Transcription
Hello, dear students. We are back once again with electrophilic substitution reactions of toluene. Today, we'll take one more electrophilic substitution. We have already undergone nitration and sulfonation. Now, we are going to take the third one, and that is the halogenation reaction of toluene.
In halogenation reactions, we can include two halogen reactions: one is chlorination, and one is bromination. Let us take the chlorination reaction of toluene. I am going to react toluene with chlorine in such a way that I get an electrophile out of it. So, to get an electrophile, a specific, precise reagent is required over here. We have FeCl3 as a catalyst. But here, FeCl3 is chosen. The reason is that Fe has got atomic number 26. Of its 26 electrons, the three valence electrons are bonded with Cl, and each Cl has seven valence electrons, right? So here, Fe shares its electrons with chlorine. Each chlorine atom, and as a result, you can see each chlorine completes its octet. But what about iron? Fe is yet not satisfied with its octet, so that will prompt that will allow the reaction to create an electrophile out of Cl2.
Cl2: Cl having seven valence bonded to another Cl atom having seven valence electrons; they are bonded together, and as a result, each Cl atom is able to complete its octet. Now, this Fe has six electrons after sharing with three chlorine atoms. To complete its octet, it requires two more, so it prompts one of the chlorine atoms from the chlorine molecule to bind with it. Not only that, the chlorine which is binding with Fe has to come with eight electrons. That means what happens? It takes away the electron of the neighboring chlorine atom to which it is bonded, and as a result, it forms a bond with Fe, forming a coordinate covalent bond. Whereas, on the other side, the chlorine atom which is left without an electron is now no more an atom, but it is your electrophile; that is, it has got only one, two, three, four, five, and six valence electrons. So that Cl attains a plus sign. This here is a minus sign, and this Cl+ participates in the reaction that's taught to you in. So here, overall, the temperature maintained is around 30 degrees Celsius, or you can consider it as 303 Kelvin. It is an electrophilic substitution reaction, so the H atom bonded on the carbon will have to get replaced by the chloronium ion.
But before that, we know it is the methyl group. The methyl group is an electron-donating group, and so it will direct the incoming group on ortho and para positions respectively. So here, the Cl+, which is getting on the toluene carbon, has to take ortho or para positions. So here, you can see the methyl group. I am showing over here CH3, which is ortho/para directing with respect to it. Either these are, these are ortho positions. I am taking right now. This position has also, and one more with respect to the methyl group; this position is the para position. So, as we have a mixture of ortho and para, to balance the reaction, we will make Cl2 into two moles, where one Cl+, that is chloronium ion, which attacks the benzene nucleus, which is rich with a pi electron cloud, so H leaves the carbon, leaving its electron there on carbon. So that H now converts into a proton, H+. Okay, we'll talk about that later on. Before that, this carbon is reached with electrons; it is lacking electrons. The plus and minus charges are getting nullified, and as a result, Cl gets bonded on this carbon. On the other hand, you can see the Cl can also bond on the carbon which is at the para position. So here, you get o-chlorotoluene because the positions are also at ortho. We have Cl, and basically, the compound is toluene. This is para, so we write p-chlorotoluene. So this is how you get a mixture of ortho and para, and then you can separate it by different practical methods.
Now further, if chlorination is carried on further with ferric chloride catalyst again at high temperatures, this ortho and para mixture will convert into a single compound, and you can see this is also, this is para. So in a single compound, you get both the chlorine substitutions; that is, the CH3 of toluene will be made that itself, but this ortho Cl and this ferrous here will be obtained on the same compound. That gives you 2,4-dichlorotoluene. Now, in every step, H which left, giving you H+, that is a proton. That H+ combines with this Cl- in both cases and over here. So we have two moles of HCl byproduct: one H+, one H+, and here you have Cl-, Cl-, two moles, so that forms two moles of HCl. Here, we have taken one mole of chlorine. The Cl+, you know, it has formed on the second, on the fourth position. So here also, you can have, in a similar way, HCl as a byproduct. So this is what you can see is the hydrogenation reaction, but as we have reacted with chlorine, it is your chlorination reaction.
Instead of that, we take again toluene, and we react it with bromine in the presence of ferric bromide as a catalyst. The temperature is 40 degrees Celsius, or you can write 313 Kelvin. The concept remains the same as that of Cl. So here are the two moles. The methyl group is an ortho-directing group, and so it, in the amine, directs the incoming group on both the ortho and para positions. So here you have CH3. This is, as you know, ortho. This is, you know, para. So this is Br, this is Br. So it is ortho-bromotoluene. This is para-bromotoluene. So you get a mixture of ortho and para, and two hydrogens in the form of protons will combine with 2Br-, and that will form two moles of HBr as a byproduct. If you further perform bromination with the same catalyst, FeBr3, and if you heat the reaction mixture, you will get two and four positions both acquainted with Br, and as a reason, you get 2,4-dibromotoluene. You had dichloro here; you have dibromo also, dibromo toluene. So I'll write that as 2,4-dibromo. So this is how you can carry your halogenation reaction of toluene.
In the previous episodes, we have seen the halogenation reaction of benzene. In that, we have taken much, much detail of how this electrophile is obtained from the reagent that is participating; how this catalyst is playing the role to create an electrophile. This we have taken already in the episode of the halogenation reaction of benzene, so go through that episode as well for detailed mechanism. Reaction mechanism I have given you, but it is a little bit short, but there we have detailed it, right? So this is a halogenation reaction. Prepare well, and we will continue for more topics, that is alkylation and acylation reactions of toluene. Till then, see you. Take care.