184 Applications and Occurrences of Heterocycles in Everyday Life
Picloram is a selective systemic herbicide, which kills broad-leaved weeds but not grasses. Paraquat is a non-selective
very fast-acting contact herbicide that is also highly toxic to humans. It has the advantage that it is deactivated on con-
tact with soil, which reduces collateral damage. Its mechanism is inhibition of photosynthesis, by generation of reactive
oxygen species, which damage porphins. It is one of the most widely used herbicides throughout the world, but banned
in the EU and restricted in the USA. The related diquat is less toxic.
Fungicides
Agricultural fungicides are often triazoles, such as hexaconazole, similar to those used in medicine, or benzimidazoles,
such as carbendazim. There are also some more chemically interesting heterocycles, such as tricyclazole.
Rodenticides
The most well-known rodenticide is warfarin, the action of which against rats is the same as for its medicinal use in
humans, that is, inhibition of blood coagulation. The development of resistance to its effects (in rats!) led to the devel-
opment of much more potent second-generation analogues such as fl ocoumafen and bromadiolone.
Explosives
Many compounds are explosive but relatively few of them are suitable as practical explosives. The most important
requirements are that they are stable and not easy to initiate accidentally! Friction, static electricity and impact are the
main dangers.
Much research has been carried out to develop ‘insensitive explosives’, which are almost impossible to detonate without
a proper detonator – the least sensitive will burn quietly and not react even if a bullet is fi red into them.
Commercial explosives are often mixtures, designed to have perfect oxygen balance or modifi ed properties for specifi c
purposes, such as quarry blasting, munitions or propellants (for cartridges or rockets). Solid rocket fuels are basically
explosives, modifi ed so that burning does not progress to explosion.