Chapter 5 Notes - Chapter
5
 Stereochemistry:
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Unformatted text preview: Chapter
5
 Stereochemistry:
 Chiral
Molecules
 Isomerism:
 Constitutional
Isomers
and
Stereoisomers
 Stereoisomers
are
isomers
with
the
same
molecular
formula
and
same
connectivity
of
 atoms
but
different
arrangement
of
atoms
in
space.
 Enantiomers:
stereoisomers
whose
molecules
are
nonsuperposable
mirror
images
of
 each
other.
 Diastereomers:
stereoisomers
whose
molecules
are
not
mirror
images
of
each
other.
 Example:
cis
and
trans
double
bond
isomers‐
stereoisomers
that
are
diastereomers
 Example:
cis
and
trans
cycloalkane
isomers
 Enantiomers
and
Chiral
Molecules
 Chiral
molecule:
a
molecule
which
is
not
superposable
on
its
mirror
image.

A
chiral
 molecule
and
its
mirror
image
are
called
a
pair
of
enantiomers.

Molecules
that
are
 superposable
on
their
mirror
image
are
achiral.


 Example:
2‐butanol
is
a
chiral
molecule.

I
and
II
are
mirror
images
(figures
a
and
b)
and
 are
not
superposable
(figure
c).

They
are
enantiomers.
 Example:
2‐propanol,
in
contrast,
is
not
chiral.
 A
pair
of
enantiomers
is
always
possible
for
molecules
that
contain
a
single
tetrahedral
 atom
with
four
different
groups
attached
to
it.

Such
atoms
are
called
chirality
centers.


A
 molecule
with
more
than
one
tetrahedral
carbon
bonded
to
four
different
groups
is
not
 always
chiral.

Switching
two
groups
at
the
tetrahedral
center
leads
to
the
enantiomeric
 molecule
in
a
molecule
with
one
tetrahedral
carbon.

Any
atom
at
which
an
interchange
of
 groups
produces
a
stereoisomer
is
called
a
stereogenic
center.
 Carbons
at
a
tetrahedral
stereogenic
center
are
designated
with
an
asterisk
(*).
 Example:
2‐butanol
 The
Biological
Importance
of
Chirality
 The
binding
specificity
of
a
chiral
receptor
site
for
a
chiral
molecule
is
usually
only
 favorable
in
one
way.
 Chiral
molecules
can
exhibit
their
handedness
in
many
ways.
 Tests
for
Chirality:
Planes
of
Symmetry
 Plane
of
symmetry:
an
imaginary
plane
that
bisects
a
molecule
in
such
a
way
that
the
 two
halves
of
the
molecule
are
mirror
images
of
each
other.

A
molecule
with
a
plane
 of
symmetry
cannot
be
chiral.
 Example:
 2‐Chloropropane
(a)
has
a
plane
of
symmetry
but
2‐chlorobutane

(b)
does
not.
 Nomenclature
of
Enantiomers:
The
R,S
System
 Cahn‐Ingold‐Prelog
System
 The
four
groups
attached
to
the
stereogenic
carbon
are
assigned
priorities
from
highest
 (a)
to
lowest
(d).

Priorities
are
assigned
as
follows:
Atoms
directly
attached
to
the
 stereogenic
center
are
compared.

Atoms
with
higher
atomic
number
are
given
higher
 priority.

If
priority
cannot
be
assigned
based
on
directly
attached
atoms,
the
next
layer
 of
atoms
is
examined.
 Example:
 The
molecule
is
rotated
to
put
the
lowest
priority
group
back.

If
the
groups
descend
 in
priority
(a,b
then
c)
in
clockwise
direction
the
enantiomer
is
R.

If
the
groups
 descend
in
priority
in
counterclockwise
direction
the
enantiomer
is
S.
 Groups
with
double
or
triple
bonds
are
assigned
priorities
as
if
their
atoms
were
 duplicated
or
triplicated.
 In
Class
Problem:

Are
A
and
B
identical
or
enantiomers?
 In
Class
Problem:

Are
A
and
B
identical
or
enantiomers?
 Manipulate
B
to
see
if
it
will
become
superposable
with
A
 In
Class
Problem:

Are
A
and
B
identical
or
enantiomers?
 Manipulate
B
to
see
if
it
will
become
superposable
with
A
 Exchange
2
groups
to
try
to
convert
B
into
A.

One
exchange
of
groups
leads
to
the
 enantiomer
of
B.

Two
exchanges
of
groups
leads
back
to
B
 Properties
of
Enantiomers:
Optical
Activity
 Enantiomers
have
almost
all
identical
physical
properties
(melting
point,
boiling
point,
 density).

However,
enantiomers
rotate
the
plane
of
plane‐polarized
light
in
equal
but
 opposite
directions.
 Plane
Polarized
Light
 Oscillation
of
the
electric
field
of
ordinary
light
occurs
in
all
possible
planes
perpendicular
 to
the
direction
of
propagation.
 If
the
light
is
passed
through
a
polarizer
only
one
plane
emerges.
 The
Polarimeter
 Specific
Rotation
 An
empty
sample
tube
or
one
containing
an
achiral
molecule
will
not
rotate
the
plane‐ polarized
light.

An
optically
active
substance
(e.g.
one
pure
enantiomer
)
will
rotate
the
 plane‐polarized
light
.

The
amount
the
analyzer
needs
to
be
turned
to
permit
light
 through
is
called
the
observed
rotation
α. The
standard
value
specific
rotation
[α]
can
be
 calculated.

If
the
analyzer
is
rotated
clockwise
the
rotation
is
(+)
and
the
molecule

is
 dextrorotatory.

If
the
analyzer
is
rotated
counterclockwise
the
rotation
is
(‐)
and
the
 molecule
is
levorotatory.
 The
specific
rotation
of
the
two
pure
enantiomers
of
2‐butanol
are
equal
but
opposite
 There
is
no
straightforward
correlation
between
the
R,S
designation
of
an
enantiomer
 and
the
direction
[(+)
or
(‐)]
in
which
it
rotates
plane
polarized
light.
 Racemic
mixture:
a
1:1
mixture
of
enantiomers.

There
is
no
net
optical
rotation.

 Racemic
mixtures
are
often
designated
as
(+).
 Racemic
Forms
and
Enantiomeric
Excess
 Often
a
mixture
of
enantiomers
will
be
enriched
in
one
enantiomer.

One
can
measure
 the
enantiomeric
excess
(ee).
 Example
:
The
optical
rotation
of
a
sample
of
2‐butanol
is
+6.76o.

What
is
the
 enantiomeric
excess?
 The
Synthesis
of
Chiral
Molecules
 Most
chemical
reactions
which
produce
chiral
molecules
produce
them
in
racemic
form.
 Molecules
with
More
than
One
Stereogenic
Center
 The
maximum
number
of
stereoisomers
available
will
not
exceed
2n,
where
n
is
equal
to
 the
number
of
tetrahedral
stereogenic
centers.
 There
are
two
pairs
of
enantiomers
(1,
2)
and
(3,4).

Enantiomers
are
not
easily
separable
 so
1
and
2
cannot
be
separated
from
each
other.

Diastereomers:
stereoisomers
which
are
 not
mirror
images
of
each
other‐
for
instance
1
and
3
or
1
and
4.

Diastereomers
have
 different
physical
(and
chemical)
properties
and
can
be
separated
from
one
another.
 Meso
Compounds
 Sometimes
molecules
with
2
or
more
stereogenic
centers
will
have
less
than
the
 maximum
amount
of
stereoisomers.
 Meso
compound
are
achiral
despite
the
presence
of
stereogenic
centers.

They
are
not
 optically
active.

They
are
mirror
images
which
are
superosable.

They
have
a
plane
of
 symmetry.
 Naming
Compounds
with
More
than
One
 Stereogenic
Center
 The
molecule
is
manipulated
to
allow
assignment
of
each
stereogenic
center
separately.
 This
compound
is
(2R,
3R)‐2,3‐dibromobutane.
 Fischer
Projection
Formulas
 Fischer
projection
formulas
are
2‐dimensional
representations
of
chiral
molecules.


 Vertical
lines
represent
bonds
that
project
behind
the
plane
of
the
paper.

Horizontal
 lines
represent
bonds
that
project
out
of
the
plane
of
the
paper.
 Stereoisomerism
of
Cyclic
Compounds
 Neither
the
cis
not
trans
isomers
of
1,4‐dimethylcyclohexane
is
optically
active.

Each
 has
a
plane
of
symmetry.
 The
trans
and
cis
1,3‐dimethylcyclohexane
each
have
two
stereogenic
centers.

The
cis
 compound
has
a
plane
of
symmetry
and
is
meso.

The
trans
compound
exists
as
a
pair
of
 enantiomers.
 Relating
Configurations
through
Reactions
in
which
 No
Bonds
to
the
Stereogenic
Carbon
are
Broken
 A
reaction
which
takes
place
in
a
way
that
no
bonds
to
the
stereogenic
carbon
are
 broken
is
said
to
proceed
with
retention
of
configuration.
 Relative
configuration:
the
relationship
between
comparable
stereogenic
centers
in
 two
different
molecules.

(R)‐1‐Bromo‐2‐butanol
and
(S)‐2‐butanol
have
the
same
 relative
configuration.

Absolute
configuration:
the
actual
3‐dimensional
orientation
 of
the
atoms
in
a
chiral
molecule.

Absolute
configurations
can
be
determined
from
 x‐ray
crystallographic
analysis.
 Chiral
Molecules
that
Do
Not
Possess
a
Tetrahedral
 Atom
with
Four
Different
Groups
 Atropoisomer:
conformational
isomers
that
are
stable.
 Allenes:
contain
two
consecutive
double
bonds.
 ...
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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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