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2.
Why are there
so many carbon compounds
Carbon
atom is unique amongst all the elements that are found in nature. It can
form long chain molecules. The ability of carbon to form a long chain is
called catenation. The chains are
formed because carbon atoms form tetravalent bonds with other carbon atoms.
This structure can be repeated endlessly without disturbing the stability of
the bonds and the compounds formed. The chains can form branches, and sub-branches. The carbon
atoms also form rings. The rings themselves can have more rings attached to
them. The list is endless. Most of the protein molecules, amino acids are
long chain carbon molecules.
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Variety of carbon chains formed when
carbon atoms join together. |
The carbon compounds are classified into two groups :
1. Open chain compounds or aliphatic compounds.
2. Closed chain compounds or aromatic compounds.
Organic
compounds forming carbon-carbon chains are called aliphatic compounds. They
are found in animal and vegetable fats. They do not have strong aroma. The alkanes (sp3
hybridization), alkenes (sp2
hybridization ) and alkynes (sp hybridization) fall under this category.
There are millions of alkanes, alkenes and alkynes found in nature.
Organic
compounds form closed rings along with the branches of the rings are
called cyclic or aromatic compounds. The rings are formed with 6 carbon
atoms only. As the name suggests, they give off very strong aroma. Benzene
with 6 carbon atoms ring is the best example of this category.
(For Kekule’s dream see box). The number of variation of ringed
compounds is endless. Most of the perfumes and pleasant odours from fruits
and flowers are due to such aromatic compounds.
Summary
We have seen in this chapter the difference between organic and
inorganic compounds. We have studied how carbon forms tetravalent covalent
bonds. The large number of carbon compounds in nature is basically due to
the high stability of carbon bonds. We have
studied how alkane, alkene and alkyne compounds are formed.
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The
precise chemistry of carbon compounds owes a large part to Kekule. Friedrich August Kekule was born on September 7, 1829 in
Darmstadt, Germany. It was by chance Kekule
enrolled himself for a course
in Chemistry in the winter of 1847 in the University of Giessen,
Germany.
This decision would change his life forever.
He became so interested in the carbon material that he studied their
structures very closely. He arrived in London in 1853, where he
started working on classification of organic compounds
based on their structure. There he had a "vision"
from
where the tetravalency of carbon and the ability of carbon atoms to
form chains and rings were born.
Friedrich August
Kekule
Kekule worried a lot about the structure of
benzene. One day he
fell off to sleep thinking about benzene, in front of a fire. There
he dreamt that snakes were chasing one another; ultimately one snake
put his tail in its mouth ! Thus the structure of benzene was solved
: benzene is a 6 carbon ringed compound with 6 hydrogen atoms. The
carbon bonds are alternately single and double.
Kekule’s
wrote about his dream in his diary : ....”I
was sitting writing on my textbook, but the work did not progress;
my thoughts were elsewhere. I turned my chair to the fire and dozed.
Again the atoms were gamboling before my eyes. This time the smaller
groups kept modestly in the background. My mental eye, rendered more
acute by the repeated visions of the kind, could now distinguish
larger structures of manifold conformation; long rows sometimes more
closely fitted together all twining and twisting in snake-like
motion. But look! What was that? One of the snakes had seized hold
of its own tail, and the form whirled mockingly before my eyes. As
if by a flash of lightning I awoke; and this time also I spent the
rest of the night in working out the consequences of the
hypothesis”.
Kekule died on July 13, 1896 in Bonn. |
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