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Unit 1 – Organic Chemistry
L.O. #3 The properties and
characteristics
of organic compounds
Intermolecular forces

The physical and chemical properties are
determined by the bonds and forces
between atoms and molecules.


Intramolecular forces occur between
atoms in molecules and determine a
substance’s chemical properties.
Intermolecular forces occur between
molecules and affect a substance’s
physical properties.
Intermolecular forces



The physical properties of organic
compounds are affected by the
intermolecular forces of attraction and
repulsion between molecules
Larger forces of intermolecular attraction
result in high boiling & melting points.
Weak intermolecular forces result in low
boiling & melting points producing gas
states.
Intermolecular forces



The foundation for these intermolecular
forces lies in the types of bonds that form
between atoms in molecules.
The bonding continuum ranges from nonpolar to ionic, however organic molecules
are predominantly covalent molecules
and lie in the polar to non-polar range.
Polar and non-polar regions affect
solubility
Intermolecular forces

The polarity of a bond is determined by
the electronegativity difference.
Electronegativity Difference (DE)
Non-polar
0
Polar
0.4
Organic compounds
Ionic
1.7
3.3
Intermolecular forces

The intermolecular forces are attractive
and repulsive forces between molecules
determined by factors such as:

The bond types within the molecules.



Polar vs. Non-polar
The size of the molecules.
The shape of the molecules.
Intermolecular forces


Johannes van der Waals (1837-1923)
studied these forces and they are often
referred to by his name.
We will focus on the three main types:
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

Dispersion (London) forces
Dipole-dipole forces
Hydrogen bonding
Intermolecular forces

Dispersion (London) forces


The constant vibration of electrons in
covalent bonds generates temporary
regions of charge distribution and polar
regions which result in weak forces of
attraction between all covalent molecules.
The overall strength of these forces is
proportional to;


The number of electrons in the molecule
The size and shape of the molecule
Intermolecular forces

Dipole-Dipole Forces



Polar molecules orient themselves so that
electrostatic attraction occurs between the
different charged regions of the molecules.
These forces are generally stronger than
the dispersion (London) forces.
Their strength depends upon the;

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Nature (i.e. DE difference) of the polar bonds
Number and placement of the polar bonds
Size of the molecules
Intermolecular forces

Hydrogen bonding

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
A strong form of dipole-dipole attraction between
a hydrogen atom and oxygen, nitrogen or
fluorine in a polar covalent molecule.
The high electronegative difference results in a
strong polar bond and distinct regions of charge
distribution.
Hydrogen bonding is responsible for water’s
unique characteristics and influences organic
compounds’ solubility in water.
Assessing intermolecular forces

Can molecules form hydrogen bonds?

If so:

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They have higher boiling and melting points
than similar molecules that cannot form
hydrogen bonds
They form hydrogen bonds with water and
thereby have greater solubility in water
Assessing intermolecular forces

Are they polar?

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Polar molecules tend to have higher boiling &
melting points than similar non-polar ones.
Hydrogen bonding capability increases the
trend further.
A large non-polar region (hydrocarbon) in a
molecule reduces the affect of the smaller
polar regions and are less soluble than
smaller hydrocarbon polar molecules.
Assessing intermolecular forces

How strong are the dispersion forces?

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Large hydrocarbon chains have stronger
dispersion forces.
The boiling and melting points tend to be
proportional to the number of carbons in the
hydrocarbon.


Large hydrocarbons have high boiling and melting
points.
Small hydrocarbons tend to be gases at SATP.
Hydrocarbons (CnH2n+2)
Physical Properties
Polarity of
functional
group
 The
C-H bond is non-polar so most hydrocarbons are
non-polar
 The addition of halides results in polar regions.
 The predominant intermolecular force in dispersion forces
Hydrogen
bonding
 None
Solubility in
Water
 Very
Melting &
Boiling
Points
 The
poor solubility in water due to the lack of polar and
hydrogen bonds
melting and boiling points tend to be very low
 The boiling points increase as the length of the parent
chain increases
C’s  gases @ SATP
5-20 C’s  liquids (some very viscous) @SATP
20 + C’s  solids @ SATP
1-5
Hydrocarbons (CnH2n+2)
Additional Characteristics

As the number of carbons in the parent
chain increase:

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
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The state of matter at SATP moves from
gas to liquid to solid
The melting and boiling points increase
The density increases
Popular fuel source (Methane, propane,
octane, etc.)
Source for plastics
Alcohols (R-OH)
Physical Properties
Polarity of
functional
group
 OH
is very polar
 As the parent chain grows, its non-polar nature become
more influential
 Small alcohols are more polar than larger ones
Hydrogen
bonding
 Alcohols
Solubility in
Water
 The
Melting &
Boiling
Points
 Due
experience hydrogen bonding with other
alcohols and water
capacity to form hydrogen bonds make alcohols
extremely soluble in water.
 The solubility decreases as the number of carbons
increase
to the strength of hydrogen bonds most alcohols
have higher melting & boiling points than similar alkanes.
 Most alcohols are liquids at SATP
Alcohols
Additional Characteristics

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Alcohols are extremely flammable
Most alcohols are poisonous
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Methanol can cause blindness or death.
Ethanol is consumed in moderate quantities.
Excessive consumption may cause blindness or
death.
Yeast ferment sugars to produce ethanol and carbon
dioxide
Antifreeze, cosmetics, foods, medical
preparations, solvents, antiseptics, etc.
Biological compounds – cholesterol, retinol, etc.
Ethers (R-O-R2)
Physical Properties
Polarity of
functional group
 The
C-O bond is less polar than the O-H
 Ethers are less polar than alcohols
 The bent shape around the O aids in the dipoledipole force strength
Hydrogen
bonding
 There
Solubility in
Water
 Ethers
Melting & Boiling
Points
 Much
is no hydrogen bonding in ethers
 Ethers can accept hydrogen bonds from water
are usually soluble in water
 The solubility decreases as the size of the alkyl
groups grows
lower than comparable alcohols
 Higher than similar hydrocarbons
Ethers
Additional Characteristics

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Ethers are extremely flammable
Have been used as anaesthetics


“ether” – ethoxyethane
More recent anaesthetic
(1-methoxypropane) has fewer side
effects.
Amines (R-NH2)
Physical Properties
Polarity of
functional
group
 C-N
and N-H bonds are polar
 Amines are usually polar
Hydrogen
bonding
 The
Solubility in
Water
 Low
Melting &
Boiling
Points
 The
presence of one or more N-H bonds allows hydrogen
bonding
molecular weight amines are soluble in water (<4 C)
 The solubility decreases as the number of carbons
increase
boiling points of primary and secondary amines (N-H
bonds present) are greater than tertiary amines (no N-H
bonds)
Amines
Additional Characteristics
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Widely found in nature. They are often
toxic. Many medical applications.
Low molecular weight amines have a
“fishy” smell while others are
responsible for odours associated with
decay.
Amines act as weak bases. Adding an
acid produces a salt. So, add lemon to
fish to remove fishy odour.
Aldehydes (R-CO) & Ketones
Physical Properties
Polarity of
functional
group
 C=O
is polar
Hydrogen
bonding
 No
Solubility in
Water
 Low
Melting &
Boiling
Points
 Lower
O-H bonds so no hydrogen bonding between
aldehydes and ketones
 The O atom can accept hydrogen bonds from water
molecular weight aldehydes and ketones have high
solubility
 The solubility decreases as the number of carbons
increase
than comparable alcohols
 Higher than corresponding alkanes
Aldehydes & Ketones
Additional Characteristics

Aldehydes have a strong pungent odour while
ketones smell sweet. As the size of the aldehyde
increases the odour becomes more pleasant (i.e.
cinnamon)
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Odours in nature may be used to communicate between
insects - pheromones
Since they are polar, they are used as polar solvents.
The non-polar alkyl chain permits their use as nonpolar solvents too.
Antiseptics, disinfectants (Formaldehyde), resins,
dyes, preservatives (Aldehyde)
Hormones in biology – testosterone, progesterone,
cortisone
Solvents, medication
Carboxylic acids (R-COOH)
Physical Properties
Polarity of
functional
group
 Polar
due to C=O and O-H bonds
Hydrogen
bonding
 Strong
Solubility in
Water
 Low
Melting &
Boiling
Points
 The
hydrogen bonding
molecular masses are very soluble (<4C – miscible
in water)
 The solubility decreases as the number of C’s increase
melting and boiling points for carboxylic acids are
very high due to the hydrogen bonding.
 Higher than corresponding alcohols
Carboxylic acids
Additional Characteristics

Carboxylic acids often have unpleasant odours
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butanoic acid smells like stale sweat
Sour taste in many foods & plants – vinegar, spoiled
wine, rhubarb, citrus fruits
By-product of anaerobic respiration in muscles – lactic
acid
Industrial processes – dyes, tanning, rubber recycling,
etc.
Food preservatives, Vitamin C, ASA
The OH does not behave like a base. The two O’s
have high electronegativities and carry the H’s
electron allowing the H ion to dissociate.
Boiling & Melting Points
Alcohol
Alkane
CH3CH2CH2CH2OH
0
CH3CH2CCH3
0
=
Aldehyde
CH3CH2CH2COH
=
Ketone
=
Carboxylic acid
0
CH3CH2CH2CH
CH3CH2CH2CH3
b.p. 165.5 oC
m.p. -4.5 oC
High
b.p. 117.2 oC
m.p. -89.5 oC
b.p. 79.6 oC
m.p. -86.3 oC
b.p. 75.7 oC
m.p. -99 oC
b.p. -0.5 oC
m.p. -138.4 oC
Low
Esters (R-COO-R2)
Physical Properties
Polarity of
functional
group
 Usually
polar due to C=O and C-O bonds
 Polarity is reduced with increased number of C atoms
Hydrogen
bonding
 No
Solubility in
Water
 The
Melting &
Boiling
Points
 The
O-H bonds, there fore they don’t form hydrogen bonds
with other esters
C=O bond can accept hydrogen bonds from water,
therefore low molecular mass esters are soluble
 Esters with chains greater than 3 or 4 are not soluble
melting and boiling points for esters are low due to
the lack of hydrogen bonding.
 They are usually volatile liquids at SATP
Esters
Additional Characteristics

Esters often have pleasant odours


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Low molecular mass esters have very
pleasant odours and are used in soaps,
perfumes, cosmetics, air fresheners, etc.
Natural esters are responsible for the
scents in flowers & fruits. (Pg 64 Table 2)
Food – lipids and dietary fats are
esters (triglycerides)
Amides (R-CONHR2)
Physical Properties
Polarity of
functional
group
 Polar
due to C=O,C-N and N-H bonds
 Similar physical properties to carboxylic acids
Hydrogen
bonding
 Since
Solubility in
Water
 Amides
Melting &
Boiling
Points
 Primary
primary amides have two N-H bonds they have
stronger hydrogen bonding than carboxylic acids
 Secondary amides also experience hydrogen bonding
are soluble in water
 The solubility decreases as the non-polar alkyl chain
increases in size
amides have much higher melting and boiling
points than carboxylic acids
 Many primary amides are solids at SATP
Amides
Additional Characteristics


Amides such as acetaminophen are
analgesics (pain killers)
Urea is a common amide found in
urine and fertilizers