Physical Properties of Organic Molecules
Unit: Organic Chemistry
Hydrocarbons:
Hydrocarbons are made of chains or circles of carbons with hydrogen
attached. Because of their regular shape, and low difference in electronegativity, all
hydrocarbon molecules are nonpolar. This fact is responsible for many of their
physical properties.
Due to the nonpolar nature of hydrocarbons, the molecules are held together by
weak London dispersion forces. These forces are what are responsible for the trend
that we see in the physical properties.
1. Melting point and boiling point.
The melting point and boiling point of straight chained hydrocarbons
increases as the length of the chain increases. (Why is this? You should be able to
explain it based on the London dispersion forces.) Short chained (1-4 carbons)
alkanes are colourless gases, mid-length alkanes (5-19 carbons) are colourless
liquids, and long chains (more than 20 carbons) are colourless waxy solids.
The difference in boiling points of the different hydrocarbons allows us to
separate them using their boiling points. Visit the site:
http://science.howstuffworks.com/oil-refining4.htm to learn about fractional
distillation and cracking.
2. Solubility in water
Since hydrocarbons are nonpolar, they do not dissolve in water (a polar
solvent). (What impact does this have on the environment when there is an oil spill?)
3. Density
Due to the weak forces holding the molecules together, hydrocarbons are less
dense than water. (What impact does this have on the environment when there is an
oil spill?)
Hydrocarbon derivatives:
We have looked at numerous hydrocarbon derivatives that contain oxygen
and/or nitrogen. The presence of oxygen or nitrogen in the compound will cause the
compounds to have some polarity. Depending on the length of the carbon chain,
the molecule may be polar (short chains) or nonpolar (if the carbon chain is long the
nonpolar characteristics outweigh the polar characteristics of the molecule.) Polar
molecules will have higher melting points and boiling points, and be soluble in water.
Generally, the compounds that contain the most oxygen and nitrogen (amides and
carboxylic acids) are the most polar. This is the trend:
Amide > Acid > Alcohol > Ketone ~ Aldehyde > Amine > Ester > Ether > Alkane
(Note: We can only compare compounds that have the same number of carbons, so
we know ethanoic acid is more polar than ethanol, which is more polar than ethanal,
which is more polar than ethane. )
Look here: http://www.elmhurst.edu/~chm/vchembook/213boilingpoint.html
to see diagrams showing the charge imbalance in different types of organic
molecules.
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Physical Properties of Organic Molecules
Unit: Organic Chemistry
1. Melting point and boiling point.
Polar molecules will have much higher melting points and boiling points than the
nonpolar alkanes with the same number of carbons. Remember, the more polar
the molecule, the greater the effect, so acids have a higher melting point than
alcohols, which have a higher melting point than alkanes. (Carboxylic acids are
found as solids, short chained alcohols are found as liquids, and short chained
alkanes are found as gases.)
2. Solubility in water
Short chained polar molecules will be soluble in water. This is due to the
hydrogen bonds that can occur.
Look here: http://ibchem.com/IB/ibnotes/full/bon_htm/solubility.htm
To see a clip of how the length of the carbon chain affects the solubility of
alcohols
Make sure you can explain:
 Why hydrocarbons are not soluble in water, but they are soluble in nonpolar
solvents.
 Why some organic molecules are soluble in water, and how we can predict this.
 Determine which molecule will be more soluble in water and which will have the
higher melting/boiling point, given two different molecules with the same
number of carbons.
 How we can separate the components of crude oil, and how we can convert
unusable parts of the crude oil into useful compounds.
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