I know that one of the ways to form a benzene ring is by the distillation
of coal, or made by the synthetically from petroleu or acetlene, but what
i want to know is the chemical equation for these reactions, and for the
preparation of an aromatic do you just consider the basic benzene ring
We are talking about several different reactions here. Some of them have no
single chemical equation, but are a messy combination of all sorts of reactions.
Nevertheless, here goes:
- Destructive distillation of coal
- This is a really messy process, for which there isn't a single simple
chemical equation. Coal started out as (mostly) cellulose,
(C6H12O6)n. Immense heat and
pressure in the absence of oxygen removed the oxygen as water (H2O)
and carbon dioxide (CO2). In the process the cellulose was changed
from (C6H12O6)n to something
between (C6H)n and
(C6H2)n.
The destructive distillation of coal finishes the process. Heating coal in the
absence of oxygen gives a liquid called "coal oil," a mixture of
benzene, naphthalene, anthracene and suchlike aromatic hydrocarbons with more
hydrogen per carbon than the original coal; what's left behind is pure carbon
with no hydrogen at all (coke).
As I said, this is a messy process with no particular chemical equation. Given
what I've told you I'm sure you can come up with several balanced reactions;
consider that all of them are happening at the same time. And don't forget that
coal contains a certain amount of sulfur, so that coal oil also contains
thiophene.
- Catalytic reforming of petroleum
- Normally this is done with something like straight-run gasoline, which
is a mixture of low-boiling saturated hydrocarbons like hexane. Heat and a
catalyst changes hexane into a mixture of benzene and hydrogen:
C6H14 ®
C6H6 + 4 H2
Of course the real process is somewhat messier, because you don't have pure
hexane and even if you did there'd be byproducts. This sort of reaction (it's
also done with methane and steam to produce carbon monoxide and hydrogen) is
the major industrial source of hydrogen.
- Catalytic trimerization of acetylene
- Conceptually, this is the simplest process. You take acetylene and, with a
catalyst and a certain amount of heat and pressure, presto! Out comes benzene.
Notice that three acetylenes equals one benzene:
3 C2H2 ®
C6H6
The reason this works is because at high pressure one benzene molecule takes
up less volume than three acetylene molecules. You need the high heat to
encourage the molecules to link up. (If you didn't have the catalyst, you'd need
even more heat!)
For more information, you can consult a textbook like Wittcoff and Reuben's
Industrial Organic Chemicals.
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