Download Lecture 8-physical properties.pptx

9/15/09
Organic
compounds
are
classified
into
families
(classes),
each
member
of
which
has
a
common
characteristic
chemical
behavior.
All
alkenes
react
with
bromine.
All
carboxylic
acids
are
acidic.
All
amines
are
basic.
Characteristic
chemical
behaviors
arise
because
all
of
the
members
of
a
particular
family
possess
a
common
functional
group,
which
dictates
the
behavior
of
the
molecule.
Organic
Functional
Groups
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Organic
Functional
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Organic
Functional
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Functional
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Functional
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Functional
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Functional
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Functional
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Functional
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Functional
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Functional
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Organic
Functional
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• The
only
bonds
found
in
alkanes
are
carbon–carbon
and
carbon–hydrogen
single
bonds.
• Alkenes,
alkynes,
and
aromatics
contain
multiple
bonds,
two
adjacent
carbon
atoms
that
share
more
than
one
bond
between
them.
• An
alkene
has
a
carbon–carbon
double
bond,
two
bonds
between
a
pair
of
adjacent
carbon
atoms.
• An
alkyne
has
a
carbon–carbon
triple
bond,
three
bonds
between
a
pair
of
adjacent
carbon
atoms.
• An
aromatic
has
six
carbon
atoms
in
a
cyclic
arrangement
with
alternating
single
and
double
bonds.
• An
alcohol
contains
a
hydroxyl
group,
an
OH
group
attached
to
a
carbon
atom.
• An
ether
contains
an
oxygen
attached
directly
to
two
different
carbons.
• An
amine
contains
an
amino
group,
an
NH3
group
attached
to
a
carbon
atom.
(In
some
amines,
an
NH
or
an
N
is
attached
to
two
or
three
carbon
atoms,
respectively.)
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• Aldehydes,
ketones,
carboxylic
acids,
esters,
and
amides
possess
the
carbonyl
group,
a
carbon–oxygen
double
bond,
but
differ
in
the
atom
or
group
of
atoms
connected
to
the
carbon
of
the
carbonyl
group.
• The
carbonyl
carbon
of
an
aldehyde
is
directly
connected
to
at
least
one
hydrogen.
• The
carbonyl
carbon
of
a
ketone
is
directly
connected
to
carbon
atoms,
not
hydrogen.
• The
carbonyl
carbon
of
carboxylic
acids
and
esters
is
connected
by
a
single
bond
to
an
oxygen,
but
these
families
differ
in
what
is
connected
to
that
oxygen.
• In
carboxylic
acids,
a
hydrogen
is
connected
to
the
oxygen;
in
esters,
the
oxygen
is
connected
to
a
second
carbon
atom.
• An
amide
has
a
nitrogen
connected
to
the
carbonyl
carbon.
Forces
between
molecules
at
the
molecular
level
are
called
secondary
forces.
Secondary
forces
can
be
divided
into
two
types:
• Intermolecular
forces
or
van
der
Waals
forces
between
two
molecules.
These
forces
are
those
responsible
for
holding
solids
and
liquids
together.
• Intramolecular
forces
between
different
parts
of
the
same
molecule.
These
forces
are
responsible
for
the
correct
folding
of
protein
and
other
large
biomolecules.
• Secondary
forces
are
only
10‐20%
as
strong
as
covalent
or
ionic
bonds.
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Dipole‐Dipole
Interactions
• Molecules
containing
permanent
dipoles
(polar
molecules)
attract
each
other
as
well
as
other
molecules
containing
dipole
moments.
• The
attractions
between
δ+
and
δ–
parts
of
the
molecules
are
strong
because
the
charges
are
close
together.
The
repulsive
forces
between
like
charges
are
smaller
because
the
unlike
charges
are
farther
apart.
• These
forces
are
called
dipole‐dipole
forces
and
occur
in
between
polar
molecules
such
as
methyl
chloride,
CH3Cl,
or
chloroform,
CHCl3.
• Water,
H2O,
a
polar
substance,
is
a
liquid
at
room
temperature
while
methane,
a
non‐polar
substance,
CH4,
is
a
gas.
Water
molecules
are
bent
and
have
a
large
dipole
moment
while
methane
molecules
are
symmetrical
and
have
no
net
dipole
moment.
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London
Forces
of
Interaction
• Nonpolar
molecules,
such
as
methane,
will
condense
into
liquids
at
low
temperatures,
so
some
type
of
force
must
exist
between
individual
non‐polar
molecules.
• At
any
instant
in
time,
the
electrons
moving
about
the
atoms
in
a
molecule
may
be
unequally
distributed
with
respect
to
the
protons
in
the
molecule.
This
results
in
a
temporary,
small
dipole
moment.
(Think
of
a
helium
atom
with
both
electrons
on
one
side
of
the
molecule).
• This
brief,
small
dipole
moment
affects
the
electron
distribution
in
neighboring
molecules
in
such
a
way
to
create
an
attractive
force.
• The
attractive
force
due
to
these
temporary
dipoles
is
called
a
London
force
and
occurs
in
all
molecules,
polar
or
not.
• The
strength
of
the
London
force
can
range
from
very
small
to
about
that
of
a
regular
dipole‐dipole
interaction.
• The
magnitude
of
London
forces
in
nonpolar
molecules
is
reflected
in
their
boiling
points.
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• Hydrogen
bonds
are
special
dipole‐dipole
interactions
involving
hydrogen
and
one
of
these
elements:
oxygen,
nitrogen,
or
fluorine.
• The
polarity
of
an
O‐H,
N‐H,
or
F‐H
bond
is
very
high,
which
leads
to
an
especially
strong
dipole‐dipole
interaction:
• Hydrogen
bonds
form
between
identical
molecules,
as
in
liquid
water,
or
between
different
molecules
in
mixtures,
such
as
ammonia,
NH3,
dissolved
in
water.
• Hydrogen
bonds
are
weaker
than
covalent
bonds
and
are
often
denoted
by
dotted
lines
connecting
one
molecule
to
the
other:
O–H∙∙∙N
• The
hydrogen
atom
acts
as
a
“glue”
bonding
the
oxygen
and
nitrogen
atoms
together.
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The
value‐normal
boiling
point
of
a
substance
is
largely
determined
by
the
types
of
secondary
forces
between
molecules
in
the
liquid
state:
• London
forces
• Dipole‐dipole
forces
• Hydrogen
bonds
• Hydrogen‐bonding
substances
interact
by
all
three
types
of
forces.
• Polar
substances
interact
by
dipole‐dipole
and
London
forces.
• Nonpolar
substances
interact
by
London
forces
only. 13
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In
organic
compounds,
the
strength
of
the
secondary
forces
between
molecules
depend
upon:
Family
(type
of
secondary
force)
Molecular
mass
Molecular
shape
Family:
The
family
determines
what
chemical
bonds
are
present,
whether
the
bonds
are
polar
or
non‐polar,
and
whether
there
is
hydrogen
bonding.
The
order
of
secondary
forces
is:
Hydrogen
bonding
>
dipole‐dipole
>
London
All
molecules
possess
London
forces.
Polar
molecules
possess
dipole‐dipole
forces
as
well.
Hydrogen
bonds
are
only
present
in
organic
molecules
when
O‐H
or
N‐H
bonds
are
present.
• The
highest
boiling
point
within
the
nonpolar
group
of
molecules
is
associated
with
the
largest
molecule.
• A
large
molecule
has
many
electrons
capable
of
creating
temporary
dipoles
in
different
parts
of
the
molecule
at
the
same
time.
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Trends
in
the
physical
properties
(melting
points
and
boiling
points)
of
compounds
can
be
predicted
by
answering
two
questions:
• Are
the
compounds
polar
or
nonpolar?
• Can
the
compounds
form
hydrogen
bonds?
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Molecular
mass:
For
any
series
of
compounds
in
the
same
organic
family,
London
forces
increase
with
increasing
molar
mass.
Molecular
shape:
For
compounds
within
the
same
family
and
having
the
same
or
close
molecular
masses,
the
order
of
boiling
points
is:
Cycloalkane
>
straight‐chain
alkane
>
branched
alkane
Molecules
that
are
able
to
pack
closely
together
experience
much
larger
London
forces
than
unsymmetrical
molecules.
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Density
Molecules
having
large
secondary
forces
pack
together
tightly,
resulting
in
greater
densities.
Water
(hydrogen
bonding)
has
a
density
of
1.0
g/ml.
Liquid
alkanes
have
densities
of
0.7‐0.8
g/ml,
depending
upon
molar
mass.
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Solubility
A
particular
solute
will
dissolve
in
a
solvent
only
if
the
solute‐solute
secondary
forces
are
similar
to
the
solvent‐solvent
secondary
forces.
Like
dissolves
like.
Physical
Properties
of
Organic
Compounds
•  Alkanes
–  Melting
and
boiling
points
increase
with
increasing
molecular
weight
within
a
homologous
series.
Compound
Formula
MW
(g/mol)
mp
(ºC)
bp
(ºC)
Methane
CH4
16
–182
–164
Pentane
CH3(CH2)3CH3
72
–130
36
Decane
CH3(CH2)8CH3
142
–30
174
Pentadecane
CH3(CH2)13CH3
212
10
271
Eicosane
CH3(CH2)18CH3
282
37
343
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Physical
Properties
of
Organic
Compounds
•  Alkanes
–  Boiling
points
decrease
with
chain
branching.
Physical
Properties
of
Organic
Compounds
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