© 2000 Rainer Glaser. All rights reserved.
The University of Missouri at Columbia
Chemistry 212 - Organic Chemistry II - WS00
Pointers on Web Destinations
Friends & Students!
On occasion, I will be pointing out a few WWW links to you. Many of these
links, and many more, can be found in the "Web Destinations" section of
the course web site.
All of the common amines have unpleasent smells. For example, the amine
formed by having three methyl groups attached to the nitrogen,
(CH3)3N, is called trimethyl amine and is the
compound responsible for the smell of dead fish. Two other compounds named
putracine (1,4-butanediamine) and cadaverine (1,5-pentanediamine)
are responsible for the smell of rotting flesh.
Interestingly, some of these bad smelling amines show up where one might
least expect them. It turns out that not all flowers smell like roses!
Some flowers smell like "a rotting carcass." Take a look at the unusual
and quite interesting site on
(Two words I never thought to combine in my life.)
What makes a molecule smell? Fascinating question. The article below was
taken from the web publication
"Elemental Discoveries" and this
will give you an idea about current hypotheses.
As always, enjoy, RG.
Blowing the theory of how we
An amazing theory about how we smell is set to put noses out of joint
everywhere...read on to find out why the vibrations up your nose might be
more important to the sweet smell of success than picking molecular locks.
Molecules smell. That's a fact. What they smell of is not usually obvious
from looking at their molecular structure though. For instance, some
molecules that have very different shapes smell very similar, for instance
hydrogen cyanide, trans-hex-2-enal and benzaldehyde all smell of bitter
almonds, while others such as acetophenones, which look almost identical,
can smell very different.
Some things can be predicted about a molecule's smell in general terms
from its structure. Hydrocarbons found in petrol, for instance, have a
nondescript odour while molecules containing amine groups have a fishy
smell. Both organic and inorganic compounds containing a sulfur atom
all have the sulfurous smell of rotten eggs. Bizarrely, a molecule like
gardamide, which contains a CONH2 amide group smells of
grapefruit and horse fur!
The currently accepted theory of why one molecule smells like it does is
based on a discovery made by British scientist John Amoore in the 1950s.
He found that people's perception of smell was not always the same and
some people had "blind spots" to certain smells - a condition called
anosmia. Some people for instance cannot smell the male hormone
androstenone others cannot sniff out musk or camphor. Amoore speculated
that the receptors in the nose responsible for sending the signal from
each aroma to the brain worked like a lock and key and for people with
anosmia they had some locks missing.
Amoore's lock and key idea was not to be sniffed at as it was based on
research in other fields that showed that some molecules could fit into
proteins, such as enzymes, like a key in a lock and trigger an effect.
Amoore reasoned that the combination of aroma molecules with different
shapes picked the nose locks and sent a smell to the brain depending on
the shape of the molecule. It all sounded very reasonable: the shape of
the NH2 group in amines fitted into the olfactory locks giving
of a fishy smell, the shape and size of the sulfur-hydrogen group
a certain receptor. The idea has been the basically accepted theory of
smell ever since.
However, another British scientist, Luca Turin a biophysicist and
enthusiastic perfume expert was never satisfied with this explanation of
smell. He says that the evidence just does not stack up. There are
literally millions of smelly molecules many with similar shapes that smell
very differently and others with wildly different shapes that smell almost
the same. His favourite example, which he demonstrated at the Royal
Society Exhibition in the summer of this year is that of decaborane.
Decaborane looks like the hydrocarbon camphane (the parent molecule of
camphor used in cold remedies) but with all the carbon atoms swapped for
boron atoms. The lock-key theory predicts that decaborane should smell
like camphor. It does not. Decaborane strangely smells of sulfur.
How could that be? In the lock-key theory, the bulbous sulfur-hydrogen
group (SH) is considered the only thing of the right shape and size to
trigger the smell of sulfur. Decaborane has no sulfur but still has its
characteristic stench! Turin has a theory to explain this and it could be
set to undo the lock-key principle and revolutionise the perfumes and
Turin's idea hinges not on the shape of molecules but on how they quiver
and shake. Imagine a molecule made up of tiny balls connected by springs,
twang the balls and the molecule vibrates with a set of frequencies which
can be recorded as a vibrational spectrum. More scientifically, he
believes that the vibrational spectrum of a molecule is the real property
that is detected by the nose and interpreted by the brain. The idea sounds
bizarre but two of our other senses vision and sound - are based on the
brain's interpretation of vibrations and spectra so why not smell? Turin
reckons that the array of receptors in our olfactory bulb - the organ up
our noses that detects smell - are sensitive to the vibrating springs in
different molecules and can pass on a fingerprint signal to the brain.
Decaborane contains no sulfur but its vibrational spectrum is very
similar to that of sulfur-containing compounds. The B-H bonds in
decaborane vibrate at the same frequency as the vibrations of the S-H
The ultimate test of the theory is what Turin describes as the
acetophenone experiment. Acetophenonethough contains eight hydrogen atoms but if
these are swapped for the heavier isotope of hydrogen, deuterium, the
molecule is still chemically the same and is still exactly the same shape
the only difference is that the eight hydrogens are heavier each by the
mass of a neutron. The heavier deuterium atoms do not affect the
molecules' shape but they do affect the vibrations of the molecule. This
results in a marked difference in smell between normal acetophenone and
the deuterated version.
The next stage is to design a molecule to produce a specific smell. This
should be possible with computerised molecular modelling techniques that
let the user simulate a vibrational spectrum for the molecule they design
on the computer. If a compound is designed with a spectrum similar to the
spectra of the components of chocolate, for instance, it might be possible
to create a much more authentic chocolate smell for use in food. Emulating
the spectrum of the musk or ambergris with biodegradable versions of the
synthetic odour molecules could make a far more environmentally friendly
perfume. Perfumes could also be designed with entirely new smells based on
the vibrational spectra of different synthetic molecules.
The fact that smell, according to Turin's theory, is a spectral sense
like sight and sound might also help explain a strange but very rare
condition known as synaesthesia where the "sufferer's" senses are mixed
up. Several musical composers claim to be able to hear in colours or smell
sounds and very young babies are thought to have a mixed up sense of the
world where the various inputs - sight, sound and smell - are not
processed separately by the brain. The idea that these senses are based on
spectra could be part of the explanation.
As if to prove there is nothing new under the sun. The idea of the nose
working like a spectrometer was first proposed sixty years ago by chemist
Sir Malcolm Dyson but Turin is taking the idea into the next millennium
working with ICI's fragrance division Quest International to design a
computer program that predicts smell based on a molecule's vibrations. The
idea is surely nothing to be sniffed at.
This article originally appeared in the Summer 1997 issue
of Elemental Discoveries