triple bond, the substituted acetylenes difluoroacetylene, dichloroacetylene, dibromoacetylene, and diiodoacetylene cannot be made directly from acetylene. A common workaround is to dehydrate vinyl dihaloethenols. Carbonylation Walter Reppe discovered that in the presence of catalysts, acetylene reacts to give a wide range of industrially significant chemicals. With carbon monoxide, acetylene reacts to give acrylic acid, or acrylic esters, which can be used to produce acrylic glass. Organometallic chemistry Acetylene and its derivatives (2-butyne, diphenylacetylene, etc.) form complexes with transition metals. Its bonding to the metal is somewhat similar to that of ethylene complexes. These complexes are intermediates in many catalytic reactions such as alkyne trimerisation to benzene, tetramerization to cyclooctatetraene, and carbonylation to hydroquinone: Fe(CO)5 + 4 C2H2 + 2 H2O -> 2 C6H4(OH)2 + FeCO3 at basic conditions (50–, 20–). In the presence of certain transition metals, alkynes undergo alkyne metathesis. Metal acetylides, species of the formula , are also common. Copper(I) acetylide and silver acetylide can be formed in aqueous solutions with ease due to a poor solubility equilibrium. Acid-base reactions Acetylene has a pKa of 25, acetylene can be deprotonated by a superbase to form an acetylide: HC#CH + RM -> RH + HC#CM Various organometallic and inorganic reagents are effective. Safety and handling Acetylene is not especially toxic, but when generated from calcium carbide, it can contain toxic impurities such as traces of phosphine and arsine, which give it a distinct garlic-like smell. It is also highly flammable, as are most light hydrocarbons, hence its use in welding. Its most singular hazard is associated with its intrinsic instability, especially when it is pressurized: under certain conditions acetylene can react in an exothermic addition-type reaction to form a number of products, typically benzene and/or vinylacetylene, possibly in addition to carbon and hydrogen. Consequently, acetylene, if initiated by intense heat or a shockwave, can decompose explosively if the absolute pressure of the gas exceeds about . Most regulators and pressure gauges on equipment report gauge pressure, and the safe limit for acetylene therefore is 101 kPagage, or 15 psig. It is therefore supplied and stored dissolved in acetone or dimethylformamide (DMF), contained in a gas cylinder with a porous filling (Agamassan), which renders it safe to transport and use, given proper handling. Acetylene cylinders should be used in the upright position to avoid withdrawing acetone during use. Information on safe storage of acetylene in upright cylinders is provided by