 Hello friends, good morning, welcome back to Centrum Academy again. Friends last class we have discussed about the comparison of acid, acid and base. We were doing acid and base comparison of acid and base. We have gone through a few examples of acidic order right and I have told you that we will discuss some more examples into the next session and then we will again compare the basic strength. So, we will see some more examples of acidic strength. How do we compare acidic strength? For example, you see if we compare these two, these three examples. You see here this is carboxylic acid and this is sulfonic acid. Now, in this sulfonic acid if you form the conjugate base here, we will get negative sign here and negative sign here. Here we have equal resonance right or we can also say more number of resonance because you see in this example we have both molecules shows equal resonance okay, but we have more number of resonating structure here. Hence it is more stable. So, what we can see when the molecule shows when the molecule shows equal resonance then the molecule having more RS resonating structure are more acidic right. When I say molecule having this molecule means what? This molecule is the conjugate base of the given molecule of the given molecule. The conjugate base of that given molecule like suppose here we have equal resonance. So, here we have more resonating structure possible. This is more acidic. Simply, similarly here you see phenol and CO32 minus okay. This is the another type CO32 minus is the conjugate base of this and for this the conjugate base is nothing but this phenoxide ion okay. So, here we have equal resonance and because of this equal resonance H2CO3 is more acidic okay. Now you see this molecule this one is having plus I effect because it is at meta position. You see OH is here it is not here at the top right. So, this is the meta position now this is ortho position. So, plus H effect this is the para position plus H effect right. All these positive effects will decrease the acidity okay. All these positive effects decrease the electron releasing effect decrease the acidity of molecule plus H is dominating over plus I. So, these plus H will decrease more the acidity will be least here in any one of these two cases okay. So, this one is most acidic the rank will be first this two if you compare apart from plus H we have plus I also here because plus H and plus H we cannot compare according to that. But we obviously know here that plus I is more effective over here in comparison to this okay. So, this will decrease even more first this will be second and this will be third okay. So, like this we compare. Now another example you see I am solving from this one okay. This is minus M this is minus I and this is minus M apart from minus M we have minus I also possible here right. And all these effects increases acidity okay. So, the order of acidity will be first one is this second one will be this and third one will be this okay C T 3 C S 3 the effect here it is plus H plus H and plus H okay. So, plus H is maximum for C S 3 if you remember C T 3 shows maximum plus I effect okay because of least bond strength here it is it is hyper conjugative tendencies maximum. So, C S 3 plus H maximum and because of plus H what happens because of plus H acidity decreases right. So, order of acidity will be this will be 1 this will be 2 and this will be 3 okay. Here you see O C S 3 here it is plus M this is minus M and this is plus M okay plus M minus M plus M sorry not minus M it is minus I meta position minus I okay. So, plus M decreases acidity right apart from plus M we have plus minus I also possible here right because of plus M acidity decreases because of plus M acidity decreases and minus I acidity increases right. So, here if you compare the acidity of this one is maximum then we have this and then we have this 1 2 and 3 okay like this we compare the acidic strength. Another example you see suppose we are comparing these two molecule these two molecules okay. So, now you see this NO 2 groups shows minus M effect and here also we have minus M effect right, but what happens here the tendency of NO 2 is minus M, but what happens here because of repulsion here and here NO 2 changes its plane to minimize repulsion okay and then it converts into this C S 3 right. So, one oxygen will go into the plane and other one comes out of the plane to minimize the repulsion here right. Now, because of this what happens the NO 2 groups changes its plane. So, resonance is not possible here. So, minus M effect is not effective over here, but here what we have minus I little bit of minus I because minus I distance dependent right. So, at this distance the effect of minus I will be very least or negligible okay, but if somebody ask you what kind of effect possible here minus I we can say, but this also we do not consider right. So, here minus M is not possible here minus M possible right because of minus M the acidity increases order of acidity will be this right few examples of benzoic acid will see benzoic acid and its derivative okay now you see here we have plus H here we have plus I plus H decreases acidity okay acidic order will be this. This is plus H plus H and plus H and plus H we have maximum for C S 3 acidity will be minimum order is this NO 2 minus M OH plus M C S 3 plus H. So, maximum stability we have for minus M right then plus H and then plus M plus M will decrease the acidity maximum okay. Now we compare the acidity of benzoic acid or we will discuss the acidity of benzoic acid when any group present at ortho position right any group present at ortho position. So, now you see what happens here the conjugate base of this is nothing but H right. So, here what happens in this group steric repulsion will be there now because of steric repulsion the CO 2 minus ion changes its plane changes its plane right. So, what happens when its plane changes to minimize the repulsion the CO 2 the plane of CO 2 is different here O minus double bond O and here we have C S 3 as this. Now because of this change in plane this ion is not in resonance with ring okay because we know for ring we must have planar molecule. So, when this CO 2 changes its plane one oxygen is coming towards the observer other one is going away right. So, this whole molecule is not planar now and hence this ion is not in involve in resonance with this ring okay and but what happens this negative charge is in resonance with this oxygen this pi bond here negative sigma pi okay. So, this CO 2 minus ion is in resonance with its own but it is not in resonance with the ring okay. So, what happens here since this is in different plane this is in different plane. So, this becomes a separate ion which is in resonance with each other and it shows the kind of resonance if you see here it shows equal resonance because it is equally distributed between the two oxygen atom right. Now because of this equal resonance this becomes stable right but this kind of so what happens what we say here since this CO 2 minus is not in resonance with this ring. So, we call it as a steric inhibition of resonance okay this effect because of any group present at ortho position what happens here we have a steric inhibition of resonance in short we write it as s i r sir okay means resonance is prohibited because of steric hindrance okay. So, what is this steric hindrance and s i r we call it as s i r effect or we also call it as ortho effect both are same thing means what when any group present at ortho position in the in this ben in benzoic acid then because of steric hindrance CO 2 minus changes its plane right and shows equal resonance within itself okay. So, because of this equal resonance the CO 2 minus ion here is more stable than in other cases where we have ortho sorry where we have para and meta position and hence the acidity of ortho substituted benzoic acid increases okay. So, I will write down all these things here any group present at ortho position of benzoic acid ortho position of benzoic acid shows hindrance or steric hindrance to minimize this hindrance to reduce hindrance CO 2 minus ion changes plane and shows and shows equal resonance and shows equal resonance effect of this is what hence ortho substituted ortho substituted benzoic acid is more acidic than or meta substituted then para or meta substituted. So, what you have to keep in mind simply whenever you have ortho substituted benzoic acid irrespective of the nature of the group right whatever group is attached any group irrespective of the nature of the group ortho substituted benzoic acid is always more acidic than meta or para substituted. This is what we call it as sir effect or steric inhibition of resonance or ortho effect okay. For example, few questions I will write down here CH 3 CS 3. So, it is ortho substituted. So, we have sir effect here here we have plus i and here we have plus h plus h okay. So, ordered is what sir maximum then plus i and then plus h another example you see in this you see here we have sir effect here we have minus m plus m. So, most acidic will be this sir effect then we have minus m and then we have plus m acidity order okay. So, the point is here one last question we will see this too right NO 2 because of hindrance here will show here minus i right again the effect of minus i is negligible here CN will show minus m because the cyanide group CN is linear here right. So, there will be no hindrance right and if hindrance is also there it will be linear only. So, minus m. So, because of minus m effect this is more acidic okay. So, this is how we compare the acidic strength of various compounds okay. One very important concept we have discussed here which is sir effect or ortho effect which is possible in case of benzoic acid which says ortho substituted benzoic acid is always more acidic than para or meta substituted irrespective of the nature of the group attached to it okay. So, next session friends we are going to discuss about the basicity order okay and few examples also we will see over there okay. Thank you.