Chapter 3 Notes

Chapter 3 Notes - Chapter
3
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Unformatted text preview: Chapter
3
 An
Introduction
to
Organic
 Reactions:
Acids
and
Bases
 Reactions
and
Their
Mechanisms
 There
are
four
general
types
of
organic
reactions:
 Substitutions
 Additions
 Eliminations
 Rearrangements
 Cleavage
of
Covalent
Bonds
 Homolysis
 Heterolysis
 In
Class
Problem:
 Designate
each
of
the
following
reactions
as
(a)
a
substitution,
(b)
an
elimination,
(c)
 an
addition,
(d)
a
rearrangement,
or
(e)
some
combination
of
the
former.
 Heterolytic
reactions
almost
always
occur
at
polar
bonds.

The
reaction
is
often
 assisted
by
formation
of
a
new
bond
to
another
molecule.

The
following
two
 examples
illustrate
two
types
of
heterolytic
cleavage
reactions:
 (1)
An
example
of
a
unimolecular
reaction
process:
 (2)
An
example
of
a
bimolecular
reaction
process:
 Introduction
to
Acid‐Base
Chemistry
 Brønsted‐Lowry
Definition
of
Acids
and
Bases
 Acid:
a
substance
that
can
donate
a
proton.
 Base:
a
substance
that
can
accept
a
proton.
 Example:
Hydrogen
chloride
is
a
very
strong
acid.

When
dissolved
in
water
 essentially
all
hydrogen
chloride
molecules
transfer
their
proton
to
water.
 Example:
Aqueous
hydrogen
chloride
and
aqueous
sodium
hydroxide
are
mixed.

 The
actual
reaction
is
between
hydronium
and
hydroxide
ions.
 Lewis
Definition
of
Acids
and
Bases
 Lewis
Acid:
electron
pair
acceptor
 Lewis
Base:
electron
pair
donor
 (Curved
arrows
show
movement
of
electrons
to
form
and
break
bonds.)
 Opposite
Charges
Attract
and
React
 BF3
and
NH3
react
based
on
their
relative
electron
densities.

BF3
has
substantial
 positive
charge
on
the
boron.

NH3
has
substantial
negative
charge
localized
at
the
 lone
pair
on
nitrogen.
 In
Class
Problem:
 Write
an
equation
that
shows
the
Lewis
acid
and
Lewis
base
in
the
reaction
of
 (a)  molecular
chlorine
(Cl2)
with
aluminum
chloride
(AlCl3).
 (b)  boron
trifluoride
(BF3)
with
tert‐butyl
alcohol
((CH3)3COH).
 Heterolysis
of
Bonds
to
Carbons:
 Carbanions
and
Carbocations
 Reaction
can
occur
to
give
a
carbocation
or
carbanion
depending
on
the
nature
of
Z.
 Carbocations
(Lewis
acids)
have
only
6
valence
electrons
and
a
positive
charge.
 Carbanions
(Lewis
bases)
have
8
valence
electrons
and
a
negative
charge.
 Organic
Chemistry
Terms
for
Lewis
Acids
and
Bases
 Electrophiles

(“electron‐loving ”
reagents
):

seek
electrons
to
obtain
a
stable
valence
 shell
of
electrons.

They
are
electron‐deficient
species
e.g.
carbocations.
 Nucleophiles
(“nucleus‐loving ”
reagents):
seek
a
proton
or
some
other
positively
 charged
center.

They
are
electron‐rich
species
e.g.
carbanions.
 The
Use
of
Curved
Arrows
in
Illustrating

Reactions
 Curved
arrows
show
the
flow
of
electrons
in
a
reaction
and
not
the
movement
of
 atoms.

An
arrow
starts
at
a
site
of
higher
electron
density
(a
covalent
bond
or
 unshared
electron
pair)
and
points
to
a
site
of
electron
deficiency.
 Example:
Mechanism
of
reaction
of
HCl
and
water.
 Strengths
of
Acids
and
Bases
 Ka
and
pKa
 Acetic
acid
is
a
relatively
weak
acid
and
a
0.1M
solution
is
only
able
to
protonate
 water
to
the
extent
of
about
1%.
 The
equilibrium
equation
for
this
reaction
is:
 Dilute
acids
have
a
constant
concentration
of
water
(about
55.5
M)
and
so
the
 concentration
of
water
can
be
factored
out
to
obtain
the
acidity
constant
(Ka).
 Ka
for
acetic
acid
is
1.76
X
10‐5
 Any
weak
acid
(HA)
dissolved
in
water
fits
the
general
Ka
expression.
 The
stronger
the
acid,
the
larger
the
Ka.
 Acidity
is
usually
expressed
in
terms
of
pKa.

pKa
is
the
negative
log
of
Ka.

The

pKa
for
 acetic
acid
is
4.75.


 The
larger
the
value
of
Ka
the
stronger
the
acid.

The
larger
the
pKa,
the
weaker
the
 acid.
 Aspirin
 Acetyl
Salicylic
Acid
 Neutral
and
Zwitterionic
 Amino
Acids
 In
Class
Problem:
 Acetyl
salicyclic
acid
(aspirin)
has
a
pKa
of
3.5.

What
are
the
molar
concentrations
of
 the
hydronium
ion
and
the
acetyl
salicylate
anion
in
a
0.1
molar
aqueous
solution
of
 acetyl
salicyclic
acid?
 Relative
Strenghts
of
Selected
Acids
and
Their
 Conjugate
Bases
 Predicting
the
Strengths
of
Bases
 The
stronger
the
acid,
the
weaker
its
conjugate
base.

An
acid
with
a
low
pKa
will
have
a
 weak
conjugate
base.

Chloride
ion
is
a
very
weak
base
because
its
conjugate
acid
HCl
is
 a
very
strong
acid.
 Methylamine
is
a
stronger
base
than
ammonia.

The
conjugate
acid
of
methylamine
 is
weaker
than
the
conjugate
acid
of
ammonia.
 Predicting
the
Outcome
of
Acid‐Base
Reaction
 Acid‐base
reaction
always
favor
the
formation
of
the
weaker
acid/weaker
base
pair.

 The
weaker
acid/weaker
base
are
always
on
the
same
side
of
the
equation.
 Example:
Acetic
acid
reacts
with
sodium
hydroxide
to
greatly
favor
products.
 Organic
compounds
which
are
water
insoluble
can
sometimes
be
made
soluble
by
 turning
them
into
salts
i.e.
reaction
with
sodium
hydroxide.


 Water
Solubility
as
a
Result
of
Salt
Formation
 Water
insoluble
amines
can
become
soluble
in
aqueous
hydrogen
chloride.
 The
Relationship
Between
Structure
and
Acidity
 Acidity
increases
going
down
a
 row
of
the
periodic
table.

The
 reason
is
a
combination
of
bond
 strength,
electron
affinity,
and
 solvation.
 Acidity
increases
from
left
to
right
in
a
row
of
the
periodic
table.

Increasingly
 electronegative
atoms

(1)
polarize
the
bond
to
hydrogen
and
(2)
stabilize
the
 conjugate
base.
 Overview
of
Acidity
Trends
 Hydrogens
connected
to
orbitals
with
more
s
character
will
be
more
acidic.

s
orbitals
 “feel”
the
presence
of
the
positively
charged
nucleus
more
than
p
orbitals.

As
a
 consequence,
negative
charges
(anions)
residing
in
hybrid
orbitals
with
more
s
 character
will
have
greater
stability.
 The
Effect
of
Hybridization
on
Acidity
 Electronic
effects
of
substituent
groups
are
transmitted
to
a
reaction
center
by
at
least
 two
mechanisms:
(1)
through
space
(electrostatic
field
effect)
and
(2)
through
the
 bonds
of
a
molecule
(inductive
effect).

In
ethyl
fluoride
the
electronegative
fluorine
is
 drawing
electron
density
(polarizing)
away
from
the
carbons.

Fluorine
is
termed
“an
 electron
withdrawing
group
(EWG).”

The
effect
of
the
fluorine
gets
weaker
with
 increasing
distance
from
the
reaction
center.
 Inductive
Effects
 Energy
Changes
in
Chemical
Reactions
 Energy:the
capacity
to
do
work.

 Kinetic
energy
is
the
energy
an
object
has
because
of
its
motion;
K.E.=1/2mv2.
 Potential
energy
is
stored
energy.

The
relative
stability
of
s
system
is
inversely
 related
to
its
relative
potential
energy.

The
more
potential
energy
an
object
has,
 the
less
stable
it
is.


 Potential
energy
can
be
converted
to
kinetic
energy
(e.g.
energy
of
motion).
 Potential
Energy
and
Covalent
Bonds
 Potential
energy
in
molecules
is
stored
in
the
form
of
chemical
bond
energy.

A
 convenient
way
to
represent
the
relative
potential
energies
of
molecules
is
in
terms
 of
their
relative
enthalpies,
or
heat
contents
(ΔHo).

The
change
in
enthalpy
of
a
 chemical
reaction,
ΔΗo

,
is
a
measure
of
the
change
in
bond
energies.
 Exothermic
reactions:
ΔHo
is
negative
and
heat
is
evolved.

The
potential
energy
in
 the
bonds
of
reactants
is
more
than
that
of
products.
 Endothermic
reactions:
ΔHo
is
positive
and
heat
is
absorbed.

The
potential
energy
 in
the
bonds
of
reactants
is
less
than
that
of
products.
 Example:
Formation
of
H2
from
H
atoms.

Formation
of
bonds
from
atoms
is
always
 exothermic
.

The
hydrogen
molecule
is
more
stable
than
hydrogen
atoms.
 The
Relationship
Between
the
Equilibrium
 Constant
and
Standard
Change
of
Free
Energy,
ΔGo
 ΔGo

(the
standard
free
energy
change
in
a
reaction)
is
the
overall
energy
change
of
a
 reaction.

It
is
directly
related
to
the
equilibrium
constant
of
a
reaction.

R
is
the
gas
 constant
(8.314
J
K‐1
mol‐1)
and
T
is
measured
in
degrees
Kelvin
(K).
 If
ΔGo
is
negative,
products
are
favored
at
equilibrium
(Keq
>1).

 If
ΔGo
is
positive,
reactants
are
favored
at
equilibrium
(Keq<1).

 If
ΔGo
is
zero,
products
and
reactants
are
equally
favored
(Keq
=
0).
 ΔGo
encompasses
both
enthalpy
changes
(ΔHo)
and
entropy
changes
(ΔSo
).
 ΔHo
,
the
enthalpy
change,
is
associated
with
changes
in
bonding
energy.

Ιf ΔHo
is
 negative
(exothermic)

this
makes
a
negative
contribution
to
ΔGo

(products
favored).
 
ΔSo
is
associated
with
the
relative
order
of
a
system.

More
disorder
means
greater
 entropy.

When
ΔSo
is
positive
the
system
is
going
to
a
less
ordered
state.

A
positive
ΔSo

 makes
a
negative
contribution
to
ΔGo
(products
favored).
 In
many
cases
ΔSo
is
small
and
ΔGo
is
approximately
equal
to
ΔHo
.
 The
Acidity
of
Carboxylic
Acids
 Carboxylic
acids
are
much
more
acidic
than
alcohols.

Deprotonation
is
unfavorable
 in
both
cases
(positive
ΔGo)
but
much
less
favorable
for
ethanol.
 Explanation
based
on
resonance
(delocalization)
effects
 Both
acetic
acid
and
acetate
are
stabilized
by
resonance.

Acetate
is
more
stabilized
by
 resonance
than
acetic
acid.

This
decreases
ΔGo
for
the
deprotonation.
 Neither
ethanol
nor
its
anion
is
stabilized
by
resonance.

There
is
no
decrease
in
ΔGo
 for
the
deprotonation.
 Calculated
Electrostatic
Potential
Maps
 Explanation
Based
On
Inductive
Effect
 In
acetic
acid
the
highly
polarized
carbonyl
group
draws
electron
density
away
 from
the
acidic
hydrogen.
 Also
the
conjugate
base
of
acetic
acid
is
more
stabilized
by
the
carbonyl
group.
 Inductive
Effects
of
Other
Groups
 The
electron
withdrawing
chloro
substitutent
makes
chloroacetic
acid
more
acidic
than
 acetic
acid.

In
this
case
we
are
comparing
the
effect
of
a
C‐Cl
bond
to
a
C‐H
bond.

Cl
is
 more
electronegative
than
H;
it
is
more
electron‐withdrawing
thereby
(1)
increasing
the
 “electronegativity ”
of
the
hydroxyl
oxygen
and
(2)
stabilizing
the
negative
charge
 associated
with
the
conjugate
base.
 In
Class
Problem:
 Place
the
following
carboxylic
acids
in
the
order
of
decreasing
acidity:
 The
Effect
of
Solvent
on
Acidity
 Acidity
values
in
gas
phase
are
generally
very
low.

It
is
difficult
to
separate
the
product
 ions
without
solvent
molecules
to
stabilize
them.
 Acetic
acid
has
pKa
of
130
in
the
gas
phase:
 _ A
protic
solvent
is
one
in
which
hydrogen
is
attached
to
a
highly
electronegative
atom
 such
as
oxygen
or
nitrogen
e.g.
water.
 Solvation
of
both
acetic
acid
and
acetate
ion
occurs
in
water
although
the
acetate
is
more
 stabilized
by
this
solvation.
 This
solvation
allows
acetic
acid
to
be
much
more
acidic
in
water
than
in
the
gas
phase.
 Organic
Compounds
as
Bases
 Any
organic
compound
containing
an
atom
with
a
lone
pair
(O,N)
can
act
as
a
base.
 Proton
transfer
reactions
like
these
are
often
the
first
step
in
many
reactions
of
 alcohols,
ethers,
aldehydes,
ketones,
esters,
amids
and
carboxylic
acids.
 π Electrons
can
also
act
as
bases.


π

Electrons
are
loosely
held
and
available
for
 reaction
with
strong
acids.
 A
Mechanism
for
an
Organic
Reaction
 Reaction
Mechanism:
a
description
of
the
events
that
take
place
on
a
molecular
 level
as
reactants
become
products.
 The
Substitution
Reaction
of
tert‐Butyl
Alcohol:
 All
steps
are
acid‐base
reactions:
 Step
1
is
a
Brønsted
acid‐base
reaction.
 Step
2
is
a
Lewis
acid‐base
reaction
in
reverse
with
heterolytic
cleavage
of
a
bond.

 Step
3
is
a
Lewis
acid‐base
reaction
with
chloride
acting
as
a
Lewis
base
and
the
 carbocation
acting
as
Lewis
acid.
 Acids
and
Bases
in
Nonaqueous
Solutions
 Water
has
a
leveling
effect
on
strong
acids
and
bases.

Any
base
stronger
than
 hydroxide
will
be
converted
to
hydroxide
in
water.
 Sodium
amide
can
be
used
as
a
strong
base
in
solvents
such
as
liquid
NH3.
 Alkyl
lithium
reagents
in
hexane
are
very
strong
bases.

The
alkyl
lithium
is
made
 from
the
alkyl
bromide
and
lithium
metal.
 Deuterium
(2H)
and
tritium
(3H)
are
isotopes
of
hydrogen.

They
are
used
for
labeling
 organic
compounds
to
be
able
to
track
where
these
compounds
go
(e.g.
in
biological
 systems).

An
alkyne
can
be
labeled
by
deprotonating
with
a
suitable
base
and
then
 titrating
with
T2O.
 Synthesis
of
Deuterium‐
and
Tritium‐Labeled
 Compounds
 In
Class
Problem:
 Complete
each
of
the
following
reactions:
 ...
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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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