Organic chemistry in the nineteenth century
By Rochelle Forrester
Ó All Rights Reserved
Publication Date 2006
In the early 19th
century chemists began to make a distinction between organic chemistry, which
concerned materials obtained from animal and plant sources, and inorganic
chemistry which dealt with materials from other sources. As the knowledge of
organic materials increased organic chemistry became synonymous with the
chemistry of carbon compounds.
Organic
analysis at the start of the 19th century was only capable of separating
mixtures of related substances and often these procedures resulted in
significant chemical alteration of the substances producing misleading results.
Lavoisier was the first to develop improved analytic methods for the study of
the carbon and hydrogen content of organic materials. He burnt the materials in
oxygen or air and weighed the carbon dioxide and water that was formed.
Gay-Lussac and Thenard improved the method by reacting the organic material
with the oxidizing agent potassium chlorate. The method was further improved
when copper oxide replaced the potassium chlorate. The method was still further
improved by Berzelius and in 1831 by von Liebig. Liebig’s method allowed
reliable analysis to take place and was to survive into the twentieth century.
Methods for the determination of nitrogen in organic substances were devised by
Dumas and for the determination of sulphur and halogens by Liebig.
In the early 19th
century most chemists believed the products of a living organism were produced
through the agency of a vital force present only in living plants and animals.
These products could be converted into other products in the laboratory, but
could not be created in the laboratory from their elements. This view known as
vitalism received a serious blow from Wohler in 1828 when he synthesized urea
by reacting silver cyanate with ammonium chloride. This however did not mean
the end of vitalism because although Wohler had created an organic product in
the laboratory, he had done so by the reaction of two other organic products.
Vitalism received a major set back in 1844 when Kolbe synthesised acetic acid from
non-organic materials and Berthelot in 1860 showed the possibility of the
organic synthesis of organic compounds from the elements carbon, hydrogen,
oxygen and nitrogen leading to the abandonment of vitalism. The development of
the concept of the conservation of energy in the middle of the 19th century
showed there was no need for the concept of a vital force.
The first
attempt to understand the nature of organic compounds was the theory of
radicals. Radicals were considered to be the “stable part of a substance that
retains its identity through a series of reactions even though it was known to
be a compound”. Lavoisier considered that when a radical combined with oxygen
an acid was formed. The radical was an element for mineral acids, but was a
compound containing both carbon and hydrogen for organic acids. The radicals of
different organic acids contained different quantities of carbon and hydrogen.
The idea of the radical was extended by experiments by Gay-Lussac on hydrogen
cynadide and cyanogen. The cyanide radical was observed to pass unchanged
through a series of reactions so radicals came to be seen as a particularly
stable group of atoms that reacted as a unit during chemical reactions.
Support for the
radical theory came from the work of Liebig and Wohler in 1832 on benzaldehyde.
They converted it to a number of other compounds including benzoyl chloride and
benzoic acid and throughout all the changes the group C14 H10 O2
(actually C7 H5 O) remained unchanged. They called this the benzoyl radical.
Radicals soon became known as the elements of organic chemistry and chemists
began to look for radicals in every compound.
Radical theory
however was modified when Dumas studied the reaction of chlorine, bromine and
iodine with oil of turpentine and other substances. He considered halogens,
such as chlorine, bromine and iodine, were replacing hydrogen within compounds
and a given volume of hydrogen was being replaced by the same volume of
halogens. He called this discovery the law of substitution. Dumas work was
taken up by Laurent who considered hydrocarbons to be a “fundamental radical”
by which “derived radicals” could be obtained by substitution reactions.
Derived radicals had similar properties to the fundamental radicals they
replaced. Laurents’s theory meant radicals could no longer be considered to be
an unchangeable group of atoms.
Laurents theory
was attacked by Berzelius as it conflicted with his dualistic theory. Berzelius
could not see how electro-negative chlorine could replace electro-positive
hydrogen without totally changing the compound. However the experimental
evidence, in particular the involving the chlorination of acetic acid and the
studies of the properties of the resulting trichloraacetic acid by Dumas and
Melsens lead to an abandonment of the dualistic theory as it applied to organic
compounds.
The dualistic
theory was opposed by a number of unitary theories proposed by Laurent, Dumas
and Gerhardt. Laurent’s theory was known as the “nucleus theory”, Dumas as the “theory
of types” and Gerhardt’s as the “theory of residues”. The proliferation of
theories reflected the confusion prevailing in organic chemistry in the first
half of the 19th century.
The concept of
radicals was enhanced by Bunsen who studied the reactions of a liquid known as
cacodyl. He treated cacodyl chloride with zinc and obtained what he considered
to be the free cacodyl radical. Later Kolbe isolated what he considered to be
the free methyl radical (actually it was ethane) while Frankland believed he
had isolated the free ethyl radical (actually butane) by treating ethyl iodine
with zinc.
A new type
theory was then developed due to the work of Wurtz, Hoffman, Williamson and
Gerhardt. Wurtz discovered the primary amines, methyl and ethyl amide. He
recognized these compounds were derivates of ammonia, in which hydrogen had
been replaced by methyl or ethyl. Hoffman showed the relationship of the amines
to ammonia by alternatively replacing one, then two then three hydrogen atoms
by organic radicals to create primary, secondary and tertiary amines. Hoffman
considered all these compounds to belong to an “ammonia type”.
Williamson
prepared ether by the action of ethyl iodine on potassium ethlate. This meant
that a hydrate theory of the structure of alcohols proposed by Liebig was incorrect
and Williamson suggested the concept of a water type. This type included
compounds such as water, alcohol, ether and methylethyl ether and acetic acids
and other acids of that series.
Gerhardt added
two further types. These were the hydrogen type that included hydrogen, ethane
and butane and the hydrogen chloride type which included hydrogen chloride and
ethyl chloride. Gerhardt considered that organic compounds could all be
classified as belonging to one of the four types, being the water, hydrogen,
hydrogen chloride and ammonia types.
The new type
theory was useful as a means of classifying the increasing number of organic
compounds but it did little to explain the constitution of the compounds or the
arrangement of the atoms in the molecule. The chemical formulae according to
type theory were reaction formulae. They represented methods of formulation and
decomposition and indicated there were chemical similarities between compounds
of the same type. To understand the constitution of the compounds it was
necessary to go beyond the radicals to the atoms and to look at the arrangement
of the atoms in the radical.
Progress to
understanding the constitution of compounds and radicals was made by Frankland.
His studies of the combination of organic materials with metals and other
experimental work showed that an atom of nitrogen, phosphorus, antimony and
arsenic always combined with three or five organic radicals. Mercury, oxygen
and zinc combined with two. Frankland considered “no matter what the character
of the uniting atoms may be, the combining power of the attracting element ...
is always satisfied by the same number of these atoms”. The term combining
power was eventually replaced by valency. The idea of valency was supported by
the law of definite and multiple proportions which implied that atoms had an
exact and limited capacity to combine. Valency was also suggested by the
experimental observation of substitution. It was known since 1834 that one atom
of chlorine replaced one atom of hydrogen while one atom of oxygen replaces two
atoms of hydrogen. Frankland’s theory was not immediately accepted because
formulae were uncertain and based upon uncertain atomic weights and because
chemists were still imbued with the concepts of radicals and types.
Frankland’s
work was carried on by Kekule and Couper who simultaneously in 1858 produced
improved theories of chemical structure. Their theories brought clarification
to ideas that were slowly developing amongst chemists over the previous twenty
years. Their ideas had two main themes, the quadrivalency of carbon and the
ability of carbon atoms to join together to form a carbon chain. The linking
together of carbon atoms explained the formation of organic compounds
containing many carbon atoms. Their work also showed how the linking together
of the atoms of a compound could be shown diagrammatically. The quadrivalency
of the carbon atom was later abandoned when subsequent research revealed that
in some cases carbon is divalent and trivalent.
The work of
Kekule and Couper provided an explanation for the structure of aliphatic
compounds as consisting of chains of carbon atoms, but it did not explain
aromatic compounds which always contained at least six carbon atoms in the
molecule. The simplest of the aromatic compounds was benzene, which had been
discovered by Faraday in 1825. Kekule suggested the structure of the benzene
molecule was a closed ring of six carbon atoms, each of which had a hydrogen
atom attached to it. Kekule’s theory for the benzene molecule and the nature of
aromatic compounds was later to receive experimental confirmation.
The idea of the
linking of atoms and the structural representations that were derived from it
form the basis of modern organic chemistry. These theories enabled chemists to
understand the relationships between already known compounds and to discover
and create a vast number of new compounds.
The later half
of the 19th century saw many more developments in structural chemistry. In 1862
a triple bond was discovered for acetylene and in 1864 a double bond for
ethylene. In 1870 Markovnikov’s rule was formulated concerning the influence of
neighboring groups on the reactivity of individual parts of organic molecules.
Van Hoff and Le Bel provided a picture of the valence bond as uniformly
directed in space. The three dimensional view of valence lead to the idea of
new types of isomerism, such as cis-trans isomerism.
Organic
chemistry began with the development of new and improved methods for organic
analysis. The new methods experimental results showing groups of atoms which
passed unchanged through a series of reactions lead to the development of the
radical theory. The radical theory was forced to be modified when new
experimental results showed halogens replacing hydrogen within organic
compounds leading to the law of substitution. The law of substitution spelt the
end of the dualistic theory in relation to organic compounds and gave rise to a
series of theories by Laurent, Gerhardt and Dumas. Eventually the new type
theory arose as a means of classifying organic compounds. There is an order of
discovery from improved analytic methods, to new experimental discoveries which
then resulted in new theories to explain the experimental results. This process
occurred with both the radical theory and the development of the law of
substitution and the theories created by Laurent, Gerhardt and Dumas which
attempted to explain the law of substitution. The development of the new type
theory also resulted from experimental results that showed that certain
compounds were related to each other.
The same situation is shown by the efforts of chemists to understand the structure of compounds and radicals. Frankland’s study of the combining of organic materials with metals was dependent upon the improved analytic techniques brought into organic chemistry by von Liebig. Frankland’s experimental work lead to the development of the concept of combining power or valence. Frankland’s work was carried on by Kekule and Couper who produced theories concerning the quadrivalency of carbon and the linking together of carbon atoms to form carbon chains. This then lead to Kekule’s theory of the structure of the benzene molecule and of aromatic compounds. Again one sees new analytic methods leading to experimental results, which then lead to the concept of valency, which then lead to theories concerning the valency of carbon and to the structure of the benzene molecule.
There was a specific order of discovery which was the inevitable path by which our knowledge of organic chemistry was to develop. Even theories which were later abandoned, due to being contradicted by later experimental results, were logical explanations of knowledge acquired by earlier experimental results. Due to this even incorrect theories can be said to have arisen logically from information available at a particular point in time and to have been an inevitable part of the growth in our knowledge of organic chemistry.
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