Unsaturated Hydrocarbons
When less than the full compliment of hydrogen atoms is present in a
hydrocarbon chain or ring, the hydrocarbon is said to be unsaturated. Unsaturated
hydrocarbons are characterized by having two adjacent carbon atoms linked by two or three
bonds instead of only one. These links are known as double bonds or triple bonds.
They are not stronger than single bonds. On the contrary, they are
surprisingly vulnerable, resulting in unsaturated compounds being more chemically reactive
than saturates.
Straight- or branch-chain hydrocarbons with one double bond are called
mono-olefins or alkenes. Hydrocarbons with a double bond ring are called
cyclo-olefins or cycloalkenes; those two double bonds in the structure are called
diolefins or dienes. Hydrocarbons with a triple bond are called acetylenes or
alkynes. A simple example, acetylene is shown below:
Neither olefins nor acetylenes occur in crude oil or natural gas but they are produced
in the conversion processes in the refinery and are important feedstocks for making
petrochemicals.
Ring compounds containing one or more six-membered rings with three alternate double
bonds form an important group known as "aromatics" because most of them have a
characteristic smell. More complex molecules of the aromatic series are obtained by
replacing one or more hydrogen atoms by hydrocarbon groups or by condensing or
"stacking" one or more rings. gain, a simple example of this is
naphthalene, illustrated below:
Another molecular complexity is caused by introducing elements such as
oxygen, nitrogen, and sulphur. With these, the number of different molecules based
on the carbon skeleton increases tremendously. We will now go on to look at these
non-hydrocarbons.
Molecular diagrams constructed by Foster Learning Inc. using
MoluCad.
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