Structural formula

A structural formula encodes the connectivity of atoms in a molecule, showing which atoms are bonded to which and the nature of those bonds (single, double, triple, or aromatic). Unlike a molecular formula such as C₂H₆O, which only gives atom counts, a structural formula distinguishes isomers like ethanol (CH₃CH₂OH) and dimethyl ether (CH₃OCH₃), which have identical molecular formulas but different properties.¹ This information is critical for understanding chemical behavior, as the arrangement of atoms determines functional groups and steric effects.

Structural formulas appear in several forms. Lewis structures show all valence electrons as dots or lines, while condensed formulas (e.g., CH₃CH₂OH) omit some bonds for brevity. Skeletal formulas, common in organic chemistry, represent carbon atoms as vertices and hydrogen atoms implicitly, with lines for bonds; heteroatoms are shown explicitly. Wedge-and-dash notation conveys three-dimensional geometry, indicating whether a bond points out of or into the plane.² Each convention balances detail with clarity, allowing chemists to choose the level of information appropriate for the context.

The concept of structural formulas emerged in the mid-19th century, driven by the work of chemists like August Kekulé, who proposed that carbon atoms can link to form chains, and Alexander Crum Brown, who introduced graphical representations of molecules. In 1861, the Russian chemist Alexander Butlerov used structural formulas to explain isomerism, and by the 1870s, such diagrams were widely adopted.³ The development was pivotal in establishing organic chemistry as a systematic science, enabling the prediction of new compounds and reactions.

Beyond simple connectivity, structural formulas can encode stereochemistry, such as E/Z isomerism in alkenes and R/S configuration at chiral centers, using specific notations. In inorganic chemistry, structural formulas often depict coordination geometries, like square planar or octahedral, which are not captured by simple line drawings. Additionally, computational chemistry uses graph-theoretic representations of structural formulas to calculate molecular descriptors, such as the Wiener index, which correlate with physical properties like boiling point.⁴ The development of linear notations like SMILES and InChI has translated structural formulas into machine-readable strings, enabling database searching and cheminformatics.

Molecular formula

A notation showing the number and type of atoms in a molecule, e.g., C2H6O.

Isomer

Compounds with the same molecular formula but different structural arrangements.

Skeletal formula

A simplified structural formula where carbon atoms are implied at line junctions and hydrogens are omitted.

SMILES

Simplified Molecular-Input Line-Entry System, a string notation for chemical structures.