Unit VI alkanes, alkenes, alkynes Alkanes

A chain of carbon atoms that contains the maximum number of hydrogen atoms has the general formula C_nH_2n+2 (n = number of carbon atoms). This type of hydrocarbon is called an alkane (a historical name is paraffin). Alkanes are saturated hydrocarbons, they consist only of carbon and hydrogen atoms and all bonds are single bonds. The simplest possible alkane is methane CH₄. Alkanes can exist in straight chains, chains with branching, and cycles. The carbon atom in an alkane forms four single bonds that are equidistant from each other. The shape is referred to as tetrahedral. There is no limit to the number of carbon atoms that can be linked together, the only limitation being that the molecule is acyclic, is saturated and is a hydrocarbon. When the carbon atoms of an alkane are connected in a single continuous sequence, the alkane is known as a normal hydrocarbon. These alkanes are sometimes called linear or straight-chain hydrocarbons, but the names are misleading. Really, the carbon chains are kinked, twisted and are linear only in symbolic representations. A succession of normal hydrocarbons which differ by one methylene (-CH₂-) group is known as homologous series. Alkanes belong to a homologous series of organic compounds in which the members differ by a molecular mass. Alkanes with more than three carbon atoms can be arranged in various different ways, forming structural isomers. The simplest isomer of an alkane is the one in which the carbon atoms are arranged in a single chain with no branches. This isomer is sometimes called the n-isomer (n for "normal", although it is not necessarily the most common). However the chain of carbon atoms may also be branched at one or more points. The number of possible isomers increases rapidly with the number of carbon atoms.

The two major sources of alkanes are natural gas and petroleum. Natural gas consists of approximately 90 to 95% methane, 5 to 10% ethane, and a mixture of other relatively low-boiling alkanes (propane, butane and 2-methylpropane). Petroleum is a thick, viscous, liquid mixture of thousands of compounds, most of them hydrocarbons, formed by the decomposition of marine plants and animals.

Knowledge about the names of alkanes is extremely important because the names of alkanes form the basis for the names of many other types of organic compounds. The trivial (non-systematic) name for alkanes is paraffins. Together, alkanes are known as the paraffin series. A system of nomenclature for organic compounds has been developed by the International Union of Pure and Applied Chemistry (IUPAC). The IUPAC name for an alkane consists of two parts: a prefix that shows the number of carbon atoms and the suffix "ane". The names of the first ten alkanes are: CH₄ - methane, CH₃CH₃ - ethane, CH3CH₂CH₃ - propane, CH₃CH₂CH₂CH₃ - butane, C₅H_I2 - pentane, C₆H_I4 - hexane, C₇H₁₆ - heptane, C₈H_l8 - octane, C₉H₂₀ - nonane, C₁₀H₂₂ - decane.

Physical Properties. The first four alkanes (C₁ to C₄) are all gases at room temperature. Natural gas is mostly methane. Compounds from C₅ to C₁₇ are oily liquids. Petroleum contains a wide variety of alkanes. Natural gas and petroleum alkanes are used mainly for fuels. Compounds greater than C₁₇ are solids at room temperature and pressure. They are nonpolar compounds. The only interactions between their molecules are the very week London dispersion forces. They have the low boiling points and the low melting points. As the number of carbon atoms increases, the molecules become larger and heavier. This means that their melting points and boiling points increase. In general, both boiling and melting points of alkanes decrease with increasing the number of branches (for alkanes with the same molecular weights). Alkanes are insoluble in water, but they are soluble in non-polar solvents such as toluene and tetrachloromethane. Their density usually increases with increasing number of carbon atoms but is less than that of water. The melting points of branched-chain alkanes can be either higher or lower than those of the corresponding straight-chain alkanes, again depending on the ability of the alkane in question to pack well in the solid phase. Alkanes do not conduct electricity, nor are they substantially polarized by an electric field. All alkanes are colourless and odourless.

Chemical properties. Alkanes are relatively inert. Most reactions require specialized catalysts for breaking the carbon-carbon bonds. Although they are relatively unreactive, alkanes do undergo two important kinds of reactions: reactions with oxygen (combustion) and substitution of hydrogen atoms with chlorine and other halogens (halogenation). Burning, oxidation of alkanes, is a very important reaction in using alkanes as fuel. The fuels used in engines require additives to improve the combustibility for smooth and efficient burning. In burning, the alkanes react with the oxygen in air to produce carbon dioxide and water. Methane and other alkanes react with chlorine in sunlight or at high temperature in a substitution reaction. The reaction of an alkane with other halogens (such as bromine) results in a range of substitution products called haloalkanes. Alkanes are useful as solvents for a wide range of organic substances.