Fuming sulfuric acid, or oleum, is produced by dissolving sulfur trioxide in concentrated sulfuric acid. It is used in the manufacture of explosives, drugs, and dyes. Oleum is a dehydrating agent and is used in nitration reactions and as a sulfonating agent. It is dangerous to work with due to its violent reaction with water and volatility.
Fuming sulfuric acid, more commonly called oleum, is obtained by dissolving sulfur trioxide (SO3) in concentrated sulfuric acid (H2SO4). This results in a mixture of compounds including sulfuric acid, disulfuric acid (H2S2O7) and free sulfur trioxide. Being volatile, sulfur trioxide causes acid to emit fumes as it absorbs moisture from the air, creating a cloud of tiny sulfuric acid droplets. Fuming sulfuric acid is produced by the normal industrial process used to make sulfuric acid, and most of it is converted into this chemical. A relatively small proportion, however, is preserved as oleum and used in the manufacture of explosives, drugs and dyes.
The industrial process used to produce sulfuric acid begins by oxidizing sulfur dioxide (SO2) to produce sulfur dioxide; this substance may react with water (H2O) to produce sulfuric acid. This reaction, however, is too violent to be easily controlled, and so, the sulfur trioxide is instead dissolved in the existing concentrated sulfuric acid to form oleum – fuming sulfuric acid. Most of this is then converted into sulfuric acid by carefully adding it to the appropriate volume of water. As long as the oleum is added to the water, rather than the other way around, the reaction, while exothermic, is controllable. The remaining oleum can be put to other industrial uses.
Oleum comes in different grades, depending on the amount of dissolved sulfur dioxide. Containers will indicate grade by indicating the proportion of free sulfur dioxide, such as 20%, 30% or 65%. Oleum is normally a steaming, oily liquid, but some forms are solid at room temperature.
Sulfur dioxide reacts with sulfuric acid to produce disulfuric acid, also called pyrosulfuric acid: SO3 + H2SO4 → H2S2O7. Pure disulfuric acid is solid at room temperature, but is rarely used either industrially or in the laboratory. In oleum it exists together with sulfuric acid, free sulfur trioxide and perhaps some more complex molecules.
Fuming sulfuric acid is an even more powerful dehydrating agent than sulfuric acid. Reacts very violently with water, releasing much heat and acid splashes, unless added slowly to water. Like sulfuric acid, it removes water from carbohydrates, leaving carbon, so it chars paper, wood and many other organic materials. This reaction releases so much heat that it can cause combustion.
One major application of fuming sulfuric acid is in nitration reactions. It is mixed with nitric acid to produce nitronium ions (NO2+) which add nitro groups (NO2) to organic compounds. For many of these reactions it is essential that no water is present. This is usually achieved by mixing commercial nitric acid — 68.5% — with fuming sulfuric acid so that the latter absorbs all the water. Nitration reactions are important in the production of explosives and dyes.
Oleum is also used as a sulfonating agent in organic chemistry. This means that it can add a sulfonic acid group (SO3H) to an organic compound. Sulfonated compounds include important drugs such as sulfonamides as well as detergents and dyes.
Because of its dehydrating properties, violent reaction with water, and volatility, fuming sulfuric acid is a very dangerous chemical to work with. It causes severe burns on contact with the skin and inhaling the fumes can cause serious damage to the respiratory system. In laboratories, experiments using this acid are normally performed inside a hood.
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