The alkanes are the hydrocarbons where all of the carbons are bonded to the other carbon atoms by single bonds. Alkanes may exist in a chain structure (either straight or branched) or in a ring structure. Alkanes that are acyclic (not in a ring) possess an empirical formula of CnH(2n+2). Alkanes that are cyclic possess an empirical formula of CnH2n. In discussing the alkanes as a functional group, the terms aliphatic (for the acyclic alkane substituent) and alicyclic (for the cyclic alkane substituent) are used. The alkanes were originally called paraffins, which came from the Latin phrase parum affinis, which means little affinity. This phrase was used due to the low reactivity of these hydrocarbons. When the alkanes do react the reactions are usually quite vigorous.
Alkanes are denoted by the ending -ane in their name. The simplest example is methane, CH4. The first four members of the straight-chained group--methane, ethane (CH3CH3), propane (CH3CH2CH3), and butane (CH3CH2CH2CH 3)--are all gases. The next members are all liquids, and from C16H34 on they are all solids. These states of matter are at standard temperature and pressure. The alkanes are very poorly soluble in water but are soluble with many of the other organic solvents such as chloroform and benzene.
Alkanes are produced by the reduction of alkenes, where hydrogen adds across to form a saturated double bond, or by reacting alkyl iodides with sodium in a solution of ether (known as the Wurtz reaction).
Alkanes burn readily in air to give carbon dioxide and water. If there is a restricted amount of oxygen only partial combustion takes place and carbon monoxide and carbon are produced along with the carbon dioxide and water. Some of the halogens will react with alkanes.Chlorine and bromine react quite readily, usually under free radical substitution conditions (in sunlight or in the presence of an iron catalyst).Iodine reacts to a much lesser extent with the alkanes due to the weak stability of the carbon-iodine bond. If methane is reacted with chlorine in strong sunlight the reaction is explosive and carbon and hydrochloric acid are the products. If the sunlight is less powerful a series of reactions takes place with the gradual replacement of the hydrogen molecules by chlorine. The products of this reaction are chloromethane (methyl chloride, CH3Cl), dichloromethane (methylenedichloride, CH2Cl2), trichloromethane (chloroform, CHCl3), and tetrachloromethane (carbon tetrachloride, CCl4). This is an example of a substitution reaction. Alkanes can be reacted with nitric acid in the vapor phase at 932°F (500°C) to yield nitroalkane. By careful use of appropriate conditions and catalysts alkanes can be converted into other hydrocarbons such as the alkenes and alkynes.
The majority of alkanes are used as fuels, as solvents, and as starting products for the production of other hydrocarbons. Methane reacts with steam (gaseous water) at 1650°F (900°C) over a nickel catalyst to produce carbon monoxide and hydrogen. This mixture is known as synthesis gas. Synthesis gas is used to manufacture methanal (formaldehyde, CH2=O), and also methanol (CH3OH). The reverse process can be carried out where synthesis gas is reacted with a catalyst to give high molecular weight hydrocarbons. This is a process known as the Fischer Tropsch reaction. Ethane and propane, as well as being fuels, are important precursors in the manufacture of ethene (acetylene) which is then further converted to a range of products. Higher alkanes are chiefly used as fuels or precursors of other hydrocarbons.
Isomers of alkanes exist depending on the arrangement and joining of the carbon atoms. For example C5H12 is pentane when all five carbon atoms are arranged in a straight line. If the arrangement of the molecule is such that there is a backbone of four carbon atoms then a methyl group can be added to one of these carbon atoms. If the methyl group were added to the second carbon atom in the chain then the resultant compound would be 2 methylbutane. These isomers differ in their physical properties, in the example given pentane has a melting point of 202°F (130°C) and a boiling point of 97°F (36°C), whereas the isomeric molecule 2 methylbutane has a melting point of 256°F (160°C) and a boiling point of 82°F (28°C). These differences are noticed in all of the other isomers within the series.
The alkanes are a simple group of hydrocarbons. They are found in petroleum and are most frequently used either as fuels or as precursors in the manufacture of other hydrocarbons.
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