygen acquires a negative charge. So this is how our conjugate base starts looking like. Till now, we have been studying the stability of conjugate base based on electronic effects. Let's try to understand what happens beyond electronic effects. I'm talking more like spatial arrangement. We'll be looking at how the atoms of these molecules are arranged in the 3D form. And we'll also be looking at something which also depends on spatial arrangement, which is hydrogen bonding. Let's look at the spatial arrangement of this molecule. Assume that this bluish line is a plane. Because carboxylate ion has two electronegative atoms, it attracts the electron or withdraws the electron from the benzene ring. When it pulls away the electron, the pi bond is pulled here, which pushes this pi electron to become negative charge on oxygen. This only happens because the p orbital of this carbon and the carbon right here are on the same plane, which looks like this. That means the interaction goes between this p orbital and this p orbital makes the resonance between the benzene ring and carboxylate ion possible. But now, if I put a group, any substituent, at the ortho position, there comes a small issue. The electron cloud of this group starts repelling with the electron cloud of this group. This repulsion creates instability and to minimize this repulsion, become more stable, the bond is rotated. The resulting carboxylate ion shows that the oxygen protrudes out of the plane and the other oxygen is downwards to the plane. It is very specific to each molecule how much will be the deviation or what will be the angle of deviation. But a caveat to make this happen is that group G has to be bulky. A few examples of bulky groups could be OR where the R is our alkyl ring, any alkyl which could be CH3's isopropyl and so on. The other could be nitro that is NO2 and more. Once such bulky groups are attached at the ortho position, the carboxylate rotates its bond and protrudes in a different plane. Which means that the p orbitals of this carbon are not in the same plane as the p orbital of this carbon. This effectively means that there will be no p orbital interaction between these two carbons. That means the resonance between this benzene and this carbon is now broken. Which means that our carboxylate ion which was going through two types of resonance, one where the carboxylate ion was oscillating the negative charge between these two oxygen and the other where this carboxylate was also resonating with the benzene. After removing this carboxylate out of the same plane, the resonance between this benzene and this carbon is now inhibited but the resonance that goes on between these two oxygens still remain. Instead of this negative charge being cross conjugated, it now has only one path to go. This one path is better because carboxylate ion shows a better quality of resonance. This is mainly because it creates equivalent resonating structures. Quick reminder, equivalent resonating structures are those structures which contribute equally to the resulting hybrid. This resulting hybrid is equally 50% this and 50% this. The quality of resonance is increased which means that our conjugate base is now more stabilized giving us better acidity. This effect of inhibiting resonance is called steric inhibition of resonance. It is only half right to say that the resonance has been inhibited. It has been only inhibited partially. What if we don't have a bulky group? What if we have a group like OH? It's a pretty small molecule but still if we attach it at this orthoposition it increases the acidity. Well this happens not because of steric inhibition of resonance, it happens because of something called hydrogen bonding. This oxygen and this hydrogen make a hydrogen bonding called intramolecular hydrogen bonding which improves the stability substantially. If you are wondering why this H is not the most acidic hydrogen, this is because after plucking this hydrogen the resonance which develops in this phenoxide ion is not as effective as the resonance which happens in the resulting carboxylate ion. SIR and hydrogen bonding has been given a special name. This special name is called the ortho effect. Having said all this these two are not the only reason for ortho effects. There are some reasons that we are not still able to decipher but till then you will not have to use any of them in your exams. Also ortho effect usually increases the acidity. It is not a thumb rule. But in general ortho substituted benzoic acids are more acidic than meta and para substituted benzoic acids.