Organic Chemistry
Properties of compounds
Phases
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Alkanes – first four gasses, 5 to 17
liquids and more than 17 solids at room
temperature.
Alkenes – two to four gasses, 5 to15
liquids and more than 15 are solids at
room temperature.
Alkynes – two to four gasses, 5 to 17
liquids and more than 17 solids at room
temperature.
Density & Solubility
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Alkanes, alkenes and alkynes all have
densities less than 1g.cm-1 and are
therefore less dense than water.
All three groups consists of non-polar
molecules which are very soluble in
non-polar solvents like
tetrachloromethane and benzene, but
insoluble in polar solvents like water.
Boiling points of the alkanes
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As the number of carbon atoms increase the
longer the organic molecule is.
The Van der Waals forces between the
molecules increases as the molecule
increases ( more C atoms in chain)
The stronger the Van der Waals forces, the
more energy is needed to break these forces
and the higher the boiling point.
Melting points of the alkanes
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As with boiling points, the melting
points of the alkanes increase with an
increase in the number of carbon
atoms.
The graph is not a smooth increase as
with the boiling points.
The boiling and melting points of the
alkenes and alkynes also increases as
the number of C atoms increases.
Alkanes
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Alkanes like petrol have a very low
boiling point (it is volatile, explosive,
flammable) and evaporates easily.
Volatility is the way in which a liquid
changes into vapour. Volatility
decreases as the number of C atoms
increase
Liquids and gasses have larger volatility
and therefore very characteristic smells
like for eg. petrol. Solids odourless.
Alcohols
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Boiling points increase with an increase
in the number of C atoms in chain.
Boiling points of alcohols higher than
boiling points of corresponding straight
chain alkanes, because of stronger
hydrogen bond between alcohol
molecules.
Alkylhalides
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Alkylhalides have higher melting and
boiling points than corresponding
alkanes.
Molecules are polar so apart from
London forces (Van der Waals force)
there are also dipole-dipole forces
present which is stronger than only Van
der Waals forces.
Carboxylic acids
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Molecules are polar.
Boiling points relatively higher than
corresponding alcohols.
This is caused by the fact that the
molecules are kept together by two
hydrogen bonds.
Esters
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Boiling points similar to that of
corresponding Aldehydes and Ketones.
Molecules are polar and have dipoledipole as well as Van der Waals forces
present, but do not form hydrogen
bonds.
Lower boiling points than alcohols and
carboxylic acids.