Welcome back, students. So, since we have learned quite a lot about the phenomena of resonance. Now, this would be the last module where we will learn the application of resonance and proceed to the next electronic effect that is the introduction of a Hyperconjugation, alright.

So, when you talk about the application of resonance is basically the phenomena which helps us to understand the stability in the compound. So, here we are going to learn the acidic strength of carboxylic acid. As we know the fact any compound is acid if its conjugate base is stable. So, let’s look at the structure of carboxylic acid and if it donates a proton it forms a conjugate base which is called as a carboxylate ion, as we have seen earlier. Whereas now we are having two structures one is carboxylic acid and other is carboxylate ion. We will try and draw resonating structure of both of these because both of them are having lone alternate pie condition present in them. Which is one of the condition, which make the compound eligible to show phenomena of resonance. So, look at the carboxylic acid structure, one which is to the left. If we show delocalisation here, we will get a resonating structure which can be simply drawn like this. So, since the lone pair from the oxy atom has delocalised it is experiencing loss of lone pair, so it will carry a positive charge and the double bonded action gaining one addition electron pair so carry a negative charge. Which means now the newly obtained resonating structure carries a positive and negative charge, which means this structure is a charged separated structure. Now, try to work on the phenomena of resonance in this structure right here, which is nothing but the carboxylate ion. So, we again have a lone alternate pie condition present over here. So, if we show delocalisation will get one more resonating structure, which is equivalent of this carboxylate ion and you can see that there is only negative charge which is getting dispersed, delocalised. So, I will say that in this structure, one and two, which are the equivalent structures, you will find that these equivalent structures are charged delocalised structure. And whenever you get a charged separated structure, it is relatively less stable structure, on the other hand charged delocalised which means if only charge is getting dispersed, delocalised over several atoms then that delocalised structure is always a more stable structure than the other. And now you think about why are we calling this as an application? We say that any compound is acid if its conjugate base is stable. So, in case of carboxylic acid we saw that the conjugate base was a carboxylate ion which is giving us charged delocalised structure which is more stable. The more stable the conjugate base is, more stable is the acid from which that conjugate base is obtained. So, likewise, students, if you try and work on the sulphonic acid or if you try and work on the phenol, you will find that the charged delocalised structures are going to be more stable due to phenomena of a resonance. So, phenomena of a resonance can simply justify the acidic strength of the compounds. Well, having known this, now we are going to learn or see a simple sequence of acidic strength but the most important one. So, remember if we talk about this several acids the sulphonic acid is the strongest acid among all because its resonating structures are more stable. And after sulphonic acid the next acidic is carboxylic acid followed by we have a carbonic acid and then we have phenol, after phenol the structure or the compound which comes most acidic is methanol. Check out here we have methanol then relatively weaker acid is water and then we again have any other alcohol. Students, please look at this, methanol is only exceptional alcohol which is more acidic than water, rest all other alcohols are weaker acid than water and after this we have alkyne then we have ammonia and then we get alkene followed by alkane. So, this is a simple order of acidic strength of several compounds and you got to remember this, alright, students.

Now, having said that we are going to learn application of resonance and this is what, was the application of resonance. Now, we will see introduction to phenomena of hyperconjugation and there are several more names coming up right up there. We will justify each one of thes names but for the time being let’s look at this reaction.

So, we have tertiary butyl halide, so when this tertiary butyl halide is brought in presence of a polar solvent like water, again we will learn what is meant by a polar, then the C-X bond heterolytically breaks and which means the bond pair between C and X have been completely taken by X, so, the carbon has lost bond pair and therefore produces a positively charged species like this. This positively charged species where positive charged is carried by carbon is called carbocation. Now, let’s take that carbocation over here and remove all these methyl group. We will get the structure like this. So, this is a general representation of carbocation. Let’s take a replicate of that, so we are taking four such general representations of a carbocation. And now, in structure number one, right there we are fulfilling all the valences let’s say by methyl groups. So, now that carbocationic carbon which is carrying a positive charge is attached to three more carbons. So, the nature of the carbon carrying positive charge is tertiary. So, we call it as tertiary carbocation. Same way if we take two methyl groups attached to such carbon you will call it as the secondary carbocation. Since it is attached to two other carbons. So, if we move further, if I am fulfilling two valences H’s and one with methyl the nature of carbocationic carbon is primary and I fulfil all the valences with H’s, I will call it as the methyl carbocation, since it looks like methyl. So, the nature of carbon carrying positive charge decides the nature of carbocation and it is said that among all this the order of stability is found to be tertiary the most, then secondary, then primary and methyl is least stable carbocation and why is the order so, can be proven by the phenomena of hyperconjugation.

So, in the very next module, we will try and prove this order and with respect to this we will learn the phenomena of hyperconjugation but we will stop here in this module, thank you.