Physical Properties of Organic Compounds
Covalent Bonds
Nonpolar
¾
Equal sharing of electrons
Electronegativity difference <0.5
C-H bonds
C-C bonds
Polar
¾
Unequal sharing of electrons
Electronegativity difference 0.5-1.7
C-O bonds
O-H bonds (very polar)
C-Cl bonds
C-F bonds
Boiling Points
Boiling points depend on the forces of attraction between molecules.
The greater the attractive forces between molecules, the higher the
boiling point because more energy is required to overcome the
attractive forces and separate the molecules.
All molecules have dispersion forces – the attraction of electrons in
one molecule for the protons in another molecule. Larger molecules
have more electrons, and therefore greater dispersion forces.
Polar molecules have slightly positive and slightly negative regions.
Opposite charges attract. This is in addition to the dispersion forces,
therefore polar molecules will have higher boiling points than nonpolar
molecules of similar size.
Physical Properties – Page 1
Ex.
H H
| |
H-C-C-H
| |
H H
ethane
HHH
| | |
H-C-C-C-H propane
| | |
HHH
Ethane and propane have nonpolar bonds only (C-H and C-C).
Therefore their boiling points are determined by the strength of
their dispersion forces. Propane has more electrons, therefore,
greater dispersion forces and a higher boiling point than ethane.
Ex. H H H H H
| | | | |
H-C-C-C-C-C-H
| | | | |
H HHH H
Pentane
H HHH H
| | | | |
H-C-C-O-C-C-H
| | | | |
H HHH H
H HHH H
| | | | |
H-C-C-C-C-O-H
| | | | |
H HHH H
Diethyl Ether
1-Butanol
Pentane, diethyl ether, and 1-butanol are similar in size (number of
electrons), therefore, their boiling points will be determined by
polarity. Pentane has all nonpolar bonds (C-C and C-H) and will have
the weakest intermolecular forces of attraction and the lowest boiling
point. Next is diethyl ether with two polar C-O bonds. 1-butanol also
has two polar bonds (C-O and O-H), but the O-H bond is more polar
than the C-O bond, making 1-butanol more polar than diethyl ether and
giving it a higher boiling point.
Physical Properties – Page 2
Solubility
Like dissolves like
General rule of solubility based on polarity
Polar solvents dissolve polar and ionic solutes
Nonpolar solvents dissolve nonpolar solutes
Ex.
Compare the solubility of pentane, diethyl ether, and 1-butanol
in water.
Water is a very polar molecule, therefore, the more polar the
molecule, the more soluble it will be in water.
Pentane is a nonpolar molecule, therefore it will not dissolve in water –
it is insoluble
Diethyl ether will be less soluble than 1-butanol. Both have polar and
nonpolar bonds. 1-butanol has an O-H bond which is more polar than a
C-O bond making 1-butanol more polar, and therefore more soluble in
water.
Physical Properties – Page 3
Organic Review
1. Write structural formulas for all organic compounds. State the
type of reaction. Complete and balance the reaction. Name the
products.
a) propene + hydrogen ¼
b) cyclohexane + chlorine ¼
c) benzene + iodine ¼
d) ethanol + butanoic acid ¼
e) 2-pentene + bromine ¼
f) ethene + water ¼
g) chloroethane + sodium hydroxide ¼
h) propene + bromine ¼
i) chlorobenzene + chlorine ¼
j) ethanol + hexanoic acid ¼
k) 3-pentanol + oxygen ¼
l) propyne + (2 mol) iodine ¼
m) cyclohexene + water ¼
n) ethoxyhexane + oxygen ¼
o) polymerization of chloroethene ¼
p) polymerization of propene ¼
2. Account for the variation in boiling points among the following two
carbon molecules:
ethane -88.6ºC
chloroethane 12.2ºC
ethanol 78.5ºC
ethanoic acid 117.9ºC
3. Draw structural formulas for each of the following molecules.
Predict and explain the order of their boiling points from highest
to lowest.
a) CH3F
b) C2H6
c) CH3OH
d) C2H4
Physical Properties – Page 4