Organic Chemistry
MDL233
Chapter 2
BY
Mahwash Hafeez
1
Five concepts for predicting physical properties
Physical properties such as melting point, boiling point and solubility are
largely determined by intermolecular attractive forces. Using five simple
concepts, you should be able to look at structures of group of different
organic molecules and predict which might be gases, liquids, or solids and
which might be soluble in water. You can predict the boiling point , melting
point , or solubility
1. London attractive forces (van der Waals forces)
2. Dipole – dipole attractive forces
3. Hydrogen bond
4. Ionic attractive forces
5. Competing intermolecular forces and solubility
2
Intermolecular Forces
Intermolecular forces: Interactions between molecules.
Intramolecular forces: Within the same molecule.
Intramolecular forces are also of three types:
1. Ion-ion interactions (ionic bond)
2. Covalent interactions ( Covalent bond)
3. Co-ordinate covalent interactions (co-ordinate covalent bond)
Intermolecular forces are of three types:
1) vander Waals forces ( London dispersion forces)
2) dipole-dipole interactions
3) H-bonding
3
Intramolecular Forces
1 - Ion–Ion Interactions
Ion – Ion interactions are strong intramolecular forces between oppositely
charged ions. e.g.
Na+Cl- (oppositely charged particles or ions): The attractive forces
between these oppositely charged ions or particles are extremely strong.
These are Ion – Ion interactions or electrostatic interactions.
Ion – Ion interactions are much stronger than the intermolecular forces in
covalent molecules.
4
O
O
O
NaHCO3
KOH
C
H3C
C
O
K
potassium acetate
mp 306
H
H3C
C
OH
Acetic acid
mp 17
H
H3C
O
Sodium acetate
mp ˃ 300
5
Na
OH
O Na
NaOH
H
Sodium phenolate
mp 382
phenol
mp 41 C
H3C
H3C
CH
NH2
H3C
Isopropyl amine
mp -95
HCl
CH
NH3
Cl
base
H3C
Isopropyl amine hydrochloride
mp -162
Covalent Interactions
(Covalent Bond)
These are intramolecular forces between non metallic atoms formed by mutual
sharing of electrons.
It can be between two similar atoms or two different atoms having different
electronegativities.
Examples; O2, CO2, C2H6, H2O, SiC.
It can be of two types:
a) Non – polar covalent bond.
b) Polar covalent bond.
7
Non – Polar covalent bond:
Two same atoms having same electronegativities
share one or more pairs of outer-shell electrons to
form non polar covalent bond.
Oxygen Atom
Oxygen Atom
Oxygen Molecule (O2)
Polar covalent bond:
Two different atoms having different electronegativities
share one or more pairs of outer-shell electrons to form
polar covalent bond.
More electronegative
element (atom)
Less electronegative
Element (atom)
Polar covalent Bond
9
CO- Ordinate Covalent bond
Bond formed by sharing of electron but both
the electrons come from the same elements. i.e.
The sharing is not mutual.
This bond is also called dative bond.
This bond is represented by an arrow ( )from
the donor to the acceptor atom.
Example Of Co-ordinate covalent bond
Intermolecular Forces
1. Van Der Waals Forces (London forces)
They are also called Temporary dipole-temporary dipole interactions.
A Temporary dipole is formed when there are momentary changes in
electron density in a molecule.
These are the only attractive forces(Van Der Waals forces) present in
non-polar compounds.
Example:CH4 (no net dipole)
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Temporary dipole-temporary
non-polar
dipole interactionnon-polar
Forces
e-
e-
- e-
e
e
d+
- e- edee
ee
- e- e e- e- e-
e-
Cl-Cl
TEMPORARY
DIPOLE
-
e- e e- e- e- e- e- ee
e
dee- ed+
e- e- e e
e-
Cl-Cl
INDUCED
DIPOLE
Dispersion
(weakest and very short-lived)
van der Waals Forces and Surface Area
1) All compounds exhibit van der Waals forces.
2) The larger the surface area of a mol, the stronger the van der
Waals forces.
14
Tokay Gecko:
Dispersion
Forces!
• Organic compound that contain only carbon and hydrogen
(hydrocarbon) are weekly attracted to each other by Van Der Waals
forces
• These attractive forces increase as molecular size increases.
H2
C
CH4
H3C
Mp -180
bp -162
H2
C
C
H2
hexane
Mp -95
bp +69
CH3
C
H2
van der Waals Forces and Polarizability
1) The more polarizable, the stronger the van der Waals
Forces.
2) the Polarizability:
(1) a measure of how the e- cloud around an atom responds to changes
in its electronic environment.
(2) Larger atoms (more loosely held valence e-) are more polarizable than
smaller atoms (more tightly held e-).
17
2. Dipole–Dipole Interactions
1) Dipole–dipole interactions: the permanent dipole-dipole
interactions between polar molecules.
2) much stronger than the van der Waals forces.
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Example
d+ H
Cl
d-
A DIPOLE
(it’s polar)
INDUCED
DIPOLE
e- e- e- ee- e- ed+ee- - ee- -ee- e- e e
e-
e-
Ar
d-
non-polar
Dipole – Induced Dipole
(weak and short-lived)
b/w a polar & a non-polar molecule
The attractive forces between molecules increases
when functional groups containing electronegative
atoms such as chlorine, oxygen and nitrogen are
present.
H3C
H3C
CH3
H3C
isobutan
mp -137C
bp 0C
H3C
d
d
CH
Cl
H3C
d
CH
H3C
isopropyl chloride
mp -117C
bp +35C
d
H3C
Cl
H3C
d
d
C
O
H3C
d
d
C
O
H3C
acetone
mp -94C
bp +56C
20
3. Hydrogen Bonding
1) Very strong dipole-dipole interactions.
2) Between H bonded to electronegative elements (O, N,
X) and a lone pair (2e-) on an electronegative element.
3) Hydrogen bond occurs among organic molecules
containing OH group (alcohol and carboxlic acid) and
N-H group
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Which of the following have high boiling point
H3C
H3C
H
CH3
Ethane
bp. -88 C
mp - 172
gas at room temp.
H
O
H
H3C
O
O
CH3
CH3
O
H
CH3
O
H
H3C
O
H
H
H
H3C
O
H
Hydrogen bond is an even stronger
intermolecular attractive force
H
O
O
CH3
CH3
O
CH3
ethanol
bp. +65 C
mp - 97
liquid at room temp.
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Intermolecular Forces—Summary
As the polarity of a mol increases, the strength of its
intermolecular forces increases.
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Physical Properties—bp & mp
• Boiling point (bp): liquid mol.  gas.
• Melting point (mp): solid  liquid.
• The stronger the intermolecular forces, the higher the bp & mp.
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Other Factors Affecting Boiling Points
• For compounds with similar functional groups:
• The larger the surface area, the higher the bp.
• The more polarizable the atoms, the higher the bp.
25
Effect of Symmetry on Melting Points
• The more compact and symmetrical the shape (a crystalline lactice), the
higher the mp.
Ex) Neopentane has a much higher mp than isopentane.
26
Solubility
• Solubility: the extent of solute dissolution in a solvent.
• Hydrophobic and Hydrophilic
Hydrophobic: Nonpolar part of a mol, water-insoluble (not
attracted to H2O).
Hydrophilic: Polar part of a mol, water-soluble (H-bond to H2O).
27
Solubility Trends
• “Like dissolves like.”
1) Ionic compounds & polar compounds dissolve in polar
solvents.
2) Nonpolar or weakly polar compounds dissolve in:
- nonpolar solvents (e.g., CCl4 , hexane).
- weakly polar solvents (e.g., diethyl ether)
3) Solubility of organic molecules
- Relative size of non-polar portion to a polar portion
- Water soluble: ≤ 5 C for one functional group
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Competing intermolecular forces and solubility
The sum of attractive forces tend to work together to raise
melting and boiling point s as
•the size of molecule’s hydrocarbon skeleton increases
polar, incorporated into molecule
•hydrogen bonded, incorporated into molecule
•ionic function group is incorporated into molecule
Solubility involves the interaction of two different
molecules, which may have different types of attractive
forces
Ex: Adding hydrocarbon to water (oil and water do not mix)
Water have hydrogen bond
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Hydrocarbon is nonpolar and no hydrogen bond
Solubility of Ionic Compounds
• Dissolve in water by many ion–dipole interactions.
Figure 3.4
30
number of carbon less than 5
butane
mp -138
insolube in water
NH2
number of carbon mor than 5
mp -57 insoluble
amine
mp -43
solube in water
NH2
mp -18 insoluble
O
O
ketone
mp -94
solube in water
mp -35 insoluble
OH
OH
mp -127
solube in water
mp -34 insoluble
O
OH
O
acid
mp -17
solube in water
OH
mp -2 insoluble
31
NH3
Cl
amine hydrochloride
mp 161
solube in water
NH3 Cl
amine hydrochloride
mp 242
solube in water
O
O
O
Na
O
sodium salt of carboxlic acid
solube in water
soluble in water
32
Na
Solubility of Organic Molecules
• Relative size of non-polar portion to a polar portion
Water soluble when; ≤ 5 C for one functional group
For example,
33
Solubility Properties of Representative
Compounds
34