Chemistry Reference
In-Depth Information
how to view the matter now that we are aware of the chemical differences. These
differences fall off rapidly with atomic number, and for many purposes are negli-
gibly small. But where precision is at issue, ignoring the kinetic effects of variation
in atomic weight is to ignore subtle differences in chemical features of the reactivity
of compounds. This is sufficient to count isotopic variants different substances,
however subtle the difference, by taking into consideration subtleties of which
chemists of bygone eras were unaware.
The elemental composition of a compound only settles its compositional
formula. Berzelius was led to coin the term “isomer” in the 1820s for a different
substance with the same compositional formula. Differences that first came to the
attention of chemists were chemical—dimethyl ether, (CH
3
)
2
O, is much less
reactive than ethyl alcohol, CH
3
CH
2
OH, with the same compositional formula,
C
2
H
6
O—accompanied with differences in physical properties such as melting and
boiling points. The notion of a structural formula was elaborated as a topological
structure indicating which elements are linked to which as a finer development of
compositional formulas serving to represent distinct isomers. They were seen as
structures derived from an archetypical compound by substitution. Thus, alcohols
were derived from an equivalent of water by the substitution of a(n equivalent of)
hydrogen by an alkyl group (C
n
H
n+1
) and were said to be of the water type.
Williamson famously showed in 1851 that ethers could be regarded as water
in which two equivalents of hydrogen are replaced by alkyl groups, which need
not necessarily be the same. At the same time this settled the compositional formula
of water as H
2
O, rather than, for example, HO as Dalton maintained.
Later in the century pairs of substances turned up which seemed to be alike
chemically but differed in one or two physical properties. They might form crystals
whose shapes are mirror images of one another, or they might form solutions
which rotate the plane of plane polarised light by equal amounts in opposite
directions. These were stereoisomers, calling for more developed structural formu-
las displaying the orientation of elements to one another in three-dimensional
space and differing in the one being a mirror image of the other. Internally they
were alike. Chemical reactions were subsequently discovered that are stereospe-
cific, where a given stereoisomer will only react with a specific stereoisomer of the
other reactant. This is generally the case with biochemically important reactions,
where the naturally occurring form of one of the reactants is a specific stereoisomer
that will only react with specific stereoisomeric forms of the other reactants.
Evidence of chirality, then, is a mark of distinction between substances.
This provides another reason in support of the thesis that isotopic variants are
distinct substances in conflict with the 1923 IUPAC ruling. Kokke and Oossterhoff
(
1972
,
1973
), for example, have shown that substitution of oxygen-18 for the more
common oxygen-16 isotope in an optically inactive compound gives rise to chiral
effects. The substitution of deuterium for protium gives rise to a similar but more
marked chiral effect.
For much of its history, the theory of structural formulas could be developed
without recourse to the atomic theory (Duhem
1902
; Needham
2008b
). But the
reality of the microworld was convincingly established at the beginning of the
