From apples to ylang-ylang, the range of scents and flavours that fill Nature’s vast garden of plants are formed chiefly from specific combinations of volatile organic compounds, known collectively as essential oils. For decades the tools used in chemistry were deemed ill-equipped to untangle these complex and unstable mixtures, until Otto Wallach devised ways in which to reveal the essential virtues of plants.

Through a carefully controlled series of reactions with common laboratory reagents (such as hydrogen chloride and hydrogen bromide), Wallach successfully characterised the key components from a wide range of essential oils and deduced their structures. In contrast to the widely held notion that the active components of essential oils from different plant species must be chemically different from one another, Wallach found that many of these substances are remarkably similar, belonging to a class of compounds that he called terpenes. These compounds are constructed from stitching various numbers of a basic chemical unit together; the unit in question sharing its structure with the compound isoprene, which is used in the production of synthetic rubber.

The ease with which different terpenes can transform into each other made them especially tricky and fragile to study, but Wallach’s methods for isolating pure essential oils from natural plant mixtures opened up a new field of research in organic chemistry. Chemists could now characterise increasing numbers of terpenes and closely related chemicals, which are all members of a group known as alicyclic compounds. Wallach’s pioneering work also had a tremendous impact on the chemical industry. Traditionally crude extracts of plant essential oils were used to create a wide assortment of medicines and perfumes. Armed with Wallach’s methods, chemists could begin to isolate active plant essential oils of medicinal or chemical interest, allowing them to manufacture synthetic, cheaper versions.