 So we have seen that anything that has an empty orbital can pull electron from a surrounding pi system towards itself And this will decrease the electron density of the system and this is what we call our minus r groups So if we have something like say bf2 attached to a pi system, this is going to act as a minus r group, right? Now generally when we talk about organic compounds, these minus r groups generally don't have an empty orbital in itself Instead these minus r groups are typically in the form of a double bond b where b is more electronegative than a So what happens in such a system is that because b is the more electronegative element, it can pull these electrons, these pi electrons towards itself Which can then create an empty orbital over a which can then go ahead and pull the electron cloud from the surrounding pi system So therefore typically the minus r groups look something like this in all these groups We have a pi bond that can be moved to the more electronegative element Creating an empty orbital on the surrounding atom which can then pull electrons from a double-bonded system So if you look at all of these groups like this aldehyde group or even this acidic group In all these cases, we can move the pi bond to the more electronegative oxygen atom thereby creating an empty orbital on the surrounding carbon which can then withdraw electrons from an attached pi system Now the ability of each of these minus r groups to withdraw electrons is not the same Some of these groups are really good at withdrawing electrons while others not so much Let's find out how that happens Let's start by comparing an aldehyde to a keto group Now to compare the electron withdrawing capacity, let us first move this pi electron to the more electronegative oxygen atom So if we do that, we will create an empty orbital on this carbon atom along with a positive charge So what we are basically getting is an electron-deficient carbon Which can then go ahead and pull electrons from a surrounding pi system, right? Now if you look at these two carbons carefully You'll realize that in the keto group this electron-deficient carbon also has a methyl group attached to it, right? Now if you recollect alkyl groups like this methyl group out here are electron donors, right? They can push some of the electrons in this surrounding sigma bond towards this electron-deficient carbon via induction, right? So an alkyl group is a plus i group It can push these electrons in the sigma bond towards this carbon atom thereby making this carbon less positive, right? Now if you compare this with an aldehyde group an aldehyde group only has a hydrogen attached to it Now hydrogen doesn't have any of these electron donating properties like an alkyl group Hydrogen cannot show plus i So therefore this hydrogen cannot push these electrons to this electron-deficient carbon So you don't be able to decrease this positive charge on the carbon atom like the alkyl group, right? So to summarize whenever we create an electron-deficient carbon The alkyl group in our ketone will kick in And remove some of the positive charge on this carbon atom via induction thereby making this carbon less electron-deficient compared to the carbon of the aldehyde group Because hydrogen doesn't show any of these inductive effects So this carbon it will be more electron-deficient compared to the carbon of the keto group And therefore it is going to withdraw electrons much more compared to the ketone So the electron withdrawing nature or we can say the minus r effect of an aldehyde group Is going to be greater compared to that of a keto group Let us now take this one step further Let us try and compare the minus r effect of an aldehyde and a keto group Versus these groups versus an acid and amide and a carboxylate iron Now in all these groups if you shift the pi bond you will notice that this electron-deficient carbon That's getting formed is surrounded by groups that have lone pairs on them, right? So all of these groups are plus r groups And they can donate this lone pair to the m2 orbital of carbon And in this way they can stabilize the carbon atom via resonance, right? Now because these resonance effects are much stronger compared to the inductive effects As resonance involves the actual delocalization of electrons compared to simple push and pull in case of induction So therefore this carbon atom is going to be much more stabilized by the OH And therefore it's going to be even less electron deficient It's going to be even less electron deficient compared to a keto group So therefore the ability of each of these groups to withdraw electrons from an associated pi system Will be even lower compared to a keto group, right? So therefore all of these groups will be weaker minus r compared to an aldehyde and ketone Now between these three groups even though all of them have a plus r group that's attached But the electron donation via plus r is not the same for all of these groups, right? Because the oxygen out here is negatively charged So it's relatively unstable and it will readily try and push electrons away from it So an O- is a much stronger plus r group compared to the neutral OH and NH2, right? Now between OH and NH2 the nitrogen atom is less electronegative compared to oxygen So the electrons are less tightly bound to this nitrogen atom So it can donate electrons much more easily or we can say that its plus r effect will be higher compared to an OH group, right? So therefore if we compare these three groups Because the electron donation will be the highest in case of O minus So therefore it can stabilize this carbon much more So the carbon out here will be the least electron deficient It's going to be the least electron deficient Followed by the carbon of the amide group followed by the carbon of the acidic group, right? Because OH is the weakest plus r between these three So it won't be able to stabilize this carbon as much So between these three groups this carbon is going to be most electron deficient, right? So therefore the electron withdrawing capacity of this carboxylic acid group will be the highest Followed by the amide followed by the carboxylate ion, right? So we can go ahead and say that the carboxylic acid will be a stronger minus r compared to the amide compared to the carboxylate ion Now one important thing that I'd like to add out here Is that if you look at this carboxylate ion If you look at the resonance that's happening out here You will see that the resonance out here leads to a resonating structure That's going to be identical in energy, right? So both of these are equivalent resonating structures And if you remember whenever such a thing happens whenever resonance leads to the formation of equivalent resonating structures The quality of resonance is really high, right? So therefore in this scenario whenever we move this pi electron to oxygen Whenever we do that this electron deficient carbon is much more likely to pull electrons from the other oxygen atom As it leads to a better quality of resonance compared to withdrawing electrons from an attached pi system So a carboxylate ion is much less likely to withdraw electrons from a surrounding pi system So it's actually one of the weakest minus r groups that's out there Now the groups that we have seen till now are actually moderate to relatively weak minus r groups The strongest minus r groups are in fact the nitro group the cyan group and the sulfonic acid group Now if you look at the structures of these groups You can see that these three groups have very different atoms in different numbers that are connected to each other So it's actually very difficult to make a comparative analysis and I'll try to do that But do keep in mind do make a mental note That n o 2 is one of the strongest minus r group that's out there Followed by c n followed by the sulfonic acid This is what has been found out experimentally and let me now try and give a rationale to explain why this might be happening Now it's relatively easy to explain the minus r effect of n o 2 Unlike all these groups and even c n and s o 3 h the atom that's going to withdraw electrons This nitrogen atom is already positively charged, right? So if we move this pi electrons to oxygen if you do that then this nitrogen will become even more positive Now of course the lone pair on this oxygen atom this oxygen has lone pairs. It can go and stabilize this nitrogen But even if you do that even if you shift this pi electrons and make a pi bond out here Even then this nitrogen will still have a positive formal charge, right? So because there will always be a positive charge on this nitrogen atom So this nitrogen inherently is going to be very electron deficient And it's going to pull electrons from whatever pi system it can lay its hands on So this makes n o 2 actually one of the strongest minus r groups that's out there Now coming to the cyan group now the carbon of a cyan group If you do your hybridizations correctly it will come out to be sp hybridized, right? Now if you compare it with all these carbon atoms Then all the carbon atoms out here are going to be sp2 hybridized Now an sp carbon is going to be more electronegative than an sp2 carbon, right? Because this is going to be more s character in an sp orbital So it's going to make this carbon much more electronegative compared to the carbon of a carbonyl group So therefore if we move this pi electron to nitrogen We are going to get a positive charge on a more electronegative sp hybridized carbon atom, right? Now a positive charge on a more electronegative sp carbon is going to make it much more unstable Compared to a positive charge on an sp2 carbon So this is going to put electrons much more from a surrounding pi system Compared to the carbon of these carbonyl groups So therefore the cyan group is also a strong minus r group Let's now come to the sulfonic acid group now if you move this pi bond to the oxygen atom We'll get a positive on sulfur, right? Now sulfur is a relatively large atom. It's relatively less electronegative Compared to these carbon and nitrogen atoms. So it can hold this positive charge much better compared to all of these atoms However, because this sulfur is also attached to three oxygen atoms And these highly electronegative oxygen atoms can also pull electrons towards themselves via induction So because of the presence of three oxygen atoms This sulfur actually becomes really electron deficient And the sulfonic acid group turns out to be a strong minus r group weaker only to the nitro and the cyan group So there are various factors at play when it comes to analyzing these groups So it's actually better to simply remember that NO2 is one of the strongest minus r groups followed by cyan and the sulfonic acid group