destructive distillation

Destructive distillation is the process of pyrolysis conducted in a distillation apparatus (retort) to form the volatile products, which are collected. The process led to the discovery of many chemical compounds before such compounds could be prepared synthetically. In this way, the building blocks of many natural materials were deduced from the fragments generated from their thermal degradation. Destructive distillation is not a unit operation like distillation, but a set of chemical reactions. The process entails the "cracking" (breaking up) of macromolecules into smaller, more volatile, components. Many materials give a few products, most materials crack to give complex mixtures. Destructive distillation remains a viable route to many compounds, and it has emerged as a possible route for recycling of monomers derived from waste polymers.

A historically significant example of destructive distillation is tar making. Pinewood slices, which are rich in terpenes, are heated in an airless container causing the material to decompose. The by-products are volatile turpentine, leaving a residue of charcoal. This process is still used in Scandinavia for tar-making. Coal tar pitch volatiles (CTPV) are the result of destructive distillation of bituminous coal. These CTPVs often contain polynuclear aromatic hydrocarbons (PNA's), which sublime readily. Some products are useful, some are carcinogenic, many are both, such as pyrene, whose name embodies the process of its formation. Isoprene, the precursor to natural rubber as well as useful drugs and fragrances, was first isolated by destructive distillation of natural rubber.^[1]

Other examples of substances that are commonly destructively distilled to extract chemicals and other materials include:

• Wood to give methanol, terpenes, and guaiacol.
• Coking refers to the transformation of coal or heavy oil into coke, a porous carbon-rich material. Coking typically occurs at temperatures near 1,000 °C.
• Coal (lower grades) to give Coal gas and coal tar.

References

1. ^ C. Greville Williams “On Isoprene and Caoutchine” Proceedings of the Royal Society of London, Vol. 10, (1859 - 1860), pp. 516-519. URL: http://www.jstor.org/stable/111688.

See also

• Dry distillation
• Pyrolysis
• Thermolysis

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