Hydrocyanic acid (hydrogen cyanide) is an important synthetic unit in organic chemistry and is thus justifiably placed in the series of the C₁ basic products. The two processes below are suitable for the direct manufacture of hydrocyanic acid:
1. Dehydration of
2. Oxidative or dehydrogenative reaction of NH₃ with hydro-carbons, preferably with methane.

Hydrocyanic acid is also obtained to a large extent as a byproduct in the manufacture of acrylonitrile via the ammoxidation of propene.

The ratio of synthetic to byproduct hydrocyanic acid differs greatly from country to country; however, there is an overall tendency toward making the hydrocyanic acid supply more independent of the market for acrylonitrile by specific manufacture of hydrocyanic acid.

In 1995, the production capacity for synthetic and byproduct hydrocyanic acid in the USA, in Western Europe and in Japan was about 800000, 510000, and 90000 tonnes per year, respectively. The fraction of this which was byproduct is indicated in the adjacent table.

To 1:

The dehydration of formamide is conducted in iron catalyst tubes at 380-430 °C under reduced pressure using modern vacuum processes. The tubes are filled with Fe or Al phosphate catalyst, which contains Mg, Ca, Z or Mn as promoters:

The reaction gas, with its high HCN content of 60-70 vol%, is suitable for direct liquefaction. The selectivity to HCN is 92-95%. Formamide processes were developed by BASF, Degussa and Knapsack, but today only BASF still operates a (21 000 tonnes per year, 1991) unit.

To 2:

Synthesis components other than ammonia include methane (Andrussow and Degussa processes) and higher alkanes (Shawinigan, now Gulf Oil, process). The Andrussow technology-originally developed by BASF - is currently preferred for the manufacture of HCN. In principle, it is an ammoxidation of methane:

The catalyst is usually platinum, either as a gauze or on a support, with additives such as rhodium. The reaction takes place at atmospheric pressure and 1000- 1200 °C with a very short residence time. The reaction gas is rapidly quenched in order to avoid decomposition of HCN. After an acid wash, pure HCN is obtained by distillation from the diluted aqueous solution. Selectivity to HCN reaches about 88% (CH₄) and 90% (NH₃).

Numerous variations of the Andrussow process have been developed including those by American Cyanamid, Du Pont, Goodrich, ICl, Mitsubishi, Monsanto, Montecatini, Nippon Soda, and Rohm & Haas.

and formaldehyde have also been investigated for the manufacture of HCN by, e.g., Sumitomo. The ammoxidation of methanol with Mo-Bi-P-oxide catalyst at 460°C has a selectivity to HCN of 84% (CH₃OH).

In the Degussa BMA (Blausaure-Methan-Ammoniak, or hydrocyanic acid-methane-ammonia) process, CH₄ and NH₃ are reacted without the addition of air or O₂:

Sintered corundum tubes with a layer of Pt, Ru, or Al serve as catalyst. HCN selectivities of 90- 91 % (CH₄) and 83-84% (NH₃) are reached at 1200-1300°C. The reaction of NH₃ is almost complete, and the small amount remaining is removed from the HCN/H₂ mixture with H₂SO₄^.HCN is separated by absorption in water. Production plants are being operated by Degussa (Germany, Belgium, USA) and Lonza (Switzerland).

In the Shawinigan process, hydrocarbons from CH₄ to light petrol, e.g., propane, are reacted with NH₃ at 1300-1600°C in a fluidized bed of fine coke:

The fluidized bed is electrically heated by immersed graphite electrodes. Because of the high energy consumption, this process only operates economically in locations with a cheap source of electrical energy. With propane, a HCN selectivity of about 87% (C₃H₈) is attained.

This process was operated by Gulf Oil in Canada for several years, and there is still a commercial plant in South Africa.

Hydrocyanic acid is also produced as a byproduct in the ammoxidation of propene to acrylonitrile, as for example in the extensively applied Sohio process.

Depending on the type of process, about 10-24 wt % of HCN is obtained relative to acrylonitrile. Improved catalysts (e.g., Sohio catalyst 41) lead, however, to a noticeable decrease in HCN formation. Although hydrocyanic acid is then available as an inexpensive byproduct, the supply is too inflexible when coupled with the production of, and therefore the fluctuations in demand for, acrylonitrile.