http://www.saflyermag.comHYDROCARBONS AND THEIR ISOMERS
Hydrocarbons, as the name implies,
are naturally-occurring or synthesized
compounds consisting mainly of carbon
and hydrogen atoms. There are literally
millions of different ways these two
elements can be hooked together, resulting
in millions of different hydrocarbons which
have wildly different characteristics.
Without delving too deeply into
basic chemistry, a carbon atom has four
‘valencies’, which simply means it can
chemically bond to the valencies of other
atoms up to four times. Hydrogen has
only one valency, so it can only make one
chemical bond.
The simplest basic hydrocarbon is
methane, which has one carbon atom
and four hydrogen atoms bonded to
each of carbon’s four valencies. A simple
representation of methane looks like this:
Ordinary octane, or more correctly,
n-octane looks like this:
N-octane has eight carbon atoms
strung in a line, each joined by one of its
valencies. Hydrogen atoms then bond to
the remaining valencies.
However, there are actually many
ways in which eight carbons and eighteen
hydrogens can be arranged, all bonded
together into one compound. These
compounds are known as isomers of the
basic compound. The following depiction is
just one of isomers of C8H18 and would be
known as an iso-octane:
There are many isomers of octane,
but this is the special one called
2-2-4-trimethylpentane – the hydrocarbon
used as the reference ‘100-octane’ fuel in
the laboratory. Nevertheless, each of these
isomers have exactly the same number of
carbons and hydrogens as n-octane.
Although this one’s name is something
of a mouthful, it is called a ‘pentane’
because it has five carbons in a row, but
has three ‘methyl groups’, two of them
bonded to carbon number 2, and one
methyl group bonded to carbon number 4.
As a general rule, the more carbon
atoms there are in a hydrocarbon, the
lower the octane rating and the lower the
volatility. But isomers of the same basic
compound change that: n-octane has an
‘octane rating’ of far below zero, lower than
n-heptane. But the iso-octanes have much
higher ‘octane ratings’, up to 100 for the
2-2-4-trimethylpentane variety.
This group of hydrocarbons which
includes fairly well-known substances
such as propane (C3H8), butane (C4h10),
hexane (C6H14) and so on are known
as paraffins. There are several other
groups of hydrocarbons with very different
structures and with names such as olefins,naphthenes and so on.
One other hydrocarbon group that
is worth mentioning here is known as
‘aromatics’. The most basic aromatic is
benzene, which looks like this:Note the ring structure of six carbons
with alternating double bonds. Other
aromatics used in fuels are toluene and
xylene. They have the same basic ring
structure, but toluene has one of the
hydrogens replaced with a methyl group,
i.e. one carbon and three hydrogens, while
xylene is also based on the benzene ring,
but with two methyl groups attached.
Aromatics got their name because they
have a fairly pleasant smell and benzene
was used as the base for after-shave lotions
and other cosmetics. That was until it was
discovered that benzene is highly toxic and
a known carcinogen. Benzene is therefore
banned in all fuels and only a trace amountThe Embraer Ipanema is the only
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