Posted by: Mark Foreman | July 9, 2011

SN2 chemistry experiment

One of the key differences between the SN2 and SN1 reactions is that the rate of the SN2 reaction is dependant on the nature and the concentration of the nucleophile. If we write out a typical SN2 reaction,

CH3I + PhS- –> PhSCH3 + I-

We will have a rate given by the following equation

d[PhSCH3]/dt = k[CH3I][PhS-]

If we change from the strong nucleophile (thiophenolate) to a less reactive nucleophile (how about acetate) then for the weaker nucleophile we will have a new rate equation.

d[AcOCH3]/dt = k’[CH3I][AcO-]

As the sulfur anion is more reactive than the acetate then k > k’.

The reason we have this kinetics equation is because both the nucelophile and the alkyl halide are involved in the rate determining step. If we now consider an alkyl halide which is unable to do the SN2 (for steric reasons) such as 2-methyl-2-bromopropane (tert-butyl bromide) reacting with methanol then we have a molecule which will do the SN1. This is a different reaction, here the slow step is the breaking of the carbon halogen bond.

(CH3)3CBr + CH3OH –> CH3OC(CH3)3 + HBr

This reaction has the rate equation

d[CH3OC(CH3)3]/dt = k[(CH3)3CBr]

After breaking the carbon halogen bond a carbocation is formed which rapidly reacts with any nucelophile it then comes across. If the methanol had been replaced with more nucleophilic reagent such as a solution of sodium cyanide (NaCN) then the new reaction would be

(CH3)3CBr + NaCN –> NCC(CH3)3 + NaBr

As long as the temperature and the solvent stays the same then the rate constant in the rate equation will remain constant.

d[NC-C(CH3)3]/dt = k[(CH3)3CBr]

For both of these SN1 reactions k is the same, as the nucelophile takes no role in the rate determining step.

But back to the SN2 reaction, before I went away on holiday I left some dodecyl bromide dissolved in methanol with a little potassium cyanide added. The idea was to offer two nucleophiles to the alkyl halide. The methanol is at a very high concentration but it is a weak nucelophile, the cyanide anions are at a low concentration but they are stronger as nucleophiles.

Rather than just wanting to stare at the flask, or asking you to take my word that it reacted the way I said it would I did some gas chromatography.

Some time ago I read about a medical school where no dead bodies are used, instead the medical doctors use their own bodies with things like ultrasound machines to allow themselves to look inside their bodies. In many ways I would love to introduce the chemical version of this to the chemical engineering students. I would like students to have access to things like gas chromatography from the first days of the course, this machine is like a sixth sense for an organic chemist it allows the chemist to detect a vast range of volatile and semivolatile substances.

While some detectors are very hard to use and vary greatly from compound to compound in their sensitivity the flame ionization detector is very simple. It senses how the flame gets hotter when an additional fuel is added to it. As a result for organic compounds it is close to equally sensitive for almost all classes of compound.

Here is a reference trace for the alkyl bromide, I have to confess the baseline ended up a bit wonky. In the ideal world it would be flat. The trace has four peaks, the first peak is at about one minute and is the acetone solvent burning off in the flame detector. The next peak at about four minutes is a solvent impurity (4-methyl-4-hydroxypent-2-one) which is formed by the self condensation of the acetone.

Condensation of acetone to form 4-methyl-4-hydroxy-pent-2-one

Please ignore this peak and the peak at about 19 minutes which is due to plasticiser in my acetone. NB Someone stole my super pure acetone and the reagent grade acetone has a dialkyl phthalate in it. Either the reagent grade acetone had the phthalate in it or it got in when I had to use a plastic gasoline syphon to transfer it out of the giant drum. Here is a picture of a typical dialkyl phthalate {bis(2-ethylhexyl) phthalate}.

Di-2-ethyl hexyl phthalate

Finally here is the GC trace. It is a graph of the voltage from the detector against time since the injection occurred. If you ever use a GC machine then bear in mind it needs a very small injection (5 microlitres) of a dilute solution (200 milligrams per litre), which works out as about 900 nanograms of sample.

GC trace of the alkyl bromide

The important peak to look for is the one at eight minutes, this peak is the alkyl bromide.

The next graph is the trace for some dodecyl alcohol which I stirred overnight in acetone with silver oxide and methyl iodide. This is a classic methylation system which converts some of the alcohol into the methyl ether. This is a way of making a small amount of the methyl ether which is formed when methanol reacts with the alkyl bromide used in my experiment. I ran a GC experiment with the dodecyl alcohol to confirm that I got the peaks the right way around.

Product of dodecyl alcohol with methyl iodide / silver oxide in acetone

Look in the graph for the small peak at about six minutes, the big peak at nine minutes is the unreacted alcohol.

Now on with the show, here this the trace for the product of leaving a dilute solution of potassium cyanide with the alkyl halide, note that a tiny peak for the methyl ether exists at six minutes but at twelve minutes a very strong peak for the alkyl cyanide appears.

The products from potassium cyanide and the alkyl bromide in methanol

This is a proof that the nature of the nucelophile has a great effect on the reaction rate in the SN2 reaction.

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Responses

  1. Respected sir
    i would like to appreciate the way u provide explanations in a lucid manner.i am tutor in organic chemistry & ur explanations would help me a lot

    • I am glad that this blog is of some use to you, please feel free to look at my other blog (markforeman.wordpress.com) you might also find some chemistry there are well.

      • Well please feel free to come back and see more, I have to say that much of the chemistry which I am writing is at markforeman.wordpress.com. If you have any special requests then please send in a comment and I might give you my thoughts on that topic.

  2. It is a Great Opportunity to learn About chemistry Experiments


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