The alkanes (i.e. the paraffins) are the simplest homologous series of organic compounds of hydrogen and carbon, where all atoms are linked by single bonds.
The general formula for the alkane series of hydrocarbons is CnH2n+2. The alkanes are said to be saturated because the maximum number of bonds are formed between each carbon atoms and its neighboring carbon and hydrogen atoms. The lack of any multiple bonds (i.e. double-bonds or triple-bonds) in the alkanes explain the relative chemical inertness of this series of hydrocarbons.
Methane CH4 CH4 Ethane C2H6 CH3CH3 Propane C3H8 CH3CH2CH3 Butane C4H10 CH3CH2CH2CH3 Pentane C5H12 CH3CH2CH2CH2CH3 Heptane C6H14 CH3CH2CH2CH2CH2CH3 Hexane C7H16 CH3CH2CH2CH2CH2CH2CH3 Octane C8H18 CH3CH2CH2CH2CH2CH2CH2CH3 Nonane C9H20 CH3CH2CH2CH2CH2CH2CH2CH2CH3 Decane C10H22 CH3CH2CH2CH2CH2CH2CH2CH2CH2CH3The shapes of the alkanes shows that all bonds on the carbon atoms are identical and that the bond angles are close to 109 degrees. Thus, each carbon atom is at the center of a tetrahedral structure, with either carbon atoms or hydrogen atoms at the apices of the tetrahedron. All bonds between carbon atoms (i.e. C-C bonds), or between a carbon atom and hydrogen (i.e. C-H bonds), are s bonds (sigma bonds). These bonds are formed by the end-on overlap of sp3 hydride orbitals of the carbon atoms.
The alkane with the simplest structure is methane, CH4. Each succeeding member of the alkane series has a further methylene group, -CH2-, in the chain. The methane molecule consists of a central carbon atom, surrounded by four hydrogen atoms, with a tetrahedral shape. This tetrahedral structure of methane indicates that the geometry of the orbitals of the carbon in methane molecule, on which the geometry of the bonds depend, is significantly different from the structure of the orbitals in the carbon atom. This is due to hybridisation of four atomic orbitals in carbon atom to four identical molecular orbitals in methane molecule. These molecular orbitals are called SP3 Hybrid Orbitals. The single covalent bonds joining carbon to carbon and carbon to hydrogen in alkanes are sigma bonds formed by the end-on overlap of electron orbitals, so that each carbon atom is the center of a tetrahedron. There is free rotation about all bonds in alkanes.
Methane CH4 CH4 Ethane C2H6 CH3CH3 Propane C3H8 CH3CH2CH3 Butane C4H10 CH3CH2CH2CH3 Pentane C5H12 CH3CH2CH2CH2CH3 Heptane C6H14 CH3CH2CH2CH2CH2CH3 Hexane C7H16 CH3CH2CH2CH2CH2CH2CH3 Octane C8H18 CH3CH2CH2CH2CH2CH2CH2CH3 Nonane C9H20 CH3CH2CH2CH2CH2CH2CH2CH2CH3 Decane C10H22 CH3CH2CH2CH2CH2CH2CH2CH2CH2CH3
Name State Formula MP degC BP degC Density(g/ml) ==== ===== ======= ======= ======= ======= Methane Gas CH4 -183 -162 0.42 Ethane Gas C2H6 -172 -89 0.55 Propane Gas C3H8 -190 -45 0.58 Butane Gas C4H10 -135 -0.5 0.58 Pentane Liquid C5H12 -130 36 0.63 Hexane Liquid C6H14 - 95 69 0.66 Heptane Liquid C7H16 - 91 98 0.68
Structural isomerism results from the fact that for a given formula, the carbon and hydrogen atoms can be arranged in a number of different ways while retaining the normal combining power of each element. For example, in the case of butane, C4H20, the atoms can be arranged in two different ways, yielding n-butane and iso-butane.
Because carbon has a valency of four, and because the carbon to carbon and the carbon to hydrogen covalent bonds are almost identical chemically, it is possible to arrange the linkages between the atoms in an alkane in many different ways. This leads to the concept of structural isomerism, where the isomers have identical formulae but different structural arrangements. Only one arrangement of the atoms within the molecule is possible for methane, CH4, ethane, CH3CH3, and propane, CH3CH2CH3. However, starting with butane, CH3CH2CH2CH3, alternative arrangements of the atoms is possible. The prefix n- (for normal) and iso- (for isomeric) is affixed to the name to distinguish between the isomers.
n-Butane C4H20 CH3CH2CH2CH3 Iso-Butane C4H20 CH3CHCH3 CH3
The number of possible isomers increases rapidly as the number of carbons increases. Pentane has 3 isomers, and were known as
n-Pentane C5H22 CH3CH2CH2CH2CH3 (Structure I) iso-Pentane C5H22 CH3CHCH2CH3 CH3 (Structure II) CH3 neo-Pentane C5H22 CH3CCH3 CH3 (Structure III)
Structure I is an unbranched chain alkane. Structures II and III are branched chain alkanes. Because the number of structural isomers increases rapidly with increasing number of carbon atoms in a molecule, a systematic method of naming the organic compounds has been promulgated by the International Union of Pure and Applied Chemistry, IUPAC.
In structure I, the longest continuous chain contains four carbon atoms and thus this compound is a pentane.
In structure II, the longest continuous chain contains four carbon atoms and thus this compound is a butane. As this also has a methyl group attached to the second carbon atom; it is therefore called 2-Methylbutane.
In structure III, the longest continuous chain contains three carbon atoms and thus this compound is a propane. As this also has two methyl group attached to the second carbon atom; it is therefore called 2,2-Dimethylpropane. Note that in this molecule, the central carbon atom is bonded to four carbon atoms, and the twelve hydrogen atoms are attached to the other four carbon atoms in the molecule.
Old Name Structure IUPAC Name ======== ========= ========== n-Pentane CH3CH2CH2CH2CH3 Pentane iso-Pentane CH3CHCH2CH3 2- methylbutane CH3 CH3 neo-Pentane CH3CCH3 2,2-dimethylpropane CH3