Chapter 4 Notes

Chapter 4 Notes - Chapter
4
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Unformatted text preview: Chapter
4
 Alkanes:
Nomenclature,
Conformational
 Analysis,
and
an
Introduction
to
Synthesis
 Shapes
of
Alkanes
 “Straight‐chain”
alkanes
have
a
zig‐zag
orientation
when
they
are
in
their
most
 straight
orientation.

Straight
chain
alkanes
are
also
called
unbranched
alkanes.
 Branched
alkanes
have
at
least
one
carbon
which
is
attached
to
more
than
two
 other
carbons.
 Constitutional
isomers
have
different
physical
properties
(melting
point,
boiling
 point,
densities
etc.).

Constitutional
isomers
have
the
same
molecular
formula
but
 different
connectivity
of
atoms.
 The
number
of
constitutional
isomers
possible
for
a
given
molecular
formula
 increases
rapidly
with
the
number
of
carbons.
 IUPAC
Nomenclature
of
 Alkanes,
Alkyl
Halides
and
Alcohols
 Before
the
end
of
the
19th
century
compounds
were
named
using
nonsystematic
 nomenclature.

These
“common”
or
“trivial”
names
were
often
based
on
the
source
 of
the
compound
or
a
physical
property.

The
International
Union
of
Pure
and
Applied
 Chemistry
(IUPAC)
started
devising
a
systematic
approach
to
nomenclature
in
1892.

 The
fundamental
principle
in
devising
the
system
was
that
each
different
compound
 should
have
a
unique
unambiguous
name.

The
basis
for
all
IUPAC
nomenclature
is
 the
set
of
rules
used
for
naming
alkanes.
 Nomenclature
of
Unbranched
Alkanes
 Nomenclature
of
Unbranched
Alkyl
groups


 The
unbranched
alkyl
groups
are
obtained
by
removing
one
hydrogen
from
the
alkane
 and
named
by
replacing
the
‐ane
of
the
corresponding
alkane
with
–yl.
 Nomenclature
of
Branched‐Chain
Alkanes
(IUPAC)
 Locate
the
longest
continuous
chain
of
carbons;
this
is
the
parent
chain
and
determines
 the
parent
name.
 Number
the
longest
chain
beginning
with
the
end
of
the
chain
nearer
the
substituent.

 Designate
the
location
of
the
substituent.
 When
two
or
more
substituents
are
present,
give
each
substituent
a
number
 corresponding
to
its
location
on
the
longest
chain.

Substituents
are
listed
alphabetically.
 When
two
or
more
substituents
are
identical,
use
the
prefixes
di‐,
tri‐,
tetra‐
etc.
 Commas
are
used
to
separate
numbers
from
each
other.

The
prefixes
are
used
in
 alphabetical
prioritization.

When
two
chains
of
equal
length
compete
to
be
parent,
 choose
the
chain
with
the
greatest
number
of
substituents
.
 When
branching
first
occurs
at
an
equal
distance
from
either
end
of
the
parent
chain,
 choose
the
name
that
gives
the
lower
number
at
the
first
point
of
difference
 Nomenclature
of
Branched
Alkyl
Chains
 Two
alkyl
groups
can
be
derived
from
propane.
 Four
groups
can
be
derived
from
the
butane
isomers.
 The
neopentyl
group
is
a
common
branched
alkyl
group
 Examples:
 Classification
of
Hydrogen
Atoms
 Hydrogens
take
their
classification
from
the
carbon
to
which
they
are
attached.
 Nomenclature
of
Alkyl
Halides
 In
IUPAC
nomenclature
halides
are
named
as
substituents
on
the
parent
chain.

 Halo
and
alkyl
substituents
are
considered
to
be
of
equal
ranking.
 In
common
nomenclature
the
simple
haloalkanes
are
named
as
alkyl
halides.
 Common
nomenclature
of
simple
alkyl
halides
is
accepted
by
IUPACand
still
used.
 IUPAC
Substitutive
Nomenclature
 An
IUPAC
name
may
have
up
to
4
features:
locants,
prefixes,
parent
compound
 and
suffixes.

Numbering
generally
starts
from
the
end
of
the
chain
which
is
 closest
to
the
group
named
in
the
suffix.
 IUPAC
Nomenclature
of
Alcohols:
 Select
the
longest
chain
containing
the
hydroxyl
and
change
the
suffix
name
of
the
 corresponding
parent
alkane
from
‐ane
to
–ol.

Number
the
parent
to
give
the
 hydroxyl
the
lowest
possible
number.

The
other
substituents
take
their
locations
 accordingly.
 Examples:
 Common
names
of
simple
alcohols
are
still
often
used
and
are
approved
by
IUPAC.
 Alcohols
with
two
hydroxyls
are
called
diols
in
IUPAC
nomenclature
and
glycols
in
 common
nomenclature.
 Nomenclature
of
Cycloalkanes
 The
prefix
cyclo‐
is
added
to
the
name
of
the
alkane
with
the
same
number
of
carbons.

 When
one
substituent
is
present
it
is
assumed
to
be
at
position
one
and
is
not
numbered.
 When
two
alkyl
substituents

are
present
the
one
with
alphabetical
priority
is
given
 position
1.

Numbering
continues
to
give
the
other
substituent
the
lowest
number
 Hydroxyl
has
higher
priority
than
alkyl
and
is
given
position
1.

If
a
long
chain
is
attached
 to
a
ring
with
fewer
carbons,
the
cycloalkane
is
considered
the
substituent.
 Bicyclic
compounds
 Bicyloalkanes
contain
2
fused
or
bridged
rings.

The
alkane
with
the
same
number
of
 total
carbons
is
used
as
the
parent
and
the
prefix
bicyclo‐
is
used.
 The
number
of
carbons
in
each
bridge
is
included
in
the
middle
of
the
name
in
square
 brackets.
 Nomenclature
of
Alkenes
and
Cycloalkenes
 Alkenes
are
named
by
finding
the
longest
chain
containing
the
double
bond
and
 changing
the
name
of
the
corresponding
parent
alkane
from
‐ane
to
–ene.

The
 compound
is
numbered
to
give
one
of
the
alkene
carbons
the
lowest
number
 The
double
bond
of
a
cycloalkene
must
be
in
position
1
and
2
 Compounds
with
double
bonds
and
alcohol
hydroxyl
groups
are
called
alkenols.


The
 hydroxyl
is
the
group
with
higher
priority
and
must
be
given
the
lowest
possible
 number.
 Two
groups
which
contain
double
bonds
are
the
vinyl
and
the
allyl
groups.
 If
two
identical
groups
occur
on
the
same
side
of
the
double
bond
the
compound
is
 cis.

If
they
are
on
opposite
sides
the
compound
is
trans.
 Several
alkenes
have
common
names
which
are
recognized
by
IUPAC.
 Physical
Properties
of
Alkanes
and
Cycloalkanes
 Boiling
points
of
unbranched
alkanes
increase
smoothly
with
number
of
carbons
 Melting
points
increase
in
an
alternating
pattern
according
to
whether
the
number
of
 carbon
atoms
in
the
chain
is
even
or
odd
 In
Class
Problem:
 (1)  Draw
bond‐line
formulas
and
provide
IUPAC
names
for
all
the
isomeric
alcohols
 with
the
formula
C5H12O.
 (2)  Give
IUPAC
names
for
each
of
the
following.
 Sigma
Bonds
and
Bond
Rotation
 Ethane
has
relatively
free
rotation
around
the
carbon‐carbon
bond.

The
staggered
 conformation
has
C‐H
bonds
on
adjacent
carbons
as
far
apart
from
each
other
as
 possible.

The
drawing
to
the
right
is
called
a
Newman
projection.
 The
eclipsed
conformation
has
all
C‐H
bonds
on
adjacent
carbons
directly
on
top
of
 each
other.
 The
potential
energy
diagram
of
the
conformations
of
ethane
shows
that
the
 staggered
conformation
is
more
stable
than
eclipsed
by
12
kJ
mol‐1.
 Conformational
Analysis
of
Butane
 Rotation
around
C2‐C3
of
butane
gives
six
important
conformations.

The
gauche
 conformation
is
less
stable
than
the
anti
conformation
by
3.8
kJ
mol‐1
because
of
 repulsive
van
der
Waals
forces
between
the
two
methyls.
 In
Class
Problem:
 Sketch
a
potential
energy
vs.
rotation
curve
describing
the
rotation
around
the
 carbon‐carbon
bond
of
1,2‐dichloroethane.

The
actual
values
of
the
energy
 changes
are
not
important.

Conformational
structures
should
accompany
all
 maxima
and
minima.

 The
heat
of
combustion
of
a
compound
is
the
enthalpy
change
for
the
complete
 oxidation
of
the
compound.

In
the
case
of
hydrocarbons,
it
is
the
complete
oxidation
 to
carbon
dioxide
and
water.

 Heats
of
combustion
per
CH2
unit.

The
model
assumes
that
cyclohexane
has
no
ring.

 The
ring
strain
of
the
other
cycloalkanes

is
calculated
relative
to
cyclohexane.
 The
Relative
Stabilities
of
Cycloalkanes:
 Ring
Strain
 The
Origin
of
Ring
Strain
in
Cyclopropane
and
 Cyclobutane
:
Angle
Strain
and
Tortional
Strain
 Angle
strain
is
caused
by
bond
angles
different
from
109.5o.

Tortional
strain
is
caused
 by
eclipsing
C‐H
bonds
on
adjacent
carbons.

Cyclopropane
has
both
high
angle
and
 tortional
strain.
 Cyclobutane
has
considerable
angle
strain.

It
bends
to
relieve
some
tortional
strain.


 Cyclopentane
has
little
angle
strain
in
the
planar
form
but
bends
to
relieve
some
 tortional
strain.
 Conformations
of
Cyclohexane
 It
is
assumed
that
the
chair
conformation
has
no
ring
strain.

All
bond
angles
are
109.5o
 and
all
C‐H
bonds
are
perfectly
staggered.
 The
boat
conformation
is
less
stable
because
of
flagpole
interactions
and
tortional
 strain
along
the
bottom
of
the
boat.

 The
twist
conformation
is
intermediate
in
stability
between
the
boat
and
the
chair
 conformation.
 Substituted
Cyclohexanes:
Axial
and
Equatorial
 Hydrogen
Atoms
 Axial
hydrogens
are
perpendicular
to
the
average
plane
of
the
ring.

Equatorial
hydrogens
 lie
around
the
perimeter
of
the
ring.
 The
C‐C
bonds
and
equatorial
C‐H
bonds
are
all
drawn
in
sets
of
parallel
lines.

The
axial
 hydrogens
are
drawn
straight
up
and
down.
 Methyl
cyclohexane
is
more
stable
with
the
methyl
substituent
occupying
an
equatorial
 orientation.

An
axial
methyl
has
an
unfavorable
1,3‐diaxial
interaction
with
axial
C‐H
 bonds
2
carbons
away.

A
1,3‐diaxial
interaction
is
the
equivalent
of
2
gauche
butane
 interactions.
 Disubstitued
Cycloalkanes
 1,2‐Dimethyl‐,
1,3‐dimethyl‐,
and
1,4‐dimethylcyclohexane
can
exist
as
cis‐trans
 stereoisomers.

Cis:
groups
on
same
side
of
ring.

Trans:
groups
on
opposite
side
of
ring.
 Trans‐1,4‐dimethylcylohexane
prefers
a
trans‐diequatorial
conformation.
 Cis‐1,4‐dimethylcyclohexane
exists
in
an
axial‐equatorial
conformation.
 A
very
large
tert‐butyl
group
is
required
to
be
in
the
more
stable
equatorial
position.
 In
Class
Problem:
 Draw
all
the
chair
conformations
of
cis‐1‐methyl‐4‐isopropylcyclohexane.

Which
is
 the
most
stable
conformation?
 Bicyclic
and
Polycyclic
Alkanes
 The
bicyclic
decalin
system
exists
in
non‐interconvertible

cis
and
trans
forms.
 Synthesis
of
Alkanes
and
Cycloalkanes
 Hydrogenation
of
Alkenes
and
Alkynes:
 Reduction
of
Alkyl
Halides:
 Alkylation
of
Terminal
Alkynes:

 Alkynes
can
be
subsequently
hydrogenated
to
alkanes.
 Retrosynthetic
Analysis‐Planning
Organic
Synthesis
 The
synthetic
scheme
is
formulated
working
backward
from
the
target
molecule
to
a
 simple
starting
material.

Often
several
schemes
are
possible.
 ...
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