Chemists can represent
molecules in a number of ways, all of which can be referred to as a "formula". To avoid confusion it is important to understand the
terminology.
Inorganic compounds, those not based on the bonding of carbon into chains or rings, tend to be fairly simple. The most common formula used for these compounds is the molecular formula. This shows exactly how many atoms of each element there are in the molecule or formula unit - NaCl for common salt, CO2 for carbon dioxide, CuSO4 for copper(II) sulphate and so on. An alternative to the molecular formula is the empirical formula, which simply shows the ratio of the different elements. For the three compounds above this happens to be identical to the molecular formula, but this is not always the case - hydrazine, for example, has a molecular formula of N2H4 and an empirical formula of NH2.
Organic compounds tend to be more complicated than inorganic compounds and these two types of formulae can be inadequate in providing a useful description of the molecule. The rings and chains of organic chemicals can be represented by three further fundamental types of formula.
The displayed formula of a compound shows all the atoms and all the bonds between them - it is effectively a kind of drawing. A simple example is the displayed formula of methane:
H
|
H - C - H
|
H
This indicates that one atom of
carbon is bonded separately to four
hydrogen atoms by single bonds. The displayed formula can also be used for cyclic compounds, and of course it can display
functional groups containing any kind of element. For example, the displayed formula of cyclohexanol is:
H H
\ /
H C O - H
\ / \ /
H - C C - H
| |
H - C C - H
/ \ / \
H C H
/ \
H H
Displayed formulae are the most explicit of the basic types. A form of shorthand is the
structural formula. This provides the same information but takes certain shortcuts. For straight chain compounds, each section of the molecule can be written, rather than drawn in full.
Hexane, for example, has a structural formula of CH
3CH
2CH
2CH
2CH
2CH
3, which can also be shortened to CH
3(CH
2)
4CH
3.
To represent cyclic compounds with a structural formula, a shape is drawn with a point for each carbon atom in the ring. Cyclohexane, for example, is a hexagon:
_
/ \
/ \
| |
| |
\ /
\ _ /
The compound consists of
carbon and
hydrogen, but these are not indicated by letter because they are the basic units of organic chemistry. A structural formula for a cyclic compound assumes that any atom is carbon unless otherwise indicated, and that its tetravalency (need for four bonds) is completed by
hydrogen when no other group is indicated. Other types of atoms in or attached to the ring
do have to be indicated. Electron delocalisation must also be indicated, by a circle in the ring. Thus the structural formula for
pyridine is:
_
/ \
/ \
| |
| O |
| |
\ /
\ /
N
(Maybe representing cyclic structural formulae in ASCII is not my strong point.)
Simple straight chain compounds, such as alkanes and alkenes, can also be represented by a skeletal formula. This consists simply of a zig-zag line, where each point represents a carbon atom and the requisite hydrogen atoms. Butane, for example, has the skeletal formula:
\ /\
\ / \
\/ \
Double bonds and
functional groups can also be represented, for example 1-hydroxybut-4-ene:
\\ /\
\\ / \
\\/ \-OH
All of these basic graphical formulae - displayed, structural and skeletal - provide only a limited amount of information about the molecule; they are not a realistic drawing of what the molecule really "looks like", which cannot be achieved simply with letters and lines.
In summary, the molecular formula is the number of each type of atom, the empirical formula is the ratio of each type of atom, the displayed formula is a drawing of all the atoms and bonds in an organic compound, the structural formula is a shorthand for these atoms and bonds, and the skeletal formula is simply a line representing carbon chains in organic compounds.
For the example of butanoic acid, the molecular formula is C4H8O2, the empirical formula is C2H4O, the displayed formula is:
H H H O
| | | //
H - C - C - C - C
| | | \
H H H O - H
the structural formula is CH
3CH
2CH
2CO
2H, and the skeletal formula is:
\ /\
\ / \
\/ \=O
|
OH
Not to be confused with "chemical formula" is "chemical name". A chemical, or systematic, name is a description of a
molecule that provides a certain amount of information about it. It is not a formula. Chemicals can have many possible "systematic names" (those derived from a logical system of
nomenclature), some familiar ("trivial") names, and yet more commercial names. One compound, for example, is known as "
ethanoic acid" under the formal
IUPAC nomenclature, "
acetic acid" by actual working
chemists, and "
vinegar" when
diluted sufficiently and sold in supermarkets.