Diethyl ether, or simply ether, is an organic compound in the ether class with the formula C4H10O, (CH3CH2)2O or (C2H5)2O, sometimes abbreviated as Et2O.[a] It is a colourless, highly volatile, sweet-smelling ("ethereal odour"), extremely flammable liquid. It is commonly used as a solvent in laboratories and as a starting fluid for some engines. It was formerly used as a general anesthetic, until non-flammable drugs were developed, such as halothane. It has been used as a recreational drug to cause intoxication. Most diethyl ether is produced as a byproduct of the vapor-phase hydration of ethylene to make ethanol. This process uses solid-supported phosphoric acid catalysts and can be adjusted to make more ether if the need arises.[8] Vapor-phase dehydration of ethanol over some alumina catalysts can give diethyl ether yields of up to 95%.[9] Diethyl ether can be prepared both in laboratories and on an industrial scale by the acid ether synthesis.[10] Ethanol is mixed with a strong acid, typically sulfuric acid, H2SO4. The acid dissociates in the aqueous environment producing hydronium ions, [H3O]+. A hydronium ion protonates the electronegative oxygen atom of the ethanol, giving the ethanol molecule a positive charge: A nucleophilic oxygen atom of unprotonated ethanol displaces a water molecule from the protonated (electrophilic) ethanol molecule, reforming the hydronium ion and yielding diethyl ether. This reaction must be carried out at temperatures lower than 150 °C in order to ensure that an elimination product (ethylene) is not a product of the reaction. At higher temperatures, ethanol will dehydrate to form ethylene. The reaction to make diethyl ether is reversible, so eventually an equilibrium between reactants and products is achieved. Getting a good yield of ether requires that ether be distilled out of the reaction mixture to decrease reverse reaction rate, taking advantage of Le Chatelier's principle.[citation needed] Another reaction that can be used for the preparation of ethers is the Williamson ether synthesis, in which an alkoxide (produced by dissolving an alkali metal in the alcohol to be used) performs a nucleophilic substitution upon an alkyl halide.[citation needed] Uses[edit] It is particularly important as a solvent in the production of cellulose plastics such as cellulose acetate.[8] Diethyl ether has a high cetane number of 85–96 and, in combination with petroleum distillates for gasoline and diesel engines, [11] is used as a starting fluid because of its high volatility and low flash point. Ether starting fluid is sold and used in countries with cold climates, as it can help with cold starting an engine at sub-zero temperatures. For the same reason it is also used as a component of the fuel mixture for carbureted compression ignition model engines. In this way diethyl ether is very similar to one of its precursors, ethanol.[citation needed] Since ether is heavier than air it can collect low to the ground and the vapour may travel considerable distances to ignition sources. Ether will ignite if exposed to an open flame, though due to its high flammability, an open flame is not required for ignition. Other possible ignition sources include – but are not limited to – hot plates, steam pipes, heaters, and electrical arcs created by switches or outlets.