THE REACTION BETWEEN METHYLBENZENE AND CHLORINE
A Free Radical Substitution Reaction This page gives you the facts and a simple, uncluttered mechanism for the free radical substitution reaction between methylbenzene (previously known as toluene) and chlorine. If you want the mechanism explained expl ained to you in detail, there is a link at the bottom of the page. Methylbenzene has a methyl group attached to a benzene b enzene ring. The hexagon with the circle inside is the standard symbol for this ring. There is a carbon atom at each corner of t he hexagon, and a hydrogen atom on each carbon c arbon apart from the one with the methyl group attached. Note: There is no need to worry about the bonding in the benzene ring at this point. If you are interested, you can follow the link - but it isn't important for now.
The facts The reaction we are going to explore happens between methylbenzene and chlorine in the presence of ultraviolet light - typically sunlight. This is a good example of a a photochemical reaction - a reaction brought about by light. Note: These reactions are sometimes described as examples of photocatalysis - reactions catalysed by light. It is better to use the term "photochemical" and keep t he keep the word "catalysis" for reactions speeded up by actual substances rather than light.
The organic product is (chloromethyl)benzene. The brackets in the name emphasise emphasise that the chlorine is part of the attached methyl group, and isn't on the ring. One of the hydrogen atoms in the methyl group has been replaced by a chlorine atom, so this is a substitution reaction. However, the reaction doesn't stop there, and all three hydrogens in the methyl group can in turn be replaced by chlorine atoms. Multiple substitution is dealt with on a separate page, and you will find a link to that at the bottom of this page.
Important! There is another reaction which happens between methylbenzene and chlorine in the absence of light and in the pr esence of a number of possible catalysts. In t hat one, substitution happens in the benzene ring instead of in the m ethyl group. You will find this reaction discussed under electrophilic substitution reactions.
The mechanism The mechanism involves a chain reaction. During a chain reaction, for every reactive species you start off with, a new one is generated at the end and this keeps the process going. Species: a useful word which is used in chemistry to mean any sort of particle you want it to mean. It covers molecules, ions, atoms, or (in this case) free radicals.
The over-all process is known as f ree radical substitution, or as a f ree radical chain reaction. Note: If you aren't sure about the words free radical or substitution, read the page What is free radical substitution?Use the BACK button on your browser to return quickly to this page.
C hain
initiation The chain is initiated (started) by UV light breaking a chlorine molecule into free radicals. Cl2
2Cl
C hain propagation reactions
These are the reactions which keep the chain going.
C hain termination reactions
These are reactions which remove free radicals from the system without replacing them by new ones. If any two free radicals collide, they will join together without producing any new radicals.
The simplest example of this is a collision between two chlorine radicals. 2Cl
Cl2
EXPLAINING TH E R EAC TI ON BE TW EEN M ET HY LBENZENE AND
O RI NE C HL
A
Free Radical Substitution Reaction
This page guides you through the mechanism f or the substitution of one of the hydrogen atoms in methylbenzene by one chlorine atom. Multiple substitution is covered separately, and you will f ind a link at the bottom of the page. We are going to talk through this mechanism in a very detailed way so that you get a f eel f or what is going on. You couldn't possibly do the same thing in an exam. At the bottom of the page, you will f ind the condensed down version corresponding to the sort of answer you would produce in an exam. The role of the UV light The ultraviolet light is simply a source of energy, and is being used to break bonds. I n f act, the energies in UV are exactly right to break the bonds in chlorine molecules to produce chlorine atoms.
Note: Only the outer electrons of t he chlorine are shown. Notice also that it is quite acceptable to use a simple view of at omic structure. There is no point in using a complicated model of the atom if a s imple one will do the job.
Because
we want to stress the fact that the chlorine atoms have single unpaired electrons, then we call them chlorine free radicals - or more usually just chlorine radicals. To show that a species (either an atom or a group of atoms) is a free radical, the symbol is written with a dot attached to show the unpaired electron. The
splitting of the chlorine molecule would be shown as: Cl2
2Cl
Free
radicals are formed if a bond splits evenly - each atom getting one of the two electrons. The name given to this ishomolytic f ission. What
happens to the chlorine radicals?
Reactions happen because things hit each other. In this case, you need to think about what the chlorine radicals are likely to hit, and what could happen as a result of that collision. At the moment the mixture contains y y
y
lots of methylbenzene molecules lots of chlorine molecules (only a few will have been fractured by the UV light) a few chlorine radicals
Let's start with the unproductive collisions. The least likely collision is between two chlorine radicals. There aren't very many of them in the mixture and so the chances of them hitting each other are relatively small. If they do collide, they will combine to form a chlorine molecule. That's worse than useless because it removes the active free radicals from the system. 2Cl
Cl2
A chlorine radical could also hit a chlorine molecule. If this happens there could possibly be an exchange of chlorine atoms, but nothing new would be formed. It is just a wasted collision. Cl
+ Cl-Cl
Cl-Cl + Cl
Note: There is no difference between the chlorine atoms shown in bold type or ordinary type. They are shown differently so that the exc hange is made clear.
The productive collision happens if a chlorine radical hits a methylbenzene molecule.
The chlorine radical removes a hydrogen atom from the methyl group. That hydrogen atom only needs to bring one electron with it to form a new bond to the chlorine, and so one electron is left behind on the carbon atom. A new free radical is formed - called a phenylmethyl radical. Note: Don't worry about the name of this new r adical. All that matters is t hat you can draw its structure.
What
happens to the phenylmethyl radicals?
It depends what they collide with. There are three interesting collisions which need to be explored. Two of these involve a set-back to the reaction, and only one is useful. Luckily, the two unhelpful collisions don't happen very often, because they involve collisions between two free radicals - and there won't be many of these present in the mixture at any one time. y
y
A phenylmethyl radical hits a chlorine radical. These will combine to make what you want - (chloromethyl)benzene - but the reaction removes the active free radicals from the system. That stops any further reactions happening.
Even worse, two phenylmethyl radicals could hit each other. Not only does this remove radicals from the system, but produces an unwanted side reaction.
So what is the usef ul collision? If a phenylmethyl radical hits a chlorine molecule (something that's quite likely to occur), the following change can happen:
The phenylmethyl radical takes one of the chlorine atoms to form (chloromethyl)benzene (which is what we want to make), but in the process generates another chlorine radical. This new chlorine radical can now go through the whole sequence again, and at the end will produce yet another chlorine radical - and so on and so on. The process is described as a free radical chain reaction. The chain continues because for every chlorine radical that goes in at the beginning, a new one is generated at the end. Chain termination
Does this mean that one tiny burst of UV light, splitting one chlorine molecule into two free radicals, is enough to convert a whole reactions-worth of methylbenzene and chlorine into (chloromethyl)benzene and HCl? Sadly, no! As we've seen, there are collisions which result in the removal of free radicals without producing any new ones. These radicals can only be replaced by starting the process all over again with a new burst of light energy. In practice, then, the chains propagate many thousands of times, but eventually any chain will be brought to an end by one of these chain termination processes.
Simplifying
all this for exam purposes:
The over-all process is known as f ree radical substitution, or as a f ree radical chain reaction. C hain
initiation The chain is initiated (started) by UV light breaking a chlorine molecule into free radicals. Cl2
2Cl
C hain propagation reactions
These are the reactions which keep the chain going.
C hain termination reactions
These are reactions which remove free radicals from the system without replacing them by new ones. If any two free radicals collide, they will join together without producing any new radicals. 2Cl
Cl2
Important! If you have found this mechanism difficult because of the names and structures involved it would be worth looking at t he methane and chlorine reaction. The two mechanisms are identical as far as the substitution is concerned, but the methane / chlorine one looks easier!
