cyclooctatetraenyl derivatives, such as uranocene. The large and very weakly polarising alkali metal cations can stabilise large, aromatic, polarisable radical anions, such as the dark-green sodium naphthalenide, Na+[C10H8•]−, a strong reducing agent. Representative reactions of alkali metals Reaction with oxygen Upon reacting with oxygen, alkali metals form oxides, peroxides, superoxides and suboxides. However, the first three are more common. The table below shows the types of compounds formed in reaction with oxygen. The compound in brackets represents the minor product of combustion. The alkali metal peroxides are ionic compounds that are unstable in water. The peroxide anion is weakly bound to the cation, and it is hydrolysed, forming stronger covalent bonds. Na2O2 + 2H2O → 2NaOH + H2O2 The other oxygen compounds are also unstable in water. 2KO2 + 2H2O → 2KOH + H2O2 + O2 Li2O + H2O → 2LiOH Reaction with sulfur With sulfur, they form sulfides and polysulfides. 2Na + 1/8S8 → Na2S + 1/8S8 → Na2S2...Na2S7 Because alkali metal sulfides are essentially salts of a weak acid and a strong base, they form basic solutions. S2- + H2O → HS− + HO− HS− + H2O → H2S + HO− Reaction with nitrogen Lithium is the only metal that combines directly with nitrogen at room temperature. 3Li + 1/3N2 → Li3N Li3N can react with water to liberate ammonia. Li3N + 3H2O → 3LiOH + NH3 Reaction with hydrogen With hydrogen, alkali metals form saline hydrides that hydrolyse in water. Na + H2 → NaH (at high temperatures) NaH + H2O → NaOH + H2 Reaction with carbon Lithium is the only metal that reacts directly with carbon to give dilithium acetylide. Na and K can react with acetylene to give acetylides. 2Li + 2C → Li2C2 Na + C2H2 → NaC2H + 1/2H2 (at 1500C) Na + NaC2H → Na2C2 (at 2200C) Reaction with water On reaction with water, they generate hydroxide ions and hydrogen gas. This reaction is vigorous and highly exothermic and the hydrogen resulted may ignite in air or even explode in the case of Rb and Cs. Na + H2O → NaOH + 1/2H2 Reaction with other salts The alkali metals are very good reducing agents. They can reduce metal cations that are less electropositive. Titanium is produced industrially by the reduction of titanium tetrachloride with Na at 4000C (van Arkel–de Boer process). TiCl4 + 4Na → 4NaCl + Ti Reaction with