 So, what we have out here are different functional groups that we can attach to a carbon chain. As you can see we have an alcoholic functional group as well as an immune functional group and even things like a Nitro and Cyanoh and even aldehyde and a keto group. Now I have classified all these functional groups into two categories plus R and minus R and in this video I am going to tell you exactly what I mean by this plus R and minus R and by the end of this video you should be able to tell whether a given functional group is plus R or minus R just by looking at it. You don't have to memorize all of these things if you really understand what's going on. So, let's see. Let's start by putting one of these functional groups maybe an alcoholic group out here. So, let's remove this and let me put an OH out here. Let's see what happens. Now as you can see we have this lone pair that is in conjugation with a double bond right. So, we can have a resonance out here this lone pair this lone pair of electrons can move over here while this pi electrons can move over to this carbon atom right. So, this will lead to the formation of a new resonating structure that's going to look like this right. This lone pair of electrons move over here to form a pi bond and this gives a plus one formal charge to this oxidant atom while this pi electrons move over to the carbon giving a minus one formal charge to this carbon atom right. Now let's take some moment and try and see what's exactly going on out here. So, what we have out here is an oxidant atom and this oxidant atom it has one of these lone pairs in a p orbital right. So, let me redraw this. One of the lone pairs is in a p orbital in this oxidant atom. So, we have two electrons out here and let me draw the other lone pair and the hydrogen. Now if you come to this carbon-carbon bond we also have a pi bond between the two p orbitals of these two carbon atoms right. So, we have two electrons that are being shared between these two orbitals. Now these electrons are not static they are dynamic right they are moving around everywhere and if both of these electrons move over completely to this carbon atom if such a thing happens then this pi bond is going to break both of these electrons are going to be over here and this will create an empty orbital over this carbon atom right. So, this will make this carbon positively charged while this carbon will have a minus one formal charge. Now this is an empty orbital and this carbon has an incomplete octet now so it needs electrons and it can take these electrons again from this orbital they can again overlap and combine and form a pi bond but it can also take these electrons from this oxidant atom right. So, the electrons of this oxidant atom can also overlap with this carbon and this is exactly what's happening during resonance. So, if we go back and look at our original drawing the two electrons in this pi bond can shift over this carbon atom and this will create an empty orbital out here and the lone pair of the oxidant atom can then overlap with this empty orbital and this will form a pi bond out here right. So, this is exactly what we mean when we draw our arrows. Now coming back this lone pair is again in conjugation with another double bond and again these electrons are not static but dynamic so they can move over to this orbital and this will create an empty orbital out here and both of these orbitals can then overlap. So, therefore we can have a new pi bond out here while this pi electrons can move over here right. So, this will lead to the formation of our new resonating structure that's going to look like this. So, what's happening out here is that when we have an OH group added to a double bonded system then the lone pair of this oxygen gets pushed into this double bonded system increasing the electron density of the system and bringing about these negative charges over these carbon atoms. So, if you look at our actual molecule, if you look at our resonance hybrid the actual molecule is a hybrid of all of these resonating structures you can see that there are these negative charges that get developed over my system right. So, therefore presence of an OH group to my double bonded system pushes electrons into the system it pushes electrons into the system via resonance and this increases the electron density of the system. Now we call such groups which increase the electron density of the system via resonance we call these the placer groups. So, now that you know what placer means what do you think is the condition is the necessary condition for a group to be placer. Well I'm sure you must have guessed it a placer group will need to have at least one lone pair of electron that it can push into the system via resonance right. So, if you look at all of these groups you can see that each one of this even this amide group and this ester group all of these have at least one lone pair that can be pushed into a double bonded system. So, all of these groups act as placer. Let us now turn our focus into minus r I'm sure you must have now also figured out what minus r means right minus r groups are those groups that pull electrons away from a double bonded system thereby decreasing the electron density of the system. So, how does such a thing happen? Let's see by adding one of these groups let me start by putting this particular group the bf2 group because it's easier to explain. Let's start by looking at what happens to my double bonded system if I add bf2 to it. So, let's see. Now, whenever we have a boron atom to which we have three bonds attached then this is going to be sp2 hybridized if you remember your hybridization you'll realize that this will be sp2 hybridized and this by default will have an empty p orbital right. Now, this empty orbital is connected to a pi bond. So, there are again these pi electrons that are shared between the p orbitals of the carbon atom and again these two electrons are not static they're dynamic they keep moving around and it might so happen that both of these electrons land up in this particular orbital right. So, if both the electrons come over here this pi bond is going to break and this will lead to the formation of an empty orbital out here. So, I'll get a plus one formal charge out here while this will become minus one right. Now, the moment this happens we have an empty orbital out here and we have a fully filled orbital out here so both of this can also overlap right. So, if we have an empty orbital then the pi electrons can be withdrawn from my double bonded system and this will lead to the formation of a new resonating structure that's going to look like this right. Now, again we have an empty orbital out here that's connected to this pi bond. So, again the electrons out here can move over to this orbital making this empty and again these two can overlap right. So, these pi electrons can then shift over here and this will lead to the formation of a new resonating structure that's going to look like this. So, what's happening is that this BF2 molecule or we should say that the presence of this empty orbital is actually withdrawing electrons from this double bonded system and bringing about this positive charges on these carbon atoms right. So, therefore if you look at the resonance hybrid of this particular molecule you can see that there are these positive charges that get developed on my double bonded system right. So, such kind of groups which can pull electrons which can withdraw electrons from a double bonded system via resonance and thereby decrease the electron density of the system such groups are called the minus r groups right. So, once again if I ask you what is the condition for a group to be minus r what will your answer be? Well, a minus r group needs to have an empty orbital on it right. This empty orbital can withdraw electrons from an adjacent pi bond making it minus r. So, coming back you can see that all of these plus r groups have a lone pair of electrons that they can donate while minus r groups should have an empty orbital and we have seen that BF2 indeed does have an empty orbital right. However, if you look at all these other groups like the NO2 and SO3 and so on they actually do not have an empty orbital in them. For example, if we add NO2 to a carbon chain and if you do your hybridizations and everything properly you will see that NO2 doesn't have an empty pure orbital but it instead has a pi bond between this nitrogen and oxidant right. So, there are these two electrons that are moving around between this nitrogen and oxidant. So, how can this group act as minus r? Well, for it to show minus r we need to have an empty orbital over this nitrogen atom so that it can pull electrons from this pi bond right. So, if these electrons if these two electrons move over here then this has to be empty so that both of these overlap right. Now in NO2 it can very well happen that both of these electrons land over here making this empty and during that time if both the electrons of this pi bond also land over here then both of these can overlap and nitrogen or we should say the NO2 group can withdraw electrons making it as a minus r group right. So, can we do this? Can we like move electrons from one orbital to another based on our needs? Well, the answer is yes but we should check for electronic activities. So, if we have something like a C double bond C in which electronic activity of both these atoms is exactly the same then we can move this pi bond this pi electrons either ways you can move it either here or there so both of these are valid. However, if we have something like NO2 let's just consider the nitrogen-oxygen bond. So, if we have a pi bond out here then because this oxygen is more electronegative than nitrogen so if you ever need to shift these electrons to one orbital you can only ever shift it to the more electronegative element which in this case is the oxygen atom. So, you can do this but you can't ever do the other way round you can't take the pi electrons and move it to the less electronegative nitrogen atom this is not valid. So, therefore, if we have an x double bond y we can move this pi electron both ways if both of them are the same atom both of them have the same electronegativities but if one of them has a higher electronegativity if electronegativity of y is greater than electronegativity of x then we can only move these pi electrons towards the more electronegative element as it loves electrons more this would be like a correct transformation but we can never move electrons to the other less electronegative atom. So, if you look at all of these groups so if I look at say NO2 so NO2 is connected to a more electronegative oxygen atom so we can move these electrons out here leaving to the formation of an m-tier battle out here and this can then pull electrons from my double bonded system so this can act as a minus m group and if you look at all these other functional groups like if you look at CN the cyanide group then the nitrogen is more electronegative than carbon so we can move this pi bond out here this will lead to the formation of an m-tier battle over this carbon atom and then this can pull electrons from the double bonded system right so when this can act as a minus m group similarly if you look at our aldehyde group or our keto group so both of this have this double bond O and oxygen is more electronegative than carbon so we can push this electrons towards the oxygen atom and this will lead to the formation of an m-tier battle over this carbon which can then pull more electrons from a double bonded system so therefore even if something doesn't explicitly have an empty orbital but if it's in the form of x double bond y where the electronegative video of y is greater than x then even then it can act as a minus r group