MORISSON   Organic Chemistry

MORISSON Organic Chemistry


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interested in the mechanisms of reactions? As an important part
of the theory of organic chemistry, they help make up the framework on which
we hang the facts we learn. An understanding of mechanisms will help us to see a
pattern in the complicated and confusing multitude of organic reactions. We shall
find that many apparently unrelated reactions proceed by the same or similar
mechanisms, so that most of what we have already learned about one reaction may
be applied directly to many new ones.
By knowing how a reaction takes place, we can make changes in the experi-
mental conditions not by trial and error, but logically that will improve the
yield of the product we want, or that will even alter the course of the reaction
completely and give us an entirely different product. As our understanding of
reactions grows* so does our power to control them.
2.12 Mechanism of chlorination. Free radicals
It will be worthwhile to examine the mechanism of chlorination of methane
in some detail. The same mechanism holds for bromination as well as chlorina-
tion, and for other alkanes as well as methane; it even holds for many compqunds
which, while not alkanes, contain alkane-like portions in their molecules. Closely
SEC. 2.12 MECHANISM OF CHLORINATION 47
related mechanisms are involved in oxidation (combustion) and other reactions of
alkanes. More important, this mechanism illustrates certain general principles
that can be carried over to a wide range of chemical reactions. Finally, by studying
the evidence that supports the mechanism, we can learn something of how a chemist
finds out what goes on during a chemical reaction.
Among the facts that must be accounted for are these: (a) Methane and
chlorine do not react in thr dark at room temperature, (b) Reaction takes place
readily, however, in the dark at temperatures over 250, or (c) under the influence
of ultraviolet light at room temperature, (d) When the reaction is induced by light,
many (several thousand) molecules of methyl chloride are obtained for each
photon of light that is absorbed by the system, (e) The presence of a small amount
of oxygen slows down the reaction for a period of time, after which the reaction
proceeds normally; the length of this period depends upon how much oxygen is
present.
The mechanism that accounts for these facts most satisfactorily, and hence is
generally accepted, is shown in the following equation :
(1) C12
heatorlight
> 2C1-
(2) Cl- + CH 4 > HC1 + CH 3 -
(3) CH 3 - + C12 > CH 3C1 + Ci-
then (2), (3), (2), (3), etc.
The first step is the breaking of a chlorine molecule into two chlorine atoms.
Like the breaking of any bond, this requires energy, the bond dissociation energy,
and in Table 1.2 (p. 21) we find that in this case the value is 58 kcal/mole. The
energy is supplied as either heat or light.
energy + :CI:C1: > :C1- + -Cl:
The chlorine molecule undergoes homolysis (Sec. 1.14): that is, cleavage of
the chlorine-chlorine bond takes place in a symmetrical way, so that each atom
retains one electron of the pair that formed the covalent bond. This odd electron
is not paired as are all the other electrons of the chlorine atom; that is, it does not
have a partner of opposite spin (Sec. 1.6). An atom or group ofatoms possessing an
odd (unpaired) electron is called a free radical. In writing the symbol for a free
radical, we generally include a dot to represent the odd electron just as we include
a plus or minus sign in the symbol of an ion.
Once formed, what is a chlorine atom most likely to do? Like most free
radicals, it is extremely reactive because of its tendency to gain an additional electron
and thus have a complete octet; from another point of view, energy was supplied
to each chlorine atom during the cleavage of the chlorine molecule, and this
energy-rich particle tends strongly to lose energy by the formation ofa new chemical
bond.
To form a new chemical bond, that is, to react, the chlorine atom must collide
with some other molecule or atom. What is it most likely to collide with? Ob-
viously, it is most likely to collide with the particles that are present in the" highest
concentration: chlorine molecules and methane molecules. Collision with another
48 METHANE CHAP. 2
chlorine atom is quite unlikely simply because there are very few of these reactive,
short-lived particles around at any time. Of the likely collisions, that with a
chlorine molecule causes no net change; reaction may occur, but it can result
only in the exchange of one chlorine atom for another:
: Cl- + : Cl . Cl : > : Cl : Cl : + : Cl- Collision probable but not productive
Collision of a chlorine atom with a methane molecule is both probable and
productive. The chlorine atom abstracts a hydrogen atom, with one electron, to
form a molecule of hydrogen chloride :
H H
H :C : H + Cl : > H : Cl : + H : C - Collision probable and productive
H H
Methane Methyl radical
Now the methyl group is left with an odd, unpaired electron; the carbon atom has
only seven electrons in its valence shell. One free radical, the chlorine atom, has
been consumed, and a new one, the methyl radical, CH 3 -, has been formed in its
place. This is step (2) in the mechanism.
Now, what is this methyl radical most likely to do? Like the chlorine atom,
it is extremely reactive, and for the same reason: the tendency to complete its octet,
to lose energy by forming a new bond. Again, collisions with chlorine molecules
or methane molecules are the probable ones, not collisions with the relatively
scarce chlorine atoms or methyl radicals. But collision with a methane molecule
could at most result only in the exchange of one methyl radical for another:
H H H H
H : C : H + C : H > H : C
J
-f H : C : H Collision probable but not productive
H H H H
The collision of a methyl radical with a chlorine molecule is, then, the impor-
tant one. The methyl radical abstracts a chlorine atom, with one of the bonding
electrons, to form a molecule of methyl chloride:
H H
H :C - -f : Cl : Cl : > H :C : Cl : + : Cl Collision probable and productive
H H
Methyl Methyl chloride
radical
The other product is a chlorine atom. This is step (3) in the mechanism.
Here again the consumption of one reactive particle has been accompanied by
the formation of another. The new chlorine atom attacks methane to form a
methyl radical, which attacks a chlorine molecule to form a chlorine atom, and so
the sequence is repeated over and over. Each step produces not only a new reactive
particle but also a molecule of product: methyl chloride or hydrogen chloride.
This process cannot, however, go on forever. As we saw earlier, union of two
r^^rt \\*,aA folotlyglv crarrf* nartirh*c is not Itkelv ! but CVerV SO often it dOCS happen
SEC. 2.14 INHIBITORS 49
and when it does, this particular sequence of reactions stops. Reactive particles
are consumed but not generated.
:C1- + -Cl: > :C1:C1:
CH 3 - + -CH 3 > CH3 :CH 3
CH 3 - + -Cl: > CH3 :CJ:
It is clear, then, how the mechanism accounts for facts (a), (b), (c), and (d)
on page 47 : either light or heat is required to cleave the chlorine molecule and form
the initial chlorine atoms; once formed, each atom may eventually bring about the
formation of many molecules of methyl chloride.
2.13 Chain reactions
The chlorination of methane is an example of a chain reaction, a reaction that
involves a series of steps, each of which generates a reactive substance that brings
about the next step. While chain reactions may vary widely in their details, they all
have certain fundamental characteristics in common.
(1) C12
heatorli8ht
> 2C1 Chain-initiating step
(2) Cl- + CH 4 HC1 + CH 3 - 1
> Chain-propagating steps
(3) CHr + C1 2 > CH3C1 + Cl- J
then (2), (3), (2), (3), etc., until finally:
(4) Cl- + -Cl > C1 2
or
(5) CH3 - + -CH 3 > CH 3CH 3 }>