Oleum

Oleum (Latin oleum = "oil"), or fuming sulfuric acid refers to a solution various compositions of sulfur trioxide in sulfuric acid or sometimes more specifically to pyrosulfuric acid, disulfuric acid.

Oleums can be described by the formula xSO3.H2O where x is the total molar sulfur trioxide content. The value of x can be varied, to include different oleums. They can also be described by the formula H2SO4.xSO3 where x is now defined as the molar free sulfur trioxide content. Oleum is generally assayed according to the free SO3 content by weight.

A value for x of 1 gives us the empirical formula H2S2O7 for pyrosulfuric acid also called disulfuric acid. Pure pyrosulfuric acid itself is a solid at room temperature, melting at 36 °C and rarely used either in the laboratory or industrial processes.

Production
Oleum is produced in the contact process. Industrially, the biggest producer of oleum came from the distillation of iron sulfates at Nordhausen, from which the historical name Nordhausen sulfuric acid is derived.

The lead chamber process for sulfuric acid production was notable, because it could not produce sulfur trioxide or even concentrated sulfuric acid directly due to corrosion of the lead, and absorption of NO2 gas. Until this process was made obsolete by the contact process, oleum had to be obtained through multiple processes.

Sulfuric acid production
Oleum is an important intermediate in the manufacture of sulfuric acid, due to its high enthalpy of hydration. When SO3 is added to water, rather than simply dissolving, it tends to form a fine mist of sulfuric acid, which is difficult to manage. However, SO3 added to concentrated sulfuric acid readily dissolves, forming oleum which can then be diluted with water to produce additional concentrated sulfuric acid.

As an intermediate for transportation
Oleum is a useful form for transporting sulfuric acid compounds, typically in rail tankcars, between oil refineries (which produce various sulfur compounds as a byproduct of refining) and industrial consumers.

Certain compositions of oleum are solid at room temperature, and thus is safer to ship than when liquid. Solid oleum can then be converted into liquid at the destination through steam heating or dilution or concentration. This requires some care to prevent overheating and evaporation of sulfur trioxide. To extract it from a tank car requires careful heating using steam conduits within the tank car. Great care must be taken to avoid overheating, as this can increase the internal pressure within the tank car to a value exceeding the limit of the tank's safety valve. (In Richmond, California in 1993 a significant release occurred due to overheating, causing a release of sulfur trioxide that absorbed moisture from the atmosphere, creating a mist of micron-sized sulfuric acid particles that formed an inhalation health hazard . This mist produced adverse health effects on residents and workers over a wide area. )

Organic chemistry
Oleum is a harsh reagent, and is highly corrosive. One important use of oleum as a reagent is the secondary nitration of nitrobenzene. The first nitration can occur with nitric acid or sulfuric acid, but this deactivates the ring towards further electrophilic substitution. A stronger reagent, oleum, is needed to introduce the second nitro group onto the aromatic ring.

Oleum can also be used in the preparation of trinitrotoluene where it can oxidize the dinitrotoluene ring and promote the substitution of a third nitro group.

Reactions
Like concentrated sulfuric acid, oleum is such a strong dehydrating agent that if poured onto powdered glucose, or virtually any other sugar, it will draw the elements of water out of the glucose in an exothermic reaction, leaving nearly pure carbon as a solid. This carbon expands outward, hardening as a solid black substance with gas bubbles in it.