 Which of these should be more stable? The phenoxide ion or the acidity ion? Well, if you draw the resonating structures of phenoxide You will get four more resonating structures, right? The lone pair of this oxygen atom can come over here and this pi electrons. They can move over here So this will lead to our first resonating structure or we should say the second resonating structure because remember None of these are the actual molecule. So in this resonating structure this negative charge This lone pair of electrons that we have over this carbon This can come over here and this pi electrons can move over here and this can go on and So we are going to get a total of five different resonating structures, right? If you look carefully these two Structures are also slightly different. The arrangement of these pi bonds are different So even these two are two different resonating structures, right? Now if we come to the acetate ion then acetate will only have one other resonating structure, right? This lone pair of oxygen atom can go over here and these pi electrons can shift over here and This is the only possibility So we'll have a total of only two resonating structures in case of acetate, right? Now based on whatever we have seen till now a greater number of resonating structures will make the molecule more stable, right? A greater number of resonating structures implies that the charge is getting delocalized more thereby reducing the charge density and making it more stable, right? So if you look at the resonance hybrid of phenoxide, which is closer to the actual molecule The charge is not localized over the oxygen atom, but it's getting distributed over oxygen and three other carbon atoms, right? So the charge is getting spread out over four atoms in phenoxide But if you look at the resonance hybrid of acetate The negative charge gets spread out only between two oxygen atoms, right? So therefore the spreading out of negative charge is not going to be as much in acetate Compared to phenoxide. So therefore the phenoxide ion The phenoxide ion should be more stable More stable while the acetate ion should be less stable, right? However, it turns out That this is actually not true a phenoxide ion is in fact less stable It's much less stable compared to the acetate ion Now one of the ways in which we can actually test this in which we can actually figure out which one of this is more stable One of the ways is by looking at the acidic strength of their corresponding acids phenol and acetic acid The strength of an acid is a measure of how easily these acids can give out this hydrogen in the form of H plus and Chemists measure this acidic strength in terms of something called the pKa value In fact lower the value of pKa means stronger will be the acid So as you can see because the acidic acid has a lower pKa So this basically means that it's actually much easier to break this acidic acid in the form of acetate and H plus Compared to phenol phenol is a much weaker acid So it's much more difficult to break phenol into phenoxide and H plus So what this basically tells us is that this acidity ions are much more stable So it's easier to break this acidic acid into acetate and H plus Compared to phenol phenoxide is not that stable. So it's more difficult to break the phenol into phenoxide and H plus So therefore because acidic acid is the stronger acid So the acetate ions that are formed this acetate ions have to be more stable than phenoxide, right? Now why is this happening? Shouldn't having a greater number of resonating structures mean a greater delocalization of charge and therefore greater stability Shouldn't that happen? Well, yeah, it does but if you look at this phenoxide ion closely You'll realize that this delocalization of electrons actually builds up a negative charge over these carbon atoms, right? Now having a negative charge on oxygen, which is a highly electronegative element. It loves electrons Having a negative charge on oxygen leads to a much greater stability Compared to having negative on this less electronegative carbon atoms, right? So energetically all of these resonating structures are not equivalent One and five are in fact going to be much more stable compared to two, three and four, right? So therefore the contribution of one and five to my resonance hybrid will be much greater compared to two, three and four In other words, this hybrid the actual molecule the actual phenoxide ion will resemble more of structure one and five Rather than two, three and four So what this basically means is that in phenoxide delocalization can happen and does happen and we can have many different resonating structures But because this delocalization leads to a buildup of negative charge on the carbon atom So therefore this delocalization the extent of this delocalization won't be much So in the resonance hybrid most of the negative charge will still be over the oxygen atom So the oxygen atom will carry almost all of the negative charge While only a small amount of charge will get developed over this carbon atoms Okay So even though we can draw many different resonating structures even though the quantity of Resonating structures will be high in phenoxide, but the quality But the quality or the extent of this Delocalization will be low the quantity will be high, but the quality will be low So even though the charge this negative charge will spread out, but it will only spread out to a very low extent On the other hand if you look at the acetate ion if you look at the resonating structures of acetate Then you can see that both of these Resonating structures are exactly identical in energy, right? In both of these the negative charge is over the oxygen atom So the quality of resonance is going to be super high out here And this negative charge is going to get distributed evenly between both of these oxygen atom So therefore in acetate even though the quantity of resonating structures is low But the quality of resonance is high It leads to the formation of resonating structures which are equivalent in energy So this delocalization can be super smooth. It can happen very effectively So therefore the reduction in charge density of this oxygen atom turns out to be much greater in acetate Compared to phenoxide making it much more stable compared to the phenoxide So going ahead This quality of resonance that's happening is something that you should be mindful of while comparing the stabilities due to resonance