I hope that you have enjoyed the SN2 reaction. I hold the view that the SN2 reaction is a thing of real joy it is like a graceful song bird soaring through the sky while emitting a delightful song, while the SN1 reaction is a bit of a lumbering ugly dinosaur which makes the windows on your house rattle. While SN2 “song bird” is an agile thing which swerves through the sky with perfect control the SN1 raptor lacks some of the fine control of the “song bird” but for some things the raptor is better than the graceful bird.
Now lets get on with the chemistry.
At the bottom is one example of the sulphur based mustard "gases", you might wonder why I am showing you a mustard. The reason is that in common with the SN1 reaction the classic way in which a mustard reacts is to undergo a slow step which makes a cation which then rapidly reacts with a nucleophile. By the way the classic way to deal with sulphur mustard is to use chlorine bleach. This is because by oxidation of the sulphur atoms the molecule changes so that it is no longer a strong alkylation agent
Before we get going with the SN1 you need to make sure that you understand stereochemistry and stwereocentres well, here are three examples for you.
Consider the two reactions, look at the equation which gives the rate
Here is the mechanism of the reaction, note the differences between this and the SN2
The stability of the carbocation controls the rate of the SN1 reaction
Note the loss of stereocontrol
A range of alkyl groups and a reminder of the SN2 reaction rates, note that in general when the SN2 is slow then the SN1 is fast. The neopentyl is a special case as both reactions are slow. If you can not work out why then think about it for a while.
Recap for last lesson, the branching not on the carbon bearing the halogen has less effect than putting the methyls onto the carbon with the halogen
The next thing for you is to consider which of the following solvents are aprotic. Aprotic means that the solvent does not autoionise to form solvated protons when it is a pure solvent.
Now I would like to make you consider the question of which solvent will make SN2 chemistry go faster, a protic polar one such as ethanol or a aprotic one such as DMF, DMSO or HMPA.
Consider the example of sodium cyanide reacting with propyl bromide as our example.
The answer is that the aprotic solvent will make the reaction go faster, the reason is that the aprotic solvents tend to only solvate the cation leaving the anions free and more able to react than they are in a protic solvent such as ethanol.
Word of advice, do not use DMF it is a toxic and viscous solvent. It is known to cause birth defects when women are exposed to it while in men it has been linked to testicular cancer (cancer of the unmentionable bits). HMPA is even stronger as a carcinogen and it has been totally banned in Sweden, my advice is to use DMSO which is a much less toxic solvent than either DMF or HMPA. Gaylord chemicals (the world leading maker of DMSO) claim that DMSO is close to harmless.
I will put up the slides for the elimination reaction shortly.
Filed under: organic reactions |