Today I got a good question, I want to make a point of posting the best question from the day’s teaching and my answer.
The student asked “Why do vinyl halides not do the SN2 reaction ?”
My answer was that two reasons exist for why the vinyl halide will not react with a nucleophile. Firstly if the nuclophile comes in on the SN2 path it will bump into a hydrogen or other group which is trans to the leaving group.
The steric reason for the vinyl halide not reacting
The second reason is that the nuclophile will be repelled by the pi electron clouds of the alkene. As a result the SN2 reaction will not occur.
Now I imagine that you will have found the SN2 reaction very interesting, but now it is time for something else. The new thing is the SN1 reaction. This is called the SN1 reaction because the rate only depends on the concentration of the alkyl halide.
Rate = k [Alkyl halide]
The thing which controls the rate of the reaction is the stability of the carbocation intermediate, there are things which can be done to alter the stability of the carbonation such as altering the solvent but the thing you need most to know about is the relationship between the structure of the alkyl halide and the stability of the carbocation.
Let us start with a methyl cation, the charges on the atoms were calculated using Huckel theory, and the carbon atom ends up with a charge of + 0.87 while each hydrogen ends up with a charge of 0.05. This is a very unstable carbocation, it is impossible to relocate (delocalize) any of the positive charge through resonance from the carbon to another atom. A little of the positive charge is moved onto the hydrogens through inductive effects but this is only a tiny amount. The more red the atom is the more positive it is, the more blue an atom the more negative.
Methyl cation
Next cation is the ethyl cation, again here is a picture of it. Those of you with sharp eyes will have noticed that the hydrogen at the top right corner of the ion is less green than many of the others. This is because this hydrogen (and the one hiding behind it) have now got some of the positive charge delocalized onto them.
Ethyl cation
The empty p orbital of the carbocation is able to interact with the sigma orbitals associated with the C-H bonds. That is as long as the C-H bond is not in the plane formed by the three groups bonded to the carbon formally bearing the charge.
For those of you who like numbers here are the atomic charges
Carbon with carbocation, 0.75
Alkyl carbon -0.085
Hydrogens bonded to the carbonium carbon 0.036
Bottom right hydrogen on alkyl 0.045 (C-H bond at 90 degrees to p orbital of the carbocation)
Top right hydrogens 0.11
Now if we move to the isopropyl carbocation we can see that the charge on the carbon has gone down to only 0.72. Below I have shown this ion in all its glory.
One view of the isopropyl cationTop view of the isopropyl cation
Add the last methyl group and we can see how the charge is now spread out over even more hydrogen atoms. I was going to show you the wonderful proof done by calculation but the software I used got it slightly wrong. The software estimated that the carbon has a charge of + 0.73. This is an interesting point.
Be as careful of computational chemistry as you would be of any other chemistry, sometimes it is possible for a calculation to be wrong due to the limits of the software. Also watch out for problems associated with a wrong starting geometry.
Organic chemistry teaching 