an alkane. To do so, the best-known methods are hydrogenation of alkenes: (R = alkyl) Alkanes or alkyl groups can also be prepared directly from alkyl halides in the Corey–House–Posner–Whitesides reaction. The Barton–McCombie deoxygenation removes hydroxyl groups from alcohols e.g. and the Clemmensen reduction removes carbonyl groups from aldehydes and ketones to form alkanes or alkyl-substituted compounds e.g.: Preparation from other organic compounds Alkanes can be prepared from a variety of organic compounds. These include alkenes, alkynes, haloalkanes, alcohols, aldehydes, ketones and carboxylic acids. From alkenes and alkynes Addition of molecular hydrogen across the π bond(s) of alkenes and alkynes gives alkanes. This hydrogenation reaction is typically performed using a powdered metal catalyst, such as palladium, platinum, or nickel. The reaction is exothermic because the product alkane is more stable. This is an important process in several fields of industrial and research chemistry. From haloalkanes Several methods produce alkanes from haloalkanes. In the Wurtz reaction, a haloalkane is treated with sodium in dry ether to yield an alkane having double the number of carbon atoms. This reaction proceeds through a free radical intermediate and has the possibility of alkene formation in case of tertiary haloalkanes and vicinal dihalides. 2 R−X + 2 Na → R−R + 2 Na+X In Corey–House synthesis, a haloalkane is treated with dialkyl lithium cuprate, a Gilman reagent, to yield a higher alkane: Li+[R–Cu–R]– + R'–X → R–R' + R–Cu + Li+X Haloalkanes can be reduced to alkanes by reaction with hydride reagents such as lithium aluminium hydride. R−X + H– → R−H + X– Applications The applications of alkanes depend on the number of carbon atoms. The first four alkanes are used mainly for heating and cooking purposes, and in some countries for electricity generation. Methane and ethane are the main components of natural gas; they are normally stored as gases under pressure. It is, however, easier to transport them as liquids: This requires both compression and cooling of the gas. Propane and butane are gases at atmospheric pressure that can be liquefied at fairly low pressures and are commonly known as liquified petroleum gas (LPG). Propane is used in propane gas burners and as a fuel for road vehicles, butane in space heaters and disposable cigarette lighters. Both are used as propellants in aerosol sprays. From pentane to octane the alkanes are highly volatile liquids. They are used as fuels in internal combustion engines,