CHM 266 Experiment 4 Handout
Free Radical Chlorination
Spring 2007
Reference:
J. A. Moore, D. L. Dalrymple, O. R. Rodig, "Experimental Methods in Organic
Chemistry, 146, 3rd Ed.,
Introduction
This experiment involves a reaction
that is covered in Chapter 4 Radical Reactions of the CHM 261 course. Actually, this is exactly the type of
experiment that led chemists to come up with the mechanism and principles of
radical reactions that will be described in lecture
Reaction and Mechanism
Although saturated hydrocarbons are inert to
most acidic and basic reagents, they can be "halogenated" in the
presence of what is called a free radical initiator. As shown for cyclohexane, a
chlorine replaces a single hydrogen.
As described in lecture, the number of hydrogens replaced will depend on
the conditions. This type of
halogenation occurs by a mechanism known as a radical chain reaction. The mechanism is provided below the reaction.
Initiation
Propagation
Termination
Chlorine gas is toxic, corrosive,
and can be difficult to handle on the bench top scale. A safer method uses sulfuryl chloride and
benzoyl peroxide (yes, the zit medicine).
At relatively low temperatures, the O-O bond breaks to form benzoate
radicals.
benzoyl peroxide
Ultimately this leads
to the formation of other radicals generically represented as R’·. The chlorine atoms needed for the reaction
are then generated in the following reaction.
Safer doesn't mean
without hazard. Sulfuryl chloride is also toxic and corrosive. The carbon tetrachloride that is used as a
solvent for the sulfuryl chloride is carcinogenic. Benzoyl peroxide is an explosion hazard. The SO2 generated in the reaction
is toxic. After all this, it may
be hard to believe that this is safer, but it is. You will be working with such small amounts
that someone would have to be fairly careless to hurt themselves or anyone
else. Of course being
careless means working with most of this material outside the hoods.
When a molecule contains more than
one type of hydrogen atom (i.e. 1°, 2°, or 3°), as in 2-methylbutane, a mixture of alkyl chlorides is
obtained. Even if only monochlorination
occurs, there should be four alkyl chloride products. If you look at the 2-methylbutane structure
shown, the H atoms are labeled as 1, 2, 3, or 4. The products obtained from replacing those H
atoms are labeled as A, B, C, and D respectively. Assuming that the H atoms are equally
reactive, then the H1's should be 6 times more likely to be replaced as H2
since there are 6 H1's and only 1 H2.
This would be the statistical prediction. The table below the reaction shows how much
of each isomer should be formed based on the statistical prediction. Then it shows how much of each actually
formed. It doesn't match.
|
Isomer |
A |
B |
C |
D |
|
Statistical
prediction % |
50 |
8 |
17 |
25 |
|
Observed % |
34 |
22 |
28 |
16 |
Too little of the
primary chlorides are obtained, and too much of the secondary and
tertiary. That's because the H atoms
don't react at equal rates. The
difference in reactivity has to do with radical stability (in Chapter 4). Based on these results and many other
experiments, it was determined that the relative rates of "radical"
reaction with hydrogens 1 through 4 (abstraction) are 1.0:4.0:2.5:1.0. The order of reactivity is tertiary >
secondary > primary. To test whether
these ratios are applicable to other hydrocarbons, you will perform the
chlorination reaction on 2,4-dimethylpentane using
sulfuryl chloride and benzoyl peroxide to generate chlorine atoms. The product
mixture will be analyzed by GC.