Chapter 10 Notes - Chapter
10
 Radical
Reactions
...

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Unformatted text preview: Chapter
10
 Radical
Reactions
 Introduction
 Homolytic
bond
cleavage
leads
to
the
formation
of
radicals
(also
called
free
radicals).

 Radicals
are
highly
reactive,
short‐lived
species.

Single‐barbed
arrows
are
used
to
show
 the
movement
of
single
electrons.
 Production
of
Radicals:

Homolysis
of
relatively
weak
bonds
such
as
O‐O
or
X‐X
bonds
 can
occur
with
addition
of
energy
in
the
form
of
heat
or
light.
 Reactions
of
Radicals:
Radicals
tend
to
react
in
ways
that
lead
to
pairing
of
their
 unpaired
electron.

Hydrogen
abstraction
is
one
way
a
halogen
radical
can
react
to
pair
 its
unshared.

 Homolytic
Bond
Dissociation
Energies:
Atoms
have
higher
energy
(are
less
stable)
than
 the
molecules
they
can
form.

The
formation
of
covalent
bonds
is
exothermic.

Breaking
 covalent
bonds
requires
energy
(i.e.
is
endothermic).
 The
homolytic
bond
dissociation
energy
is
abbreviated
DHo.
 Homolytic
Bond
Dissociation
Energies
and
Heats
of
Reaction:
Homolytic
bond
 dissociation
energies
can
be
used
to
calculate
(estimate)
the
enthalpy
change
(ΔHo)
for
a
 reaction.

DHo
is
positive
for
bond
breaking
and
negative
for
bond
forming.
 Example:

 This
reaction
below
is
highly
exothermic
since
ΔHo
is
a
large
and
negative.

ΔHo
is
not
 dependant
on
the
mechanism;
only
the
initial
and
final
states
of
the
molecules
are
 considered
in
determining
ΔHo.
 In
Class
Problem:
 Estimate
the
heat
of
reaction,
ΔHo,
for
each
of
the
following
reactions.
 Homolytic
Bond
Dissociation
Energies
and
the
Relative
Stabilities
of
Radicals:
The
 formation
of
different
radicals
from
the
same
starting
compound
offers
a
way
to
estimate
 relative
radical
stabilities.
 Examples:
 The
propyl
radical
is
less
stable
than
the
isopropyl
radical.
 Likewise
the
tert‐butyl
radical
is
more
stable
than
the
isobutyl
radical.
 The
energy
diagrams
for
these
reactions
are
shown
below.
 The
relative
stabilities
of
radicals
follows
the
same
trend
as
for
carbocations.

The
most
 substituted
radical
is
most
stable.

Radicals
are
electron
deficient,
as
are
carbocations,
 and
are
therefore
also
stabilized
by
hyperconjugation.
 The
Reactions
of
Alkanes
with
Halogens
 Alkanes
undergo
substitution
reactions
with
halogens
such
as
fluorine,
bromine
and
 chlorine
in
the
presence
of
heat
or
light.
 Multiple
Substitution
Reactions
versus
Selectivity:

Radical
halogenation
can
yield
a
 mixture
of
halogenated
compounds
because
all
hydrogen
atoms
in
an
alkane
are
 capable
of
substitution.

In
the
reaction
above
all
degrees
of
methane
halogenation
will
 be
seen.

Monosubstitution
can
be
achieved
by
using
a
large
excess
of
the
alkane.

A
 large
excess
of
methane
will
lead
to
predominantly
monohalogenated
product
and
 excess
unreacted
methane.
 In
Class
Problem:
 List
the
following
organic
radicals
in
the
order
of
increasing
stability.
 Chlorination
of
higher
alkanes
leads
to
mixtures
of
monochlorinated
product
(and
more
 substituted
products).

Chlorine
is
relatively
unselective
and
does
not
greatly
 distinguish
between
type
of
hydrogen.

 Molecular
symmetry
is
important
in
determining
the
number
of
possible
substitution
 products.
 Bromine
is
less
reactive
but
more
selective
than
chlorine
(Sec.
10.6A).
 Chlorination
of
Methane:
Mechanism
of
Reaction
 The
reaction
mechanism
has
three
distinct
aspects:
(1)
chain
initiation,
(2)
chain
 propagation
and
(3)
chain
termination.
 Chain
initiation:
 Chlorine
radicals
form
when
the
reaction
mixture
is
subjected
to
heat
or
light.

Chlorine
 radicals
are
used
in
the
chain
propagation
steps
below.
 Chain
propagation: 

 A
chlorine
radical
reacts
with
a
molecule
of
methane
to
generate
a
methyl
radical.

A
 methyl
radical
reacts
with
a
molecule
of
chlorine
to
yield
chloromethane
and
regenerate
 chlorine
radical.

A
chlorine
radical
reacts
with
another
methane
molecule,
continuing
the
 chain
reaction.

A
single
chlorine
radical
can
lead
to
thousands
of
chain
propagation
cycles.
 The
entire
mechanism
is
shown
below.
 Chain
reaction:
a
stepwise
mechanism
in
which
each
step
generates
the
reactive
 intermediate
that
causes
the
next
cycle
of
the
reaction
to
occur.
 Chain
termination:
 Occasionally
the
reactive
radical
intermediates
are
quenched
by
reaction
pathways
that
 do
not
generate
new
radicals.

The
reaction
of
chlorine
with
methane
requires
constant
 irradiation
to
replace
radicals
quenched
in
chain‐terminating
steps.
 Chlorination
of
Methane:
Energy
Changes 

 The
chain
propagation
steps
have
overall
ΔHo=
‐101
kJ
mol‐1
and
are
highly
exothermic.
 The
Overall
Free‐Energy
Change:
ΔGo
=
ΔHo
‐
T
(ΔSo)
 

 In
radical
reactions
such
as
the
chlorination
of
methane
the
overall
entropy
change
(ΔSo)
in
 the
reaction
is
small
and
thus
it
is
appropriate
to
use
ΔHo
values
to
approximate
ΔGo
values
 ΔGo
=
‐102
kJ
mol‐1
and
ΔHo
=
‐101
kJ
mol‐1
for
this
reaction.
 Activation
Energies:
 When
using
enthalpy
values
(ΔHo)
the
term
for
the
difference
in
energy
between
starting
 material
and
the
transition
state
is
the
energy
of
activation
(Eact).

Recall
when
free
energy
 of
activation
(ΔGo)
values
are
used
this
difference
is
ΔG‡.

For
the
chlorination
of
methane
 the
Eact
values
have
been
calculated.
 Energy
of
activation
values
can
be
predicted.

A
reaction
in
which
bonds
are
broken
will
 have
Eact
>
0
even
if
a
stronger
bond
is
formed
and
the
reaction
is
highly
exothermic
 Bond
forming
always
lags
behind
bond
breaking.

An
endothermic
reaction
which
 involves
bond
breaking
and
bond
forming
will
always
have
Eact
>
ΔHo.
 A
gas
phase
reaction
in
which
only
bond
homolysis
occurs
has
ΔHo
=
Eact.
 A
gas
phase
reaction
in
which
small
radicals
combine
to
form
a
new
bond
usually
has
Eact
 =
0.
 Reaction
of
Methane
with
Other
Halogens
 The
order
of
reactivity
of
methane
substitution
with
halogens
is:
fluorine

>
chlorine
>
 bromine
>
iodine.

The
order
of
reactivity
is
based
on
the
values
of
Eact
for
the
first
step
of
 chain
propagation
and
ΔHo
for
the
entire
chain
propagation.

Fluorination
has
a
very
low
 value
for
Eact
in
the
first
step
and
ΔHo
is
extremely
exothermic
therefore
fluorination
 reactions
are
explosive.

Chlorination
and
bromination
have
increasingly
higher
values
of
 Eact
and
lower
overall
ΔHo
valueswhich
makes
these
halogenation
reactions
less
vigorous.

 Iodinination
has
a
prohibitively
high
value
for
Eact
of
the
first
step
and
the
reaction
does
not
 occur.

The
energy
values
of
the
initiation
step
are
unimportant
since
they
occur
so
rarely.
 On
the
basis
of
ΔHo

values
for
the
initiation
step
iodination
should
be
most
rapid.
 Halogenation
of
Higher
Alkanes
 Selectivity
of
Chlorine:
Monochlorination
of
alkanes
proceeds
to
give
some
selectivity.

 Teritiary
hydrogens
are
somewhat
more
reactive
than
secondary
hydrogens
which
are
 more
reactive
than
primary
hydrogens.

Eact
for
abstraction
of
a
tertiary
hydrogen
is
lower
 because
of
increased
stability
of
the
intermediate
tertiary
radical.

The
differences
in
rate
 of
abstraction
are
not
large
and
chlorination
occurs
so
rapidly
it
cannot
distinguish
well
 between
classes
of
hydrogen
and
so
is
not
very
selective.
 Selectivity
of
Bromine:
Bromine
is
much
less
reactive
but
more
selective
than
chlorine
in
 radical
halogenation.

Fluorine
shows
almost
no
discrimination
in
replacement
of
 hydrogens
because
it
is
so
reactive.
 In
Class
Problem:
 The
energy
of
activation
for
the
hydrogen
atom
abstraction
step
in
the
chlorination
of
 ethane
is
4.2
kJ
mol‐1.

 (1)
Write
the
equation
describing
this
reaction.
 (2)
Estimate
the
ΔHo
for
this
step.
 (3)
Draw
a
potential
energy
diagram
describing
the
reaction.
 Reactions
that
Generate
Tetrahedral
 Stereogenic
Carbons
 A
reaction
of
achiral
starting
materials
which
produces
a
product
with
a
stereogenic
 carbon
will
produce
a
racemic
mixture.
 Generation
of
a
Second
Stereogenic
Carbon
in
a
 Radical
Halogenation
 When
a
molecule
with
one
or
more
stereogenic
carbons
undergoes
halogenation
to
 create
another
stereogenic
carbon,
the
two
diastereomeric
products
are
not
produced
 in
equal
amounts.

The
intermediate
radical
is
chiral
and
reactions
on
the
two
faces
of
 the
radical
are
not
equally
probable.
 In
Class
Problem:
 (1)  Draw
the
stereochemical
structures
for
the
reactant
and
product
for
the
 reaction
of
(S)‐2‐chloropentane
with
chlorine.
 (2)  Indicate
the
stereochemical
configuration
at
each
of
the
chiral
carbons
in
the
 products.
 (3)  Are
the
products
optically
active?
 (4)  Can
the
products
be
separated
by
conventional
means?
 Radical
Addition
to
Alkenes:
The
anti‐Markovnikov
 Addition
of
Hydrogen
Bromide
 Addition
of
hydrogen
bromide
in
the
presence
of
peroxides
gives
anti‐Markovnikov
addition.

 The
other
hydrogen
halides
do
not
give
this
type
of
anti‐Markovnikov
addition.
 Steps
1
and
2
of
the
mechanism
are
chain
initiation
steps
which
produce
a
bromine
radical.
 In
step
3,
the
first
step
of
propagation,

a
bromine
radical
adds
to
the
double
bond
to
give
 the
most
stable
of
the
two
possible
carbon
radicals
(in
this
case,
a
2o
radical).

Attack
at
 the
1o
carbon
is
also
less
sterically
hindered.
 Step
4
regenerates
a
bromine
radical
which
then
reacts
with
another
alkene
molecule.
 Radical
Polymerization
of
Alkenes:
 Chain‐Growth
Polymers
 Polymers
are
macromolecules
made
up
of
repeating
subunits.

The
subunits
used
to
 synthesize
polymers
are
called
monomers.

Polyethylene
is
made
of
repeating
subunits
 derived
from
ethylene.

Polyethylene
is
called
a
chain‐growth
polymer
or
addition
 polymer.
 Polystyrene
is
made
in
an
analogous
reaction
using
styrene
as
the
monomer.
 A
very
small
amount
of
diacyl
peroxide
is
added
in
initiating
the
reaction
so
that
few

but
 very
long

polymer
chains
are
obtained.
 The
propagation
step
simply
adds
more
ethylene
molecules
to
a
growing
chain.
 Chain
branching
occurs
by
abstraction
of
a
hydrogen
atom
on
the
same
chain
and
 continuation
of
growth
from
the
main
chain.
 ...
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This note was uploaded on 06/19/2009 for the course CHEM 2311 taught by Professor Tyson during the Fall '07 term at Georgia Institute of Technology.

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