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O.C. Physical properties
SCH4U
Physical Properties and Forces between Molecules
Organic compounds that have the same functional group often have similar physical
properties, such as boiling points, melting points, and solubilities. Physical properties are
largely determined by intermolecular forces, the forces of attraction and repulsion
between particles. Three types of intermolecular forces are introduced below.
Hydrogen bonding is a strong intermolecular attraction between the hydrogen atom
from an N-H, O-H, or F-H group on one molecule, and a nitrogen, oxygen, or fluorine
atom on another molecule.
dipole-dipole interactions.
These forces cause polar molecules to cling to each other.
Dispersion forces are attractive forces that occur between all covalent molecules.
These forces are usually very weak for small molecules, but they strengthen as the size of
the molecule increases.
A molecule with a greater number of carbon atoms usually has a higher boiling point than
the same type of molecule with fewer carbon atoms. For example, hexane,
CH3CH2CH2CH2CH2CH3 has a higher boiling point than ethane, CH3CH3. The melting
points of organic compounds follow approximately the same trend as their boiling points.
Molecules that can form hydrogen bonds with themselves have a higher boiling point
than similar molecules that cannot form hydrogen bonds with themselves. For example,
alcohols can form hydrogen bonds, but alkanes cannot. Therefore, alcohols have higher
boiling points than alkanes.
Molecules that can form hydrogen bonds with water, or can accept hydrogen bonds from
water, are usually soluble in water. For example, many alcohols are soluble in water
because they can form hydrogen bonds with water.
Polar molecules usually have a higher boiling point than similar non-polar molecules.
Also, polar molecules that can form hydrogen bonds have an even higher boiling point
than polar molecules that cannot form hydrogen bonds. For example, ethanol,
CH3CH2OH, is polar. Its molecules can form hydrogen bonds.
Methoxymethane, CH3OCH3, is an isomer of ethanol. It is also polar, but its molecules
cannot form hydrogen bonds. Thus, ethanol has a higher boiling point than
methoxymethane. Both of these compounds have a higher boiling point than the nonpolar molecule ethane, CH3CH3.
Polar molecules with a large non-polar hydr ocarbon part are less polar than polar
molecules with a smaller non-polar hydrocarbon part. For example, octanol,
CH3CH2CH2CH2CH2CH2CH2CH2OH, is less polar than ethanol, CH3CH2OH.
Polar molecules with a large hydrocarbon part are less soluble in water than polar
molecules with a smaller hydrocarbon part. For example, octanol,
CH3CH2CH2CH2CH2CH2CH2CH2OH, is less soluble in water than ethanol, CH3CH2OH.