Chemistry Reference
In-Depth Information
1
Molecular representations
and nomenclature
1.1 Molecular representations
lone pair
non-
bonded
electrons
H
H
H
From the beginnings of chemistry, scientists have
devised means of representing the materials they are
discussing, and have gradually developed a compre-
hensive range of shorthand notations. These cover
the elements themselves, bonding between atoms,
the arrangement of atoms in molecules, and, of
course, a systematic way of naming compounds that
is accepted and understood throughout the scien-
tific world.
The study of carbon compounds provides us with
the subdivision 'organic chemistry', and a few simple
organic compounds can exemplify this shorthand
approach to molecular representations. The primary
alcohol propanol (systematically propan-1-ol or 1-
propanol, formerly
n
-propanol,
n
signifying normal
or unbranched) can be represented by a structure
showing all atoms, bonds, and lone pair or non-
bonding electrons.
Lines are used to show what we call
single bonds
,
indicating the sharing of one pair of electrons. In
writing structures, we have to remember the number
of bonds that can be made to a particular atom, i.e.
the
valency
of the atom. In most structures, carbon
is tetravalent, nitrogen trivalent, oxygen divalent,
and hydrogen and halogens are univalent. These
valencies arise from the number of electrons available
for bonding. More often, we trim this type of
representation to one that shows the layout of the
carbon skeleton with attached hydrogens or other
atoms. This can be a formula-like structure without
HCC
C
O
H
H
H
H
single bond
sharing of 1 pair
of electrons
propan-1-ol
1-propanol
n
-propanol
alternative ways of
representing propanol
formula-like structure
CH
3
CH
2
CH
2
OH
formula-like structure
showing principal bonds
CH
3
CH
2
CH
2
OH
zig-zag chain omitting
carbons and hydrogens
in the hydrocarbon
portion
OH
abbreviation for alkyl
(propyl) portion
PrOH
some common abbreviations:
Me = methyl
Et = ethyl
Pr = propyl
Ph = phenyl
2 bonds
2 hydrogens inferred
1 bond
3 hydrogens inferred
OH
2 bonds
2 hydrogens
inferred
