Posted by: Mark Foreman | January 30, 2010

Alkyl halides I

Well this year we have had a bit of a change in the order, in which we teach the organic chemistry, in the past we always started with the alkyl halides and the SN2 reaction. The SN2 reaction is one of the most important reactions in organic chemistry, as I have have always said before it shows it’s self in lots of different places. 

But before we get onto the SN2 reactions, we must first consider what an alkyl halide is.  

My own view of what an alkyl halide is this.  

An organic compound where a halogen is bonded to a sp3 carbon”  

Plenty of other compounds exist with halogens bonded to other types of carbons; we will consider those molecules in a later class. Also there are other compounds with other things bonded to sp3 carbons which do many of the reactions of the alkyl halides.  

I know that in the past many irksome alkyl halides have been made by man, and then released into the environment where they have caused harm. For instance the “Freons” are examples of alkyl halides, a classic freon (Freon-12) is dichlorodifluoromethane. This freon is able to damage the ozone layer and has been banned (quite rightly in my view!) but we will now be dwelling on the highly halogenated freons today, most of the molecules today only have a few of the hydrogen atoms replaced with halogen atoms.  

Freon-12 (Dichlorodifluoromethane)

 

Another pair of molecules which have halogen atoms (chlorines) bonded to sp3 carbons are the pesticides DDT and lindane. In the case of DDT it also has chlorine bonded to sp2 carbons but it has the key feature which defines the alkyl halides in my mind. It is important to understand that a molecule can have more than one functional group, as you will learn later sometimes these groups interact with each other and sometimes they ignore each others existence and just behave as independent groups.  

DDT on the left and lindane on the right

 

The use of both lindane and DDT have either been strongly restricted or even banned in many parts of the world. Both DDT and lindane are man made and were never on this earth until organic chemists made them.  

Now after reading about freons, DDT and lindane you might be tempted to think of the alkyl halides as a class of molecules which are man made, and can never be made by nature. But many examples exist of natural alkyl halides, for instance halomon which is found in a red algae which grows in the sea. If you want to read about these alkyl halides and other organohalogens made by ‘Mother Nature’ then please go and read ‘Natural Organohalogens’ by Gordon W. Gribble in Journal of Chemical Education 1994, Volume 71, Issue 11 (November), pages 907 to 911.  

Halmon a halogenated organic from the sea

 

I would also like to point out that some very useful drugs exist which have the halogen attached to sp3 carbon feature. For example Prozac has a CF3 group in it.  

Fluoxetine also known as prozac

 

The last general example I want to tell you about right now is PVC, much as Greenpeace love to hate the stuff it is important to note that PVC has transformed society for the good. In my other blog (MarkForeman.Wordpress.Com) I have the story of my Grandfather, the church wiring and plastics if you want to find out about one very good use of PVC then have a read of this entry. The reason I lump PVC in with the alkyl halides is that it has chlorine atoms bonded to sp3 carbons in its polymer chain.   

Now onto the SN2 reaction  

To me the most important thing about the reaction is that it is a single step and it is concerted. As bonds form other bonds break. While the SN2 reaction is normally thought of as two different molecules reacting together, there is nothing to stop both the nucleophile and the electrophilic being in the same molecule. This idea of both reagents being in the same molecule does offer a method of making rings; I have to confess I have a deep love of making rings. When I was a PhD student I seemed always to be making new rings with phosphorus atoms in them but I tended to not use SN2 chemistry to make my rings.  

But I did use SN2 chemistry on a regular basis to make many important molecules for my project. Here is a typical SN2 reaction it is the reaction of ethyl bromide with iodide anions to form ethyl iodide and bromide anions. It is important to note that the carbon bearing the bromine as its stereochemistry changed. This reaction starts with the attack of the nucleophile on the backside of the alkyl halide.  

An example SN2 reaction

 

SN2 is called the Bimolecular (2 molecule) Nucleophilic Substitution reaction because the rate determining step involves the two reagents. The rate of this reaction is given by the following equation.  

Rate = k [Nucleophile][Alkyl halide]  

When the incoming nucleophile attacks the back side of the carbon bearing the leaving group, the incoming nucleophile starts to form a bond at the same time as the leaving group starts to unbond to the carbon. For a short moment in time the carbon bearing the leaving group becomes a sp2 centre with the attacking nucleophile and the leaving group both connected to the p orbital of the carbon. As the orbital overlap with the incoming group increases the orbital overlap with the leaving group decreases as the bond breaks.  

Now we look at the same reaction, this time with special glasses on which allow us to see the molecular orbital of interest.  

The SN2 reaction with the molecular orbitals involved with the reaction shown

 

The rate of the reaction is determined by the concentration (activity to be strictly correct) of both the alkyl halide and the nucleophile and the energy of the five coordinate transition state. The energy of the transition state is strongly influenced by two things.  

1. The steric effects around the carbon bearing the leaving group.  

We will start with methyl iodide which is shown below.  

Side view of methyl iodide

 

Here is another picture of methyl iodide, you can see that it is easy to get to the carbon which bears the iodine atom. So this compound will react quickly in SN2 reactions.  

Back side view of methyl iodide

 

Now let us look at ethyl iodide. You will see how for ethyl iodide that a methyl group is pointing towards the back of the molecule. This ethyl group will oppose the approach of the incoming nucleophile. In addition the additional methyl group will increase the energy of the five coordinate transition state. 

Ethyl iodide side view

 

Now here is the back side view of ethyl iodide. 

Back side of ethyl iodide

 

To help us explore this idea of the steric effect we will consider the reaction of dimethyl sulphide (smells like decaying cabbage) with a series of alkyl iodides. Please see Alkyl halides II for details. 

2. Any pi system which is near to the carbon bearing the leaving group.  

This is for the more advanced students I may add some on this topic soon. 

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