ORGANIC CHEMISTRY PRINCIPlES
IN CONTEXT
e
Arornatic: A Word That Carne to Mean Something
Other Than Odor in the Chernical Sciences
P
ARTINGTON (SECTION 4.9), IN BIS CLASSIC "A History of
Jhe Discoyery
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of Benzene
• Michael Faraday
Chemistry" wrote about Michael Faraday: "In his time Faraday was a
model for scientific men. Ofhumble origin, he rase by his genius to the highest
rank of scientific eminence, and his moral character and integrity were on the same level."
Indeed, Faraday was ofhumble origins. He was born into a family that was not well
off. His father was a village blacksmith and young Michael at the age of thirteen, one of
four sons, had to end his formal schooling and become an apprentice to a bookbinder,
But putting a brilliant and inquisitive young man in a shop full ofbooks, in which he
asked and was granted permission to read them, was all it took to set Faraday on a path
of great accomplishment. His abilities and his intense interest in science led Faraday to
the public lectures of one of the great scientists of that time, Humphrey Davy. Faraday
not only attended the lectures but as well kept copious notes, which he wrote up in the
form of a book that he sent to Davy. This led to an interview with the great man, which
led Faraday to be offered a position with access to facilities to conduct experiments
and therefore the beginning of a remarkable story, which is worth following by simply
going to Google under the name Michael Faraday.
It was not long before Faraday's scientific accomplishments led him to became a
fellow of the Royal Society (FRS) and ayear later at the age of thirty three he read a paper
before the society on [une 16, 1825, titled "On new compounds of carbon and hydrogen,
and on certain other products obtained during the decomposition of oil by heat."
Faraday, who disliked the distinction between chemistry and physics and thought
ofhimself as a natural philosopher, had long been interested in the oil-gas increasingly
used to light streets and homes in English cities in the latter part of the eighteenth and
into the nineteenth century. David Gordon founded one of the companies praducing
oil gas, the Portable Gas Company, where Michael Faraday's older brather Robert
was employed. Mr. Gordon had asked Robert if his brother might be interested in
investigating a liquid that deposited on the portable gas valves, a liquid that interfered
with the workings of the device. Let's follow Michael Faradays own words:
"The object of the paper which 1 have the honour of submitting at this time to the
attention of the Royal Society, is to describe particularly two new compounds of carbon
and hydrogen, and generally, other products obtained during the decompositionof oil by
heat. My attention was first called to the substances formed in oil at moderate and at high
temperatures, in the year 1820; and since then 1 have endeavoured to lay hold 6f every
opportunity for obtaining information on the subject. A particularly favourable one has
been afforded me lately through the kindness of Mr. GORDON, who has furnished me
with considerable quantities of a fluid obtained during the compression of oil gas, of
which 1 had some years possessed small portions, suificient to excite great interest, but
not to satisfy it."
In the twenty six pages to follow this introduction, Faraday describes a remarkable
series of experiments and analyses, which are astonishing for what wasaccomplíshed
considering the chemical complexity of what he was studying and the tools available
in the 1820s. In fact in 1968, R. Kaiser who worked for Badische Anilin- und SodaFabrik AG, a corporation derived from the company Mr. Gordon founded, published
CHAPTER 6: Benzene, Aromaticity and the Nature of Chemical Reactions • 165
a paper entitled: ""Bicarburet of Hydrogen." Reappraisal of the Discovery of Benzene
in 1825 with. the Analytical Methods of 1968." Here Kaiser documents with modern
techniques how complex a separation and analytical problem Faraday had overcome.
From his effort, involving multiple distillations and crystallizations, Faraday was able
to isolate a liquid that he reports as crystallizing close to 32° F and which he analyzed as
having the composition C2H, hence the name bicarburet ofhydrogen. This composition
was based on an atomic weight of carbon assigned as 6 with hydrogen as 1 and therefore
corresponds to (CH\n in modern terms, where n could be any number.
He also measured the vapor density, which revealed that the "particles" of the
substance, what we would call the molecular weight, was about 39. Correcting this
for the assignment of hydrogen gas as H and therefore 1, as was believed in that
time, instead of 2, as we know today for H2, would double this number to close to the
molecular weight ofbenzene, 78, corresponding to C6H6'
With this isolation ofbenzene, Faraday set in motion a molecular mysterywhose solution
took over 100years and was not fully accepted by chemists for nearly the next 125years. Let's
follow this story and discover how benzene played a central role in the industrial processes
leading to the most important plastics of our time. The story also leads to theoretical
understanding of the special properties of benzene, which takes us to understanding the
fundamental nature of the nucleotide bases in DNA and also certain coenzymes.
Faraday described his newly isolated compound in the following manner: "Bicarburet of hydrogen appears in common circumstances as a colourless transparent
liquid, having an odor resembling that of oil gas, and partaking also of that of almonds."
Many compounds isolated in those early years of the nineteenth century from
natural sources were pleasant to the smell, among them extracts from cinnamon bark,
wintergreen leaves, vanilla beans and anise seeds, all reported as aromatic essential
oils. These compounds were analyzed in the manner Faraday used for benzene and
found to gene rally have a lower ratio of hydrogen to carbon than found for what came
to be known as alkanes where the ratio of H/C is generally greater than 2. In those
early years it is not surprising that benzene should be seen as belonging to that same
class of aromatic molecules.
PROBLEM 6.1
Use web -resources to discover the chemical structures of the molecules responsible for the
properties noted in: "Many compounds isolated in those early years of the nineteenth century
from natural sources were pleasant to the smell, among them extracts from cinnamon bark,
wintergreen leaves, vanilla beans and anise seeds, all reported as aromatic essential oils." Do
any obey the isoprene rule (section 5.2, Figure 5.6)?
F
IGURE 1.6 EXHIBITS THE STRUCTURES OF SETS of molecules
that are structural isomers. 1he formulas for the two isomeric sets in
this figure are given as C6H14and C6H140. In otherwords there are
twice as many hydrogen atoms plus two more, for the number of carbon atoms, or
that is, CnH2n+Z' For six carbon atoms one has 12+2 hydrogen atoms. This formula
corresponds to a general rule for organic molecules that contain only single bonds and
as well in which no ring is present. The formula simply expresses the four coordinate
nature of carbon seen first in the formula for methane, CH4, which fits the same formula
for n=1. All these molecules are called saturated.
Now try this formula for any of the structures of glucose in Chapters 1 and 3 and
discover that two hydrogen atoms are missing. The formula for glucose is C6H1Z06. This
formula fits if glucose is written in the cyclic form or in the open form (Figure 3.11).
For every ring in an organic molecule, as in the cyclic form of glucose, the formula
¡fe
...
. .•. ;
-A ShoDiversion
!aboutthe Ratio of
Hydrogén tocarnon
in VaiiousOrganic
Molecules.
170 . Organic Chemistry Principies In Context
FIGURE 6.1 ~
Evolution of the
Structure of Benzene
from 1865 to the
Present
It isfrom this closed chain that the substances called 'aromatic' are derived." Kekulé goes
on to write "The theory requires that only one modification of monochlorobenzene and
of pentachlorobenzene can exist, but several isomers (probably three) of di-, tri-, and
tetrachlorobenzene."
Although Kekulé had been attracted to chemistry by his teachers when he was
nineteen years old, he was, at first, a student of architecture based on his talents in
drawing and mathematics. These abilities apparently were at work in the visualization
necessary for the development of his structural theoretical ideas, which could be
called molecular architecture.
Kekulé spends more than
a page of his 1865 publication
with drawings demonstrating
his structural idea for benzene
including drawings for many,
if not all, of the derivatives
1865
of benzene that' had been
synthesized up to that time.
The second drawing of this
series, which Kekulé named
"Chaine ferrnee," is redrawn
in Figure 6.1 showing the
six carbon atoms with their
oblong shape and the six
affinity units represented by
six circles. The up -and down
1866
arrows represent the closing
of the ring, which in this first
H
publication on Kekulé's ideas
H
H
for benzene did not propose
the shape taken by this "closed
chain," The double lines and
single lines between the oblong
H
H
carbon atoms, in modern
H
terms, would represent double
benzene .
and single bonds. In another
2008
paper in 1865 he proposed a
hexagon for the closed chain
and by 1866, his final structure
showing how the bonding arrangements could be made within the hexagon structure.
Finally, in Figure 6.1 we show the structure of the benzene ring used today, evolved
from Kekulé's proposal.
In Kekulé's structures drawn in Figure 6.1, only what Kekulé called the "common
nucleus" is shown, that is only the six carbon atoms and how they are connected.
In the 1865 structure the six small circles stand for, in Kekulé's words, "the six free
affinity units," which allow the connection to the six hydrogen atoms to complete
the formula for benzene, known by that time to be C6H6. The lines extending awa
from Kekulés hexagon structure and similarly away from the modern structure
also connect to hydrogen atoms in benzene itself. However, Kekulé allowed the
possibility that these affinity units could allow connection to atoms other than
hydrogen, which he called side chains, a word still used today. It's interesting to
compare Kekulé's first presentation of these side chains with today's structures as
shown in Figure 6.2.
The first experimental evidence that Kekulés structure was correct was bis
prediction of the number of isomers, or what were called modifications, possib
when one or more hydrogen atoms in benzene are substituted for by another aro