Chapter 6 Notes

Chapter 6 Notes - Chapter
6
 Ionic
Reactions


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Unformatted text preview: Chapter
6
 Ionic
Reactions
 Nucleophilic
Substitution
and
Elimination
 Reactions
of
Alkyl
Halides
 Introduction
 The
polarity
of
a
carbon‐halogen
bond
leads
to
the
carbon
having
a
partial
positive
 charge.

In
alkyl
halides
this
polarity
causes
the
carbon
to
become
activated
to
 substitution
reactions
with
nucleophiles.
 Carbon‐halogen
bonds
get
less
polar,
longer
and
weaker
in
going
from
fluorine
to
iodine
 Structural
Types
of
Organic
Halides
 Nucleophilic
Substitution
Reactions
 In
these
types
of
reactions
a
nucleophile
(a
species
with
an
unshared
electron
pair)
 reacts
with
an
electron
deficient
carbon
bearing
a
leaving
group.


The
leaving
group
 is
displaced
by
a
nucleophile
and
separates
from
the
carbon
with
its
pair
of
electrons.
 Examples
of
nucleophilic
substitution:
 The
nucleophile
reacts
at
the
electron
deficient
(electrophilic)
carbon.
 A
nucleophile
may
be
any
negative
ion
or
neutral
molecule
that
has
at
least
one
 unshared
electron
pair.
 A
leaving
group
is
a
substituent
that
can
leave
as
a
relatively
stable
entity
with
the
 pair
of
electrons
originally
bonding
it
to
carbon.

It
can
leave
as
an
anion
or
a
 neutral
species.
 In
Class
Problems:
 Designate
the
nucleophile,
the
electrophile,
and
the
leaving
group
for
each
of
 the
following
reactions.
 Kinetics
of
a
Nucleophilic
Substitution
Reaction:
 An
SN2
Reaction
 The
initial
rate
of
the
following
reaction
is
measured.
 The
rate
is
directly
proportional
to
the
initial
concentrations
of
both
methyl
chloride
and
 hydroxide
ion.

The
rate
equation
reflects
this
dependence.

Methyl
chloride
and
 hydroxide
ion
are
involved
in
the
rate‐controlling
step
of
the
reaction
or
in
a
step
prior
to
 the
rate‐controlling
step.
 SN2
reaction:
substitution,
nucleophilic,
2nd
order
(bimolecular).
 A
Mechanism
for
the
SN2
Reaction
 A
transition
state
is
the
high
energy
state
of
the
reaction.

It
is
an
unstable
entity
with
a
 very
brief
existence
(10‐12
s).

In
the
transition
state
of
this
reaction
bonds
are
partially
 formed
and
broken.

Both
chloromethane
and
hydroxide
ion
are
involved
in
the
 transition
state
and
this
explains
why
the
reaction
is
second
order.
 In
Class
Problem:
 What
is
the
effect
of
doubling
the
concentration
of
potassium
acetate
in
the
 following
SN2
reaction?
 Transition
State
Theory:
Free‐Energy
Diagrams
 Exergonic
reaction:
negative
ΔGo
(products
favored).
 Endergonic
reaction:
positive
ΔGo
(products
not
favored).
 The
reaction
of
chloromethane
with
hydroxide
is
highly
exergonic. 
 

 The
equilibrium
constant
is
very
large.
 An
energy
barrier
is
evident
because
a
bond
is
being
broken
in
going
to
the
transition
 state
(which
is
the
top
of
the
energy
barrier).

The
difference
in
energy
between
 starting
material
and
the
transition
state
is
the
free
energy
of
activation
(ΔG‡
).

The
 difference
in
energy
between
starting
molecules
and
products
is
the
free
energy
 change
of
the
reaction,

ΔGo.
 A
Free
Energy
Diagram
of
a
Typical
SN2
Reaction
 In
a
highly
endergonic
reaction
of
the
same
type
the
energy
barrier
will
be
even
 higher
(ΔG‡
is
very
large).
 There
is
a
direct
relationship
between
ΔG‡
and
the
temperature
of
a
reaction.

The
 higher
the
temperature,
the
faster
the
rate.
 Where
e
=
2.718
and
ko
is
the
absolute
rate
constant
[the
rate
at
which
all
transition
 states
proceed
to
products
(6.2
x
1012
sec‐1)].

Near
room
temperature,
a
10oC
increase
 in
temperature
causes
a
doubling
of
rate.

Higher
temperatures
cause
more
molecules
 to
collide
with
enough
energy
to
reach
the
transition
state
and
react.
 The
energy
diagram
for
the
reaction
of
chloromethane
with
hydroxide:
 A
reaction
with
ΔG‡
above
84
kJ
mol‐1
will
require
heating
to
proceed
at
a
reasonable
 rate.

This
reaction
has
ΔG‡
=
103
kJ
mol‐1
so
it
will
require
heating.
 The
Stereochemistry
of
SN2
Reactions
 Backside
attack
of
nucleophile
results
in
an
inversion
of
configuration.

Configuration
 describes
a
particular
arrangement
of
atoms
around
the
reacting
carbon
center.
 In
cyclic
systems
a
cis
compound
can
react
and
become
trans
product.
 SN2
reactions
always
occur
with
inversion
of
configuration.
 In
Class
Problem:
 (R)‐2‐Butanol,
labeled
with
18O,
is
subjected
to
the
following
sequence
of
 reactions.

What
is
the
absolute
configuration
of
the
product?
 The
Reaction
of
tert‐Butyl
Chloride
with
 Hydroxide
Ion:
An
SN1
Reaction
 tert‐Butyl
chloride
undergoes
substitution
with
hydroxide.

The
rate
is
independent
of
 hydroxide
concentration
and
depends
only
on
concentration
of
tert‐butyl
chloride.

The
 rate
equation
is
first
order
with
respect
to
tert‐butyl
chloride
and
first
order
overall.
 SN1
reaction:
Substitution,
nucleophilic,
1st
order
(unimolecular).
 The
rate
depends
only
on
the
concentration
of
the
alkyl
halide.

Only
the
alkyl
halide
(and
 not
the
nucleophile)
is
involved
in
the
transition
state
of
the
step
that
controls
the
rate.
 Multistep
Reactions
and
the
Rate‐Determining
Step
 In
multistep
reactions,
the
rate
of
the
overall
reaction
will
be
essentially
the
same
as
the
 rate
of
the
slowest
step.

This
is
called
the
rate‐limiting
step
or
the
rate‐
determining
 step.

In
the
case
below
k1<<k2
or
k3
and
the
first
step
is
rate
determining.
 A
Mechanism
 for
the
SN1
 Reaction
 Step
1
is
rate
 determining
 (slow)
because
 it
requires
the
 formation
of
 unstable
ionic
 products.

In
 step
1
water
 molecules
help
 stabilize
the
 ionic
products.
 Carbocations
 A
carbocation
has
only
6
electrons,
is
sp2
hybridized
and
has
an
empty
p
orbital
 normal
to
the
cationic
center
and
the
three
atoms
attached
to
this
center.
 The
more
alkyl
groups
attached
to
the
carbocationic
center
the
more
stable
the
 carbocation.

The
more
stable
a
carbocation
is,
the
easier
it
is
to
form.
 Hyperconjugation
stabilizes
the
carbocation
by
donation
of
electrons
from
an
adjacent
 β
carbon‐hydrogen
or
carbon‐carbon
σ
bond
into
the
empty
p
orbital.


More
 substitution
provides
more
opportunity
for
hyperconjugation.
 The
Stereochemistry
of
SN1
Reactions
 When
the
leaving
group
leaves
from
a
stereogenic
center
of
an
optically
active
 compound
in
an
SN1
reaction
racemization
will
occur.

The
reason
is
that
an
achiral
 carbocation
intermediate
is
formed.
 Racemization:
transformation
of
an
optically
active
compound
to
a
racemic
mixture.
 The
Stereochemistry
of
SN1
Reactions
 In
Class
Problem:
 Explain
the
following
observation.

(S)‐3‐Bromo‐3‐methylhexane
reacts
in
aqueous
 acetone
to
give
racemic
3‐methyl‐3‐hexanol.
 Solvolysis
 A
molecule
of
the
solvent
is
the
nucleophile
in
a
substitution
reaction.

If
the
solvent
is
 water
the
reaction
is
a
hydrolysis.

If
the
solvent
is
methanol
or
formic
acid,
the
reaction
 is
methanolysis
or
formolysis,
repectively.
 Factors
Affecting
the
Rate
of
SN1
and
SN2
Reactions
 The
Effects
of
the
Structure
of
the
Substrate
 (1)  Structure
of
the
substrate.
 (2)  Concentration
and
reactivity
of
the
nucleophile
(for
bimolecular
reactions).
 (3)  Effect
of
solvent.
 (4)  Nature
of
the
leaving
group.
 Factors
Affecting
the
Rate
of
SN1
and
SN2
Reactions
 The
Effects
of
the
Structure
of
the
Substrate
 SN2
Reactions:
 In
SN2
reactions
alkyl
halides
show
the
following
general
order
of
reactivity.
 Steric
hinderance:
the
spatial
arrangement
of
the
atoms
or
groups
at
or
near
a
reacting
 site
hinders
or
retards
a
reaction.

In
tertiary
and
neopentyl
halides,
the
reacting
carbon
 is
too
sterically
hindered
to
react
at
an
appreciable
rate
by
the
SN2
mechanism.
 SN1
reactions
 Organic
compounds
that
undergo
reaction
by
an
SN1
path
at
a
reasonable
rate
are
those
 that
are
capable
of
forming
relatively
stable
carbocations
such
as
tertiary
halides,
allyl
 halides
and
benzyl
halides.
 The
Hammond‐Leffler
Postulate
 The
transition
state
for
an
exergonic
reaction
looks
very
much
like
starting
material.

The
 transition
state
for
an
endergonic
reaction
looks
very
much
like
product.

Generally
the
 transition
state
looks
most
like
the
species
it
is
closest
to
in
energy
 In
the
first
step
of
the
SN1
reaction
the
transition
state
looks
very
much
like
 carbocation
intermediate.

The
carbocation‐like
transition
state
is
stabilized
by
all
the
 factors
that
stabilize
carbocations.

The
transition
state
leading
to
tertiary
 carbocations
is
much
more
stable
and
lower
in
energy
than
transition
states
leading
 to
other
carbocations
such
as
secondary
carbocations.
 Vinylic
and
phenyl
halides
are
generally
unreactive
in
SN1
and
SN2
reactions.
 SN1
Reaction:
 Rate
does
not
depend
on
the
identity
or
concentration
of
nucleophile
 SN2
Reaction:
 Rate
is
directly
proportional
to
the
concentration
of
nucleophile.

Stronger
 nucleophiles
also
react
faster.

A
negatively
charged
nucleophile
is
always
more
 reactive
than
its
neutral
conjugate
acid.

When
comparing
nucleophiles
with
the
same
 nucleophilic
atom,
nucleophilicities
parallel
basicities.
 The
Effects
of
the
Concentration
and
Strength
of
 Nucleophile
 Methoxide
is
a
much
better
nucleophile
than
methanol.
 Solvent
Effects
on
SN2
Reactions:
 Polar
Protic
and
Aprotic
Solvents
 Polar
Protic
Solvents:
Polar
solvents
that
have
a
hydrogen
atom
attached
to
strongly
 electronegative
atoms.

They
solvate
nucleophiles
and
make
them
less
reactive.
 Larger
nucleophilic
atoms
are
less
solvated
and
therefore
more
reactive
in
polar
protic
 solvents.

Largernucleophiles
are
also
more
polarizable
and
can
donate
more
electron
 density.
 Relative
nucleophilicity
in
polar
solvents:
 Polar
Aprotic
Solvents:
Polar
aprotic
solvents
do
not
have
a
hydrogen
attached
to
an
 electronegative
atom.
 They
solvate
cations
well
but
leave
anions
relatively
unsolvated.
 Polar
protic
solvents
lead
to
generation
of
“naked”
and
very
reactive
nucleophiles.

 Trends
for
nucleophilicity
are
often
the
same
as
for
basicity.

They
are
excellent
solvents
 for
SN2
reactions.
 Solvent
Effects
on
SN1
Reactions:
 The
Ionizing
Ability
of
the
Solvent
 Polar
protic
solvents
are
excellent
solvents
for
SN1
reactions.

Polar
protic
solvents
 stabilize
the
carbocation‐like
transition
state
leading
to
the
carbocation
thus
lowering
 ΔG‡.

In
order
to
increase
the
solubility
of
alkyl
halides
mixed
solvent
systems
are
often
 used.

Water‐ethanol
and
water‐methanol
mixtures
are
most
common
solvent
 mixtures.
 The
Nature
of
the
Leaving
Group
 The
best
leaving
groups
are
weak
bases
which
are
relatively
stable.

The
leaving
group
can
 be
an
anion
or
a
neutral
molecule.
 Leaving
group
ability
of
halides:
 This
trend
is
opposite
to
basicity:
 Other
very
weak
bases
which
are
good
leaving
groups:
 The
poor
leaving
group
hydroxide
can
be
changed
into
the
good
leaving
group
water
by
 protonation.
 Summary
SN1
vs.
SN2
 In
both
SN2
and
SN1
reactions
alkyl
iodides
react
the
fastest
than
alkyl
bromides
which
 react
faster
than
alkyl
chlorides
because
iodide
is
a
leaving
group
than
bromide
which
is
 better
than
chloride.
 In
Class
Problems:
 For
each
of
the
following
pairs
of
reactions,
predict
which
one
is
faster
and
 explain
why.
 Organic
Synthesis:
 Functional
Group
Transformations
Using
SN2
Reactions
 Stereochemistry
can
be
controlled
in
SN2
reactions.
 Elimination
Reactions
of
Alkyl
Halides
 Elimination
reactions
of
alkyl
halides
compete
with
substitution
reactions.

In
a
typical
 elimination
reaction
the
fragments
of
some
molecule
(YZ)
are
removed
from
adjacent
 atoms
of
the
reactant.

This
results
in
the
formation
of
a
multiple
bond.
 Dehydrohalogenation:
Used
for
the
synthesis
of
alkenes.

Elimination
competes
with
 substitution
reaction.
 Strong
bases
such
as
alkoxides
favor
elimination.
 The
alkoxide
bases
are
made
from
the
corresponding
alcohols.
 The
E2
Reaction
 E2
reaction
involves
concerted
removal
of
the
proton,
formation
of
the
double
bond,
 and
departure
of
the
leaving
group.

Both
alkyl
halide
and
base
concentrations
affect
 rate
and
therefore
the
reaction
is
2nd
order.
 The
E1
Reaction
 The
E1
reaction
competes
with
the
SN1
reaction
and
likewise
goes
through
a
carbocation
 intermediate
 Substitution
versus
Elimination
 SN2
versus
E2
 Primary
substrate:
If
the
base
is
small,
SN2
competes
strongly
because
approach
at
 carbon
is
generally
unhindered.
 Secondary
substrate:
Approach
to
carbon
is
generally
sterically
hindered
and
E2
 elimination
is
favored.
 Tertiary
substrate:
Approach
to
carbon
is
extremely
hindered
and
elimination
 predominates
especially
at
high
temperatures.
 Temperature:
Increasing
temperature
favors
elimination
over
substitution.
 Size
of
theBase/Nucleophile:
Large
sterically
hindered
bases
favor
elimination
because
 they
cannot
directly
approach
the
backside
of
the
carbon
halogen
bond
closely
enough
 to
react

in
a
substitution.

Potassium
tert‐butoxide
is
an
extremely
bulky
base
and
is
 routinely
used
to
favor
E2
reaction.
 In
Class
Problem:
 Give
the
product
(or
products)
formed
in
each
of
the
following
reactions.

In
each
 case
give
the
mechanism
(SN1,
SN2,
E1,
E2)
by
which
the
product
is
formed.

 Indicate
the
major
and
minor
products
for
each
reaction.
 ...
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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 Tech.

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