Science and Technology for UPSC Examinations [Chemistry] Part 15- Alkanes

Chain compounds in which all carbon-to-carbon bonds are only simple single bonds are called ALKANES. These compounds are also called saturated hydrocarbons, because each carbon-to-carbon bond is a single bond, and the valence of the carbon atom is, therefore, saturated.

No more atoms can be bonded to the atoms in the compound, without breaking the compound into two or more fragments. If it contains one or more bonds which can react with hydrogen it is called an unsaturated hydrocarbon. Almost all other organic compounds can be named as derivatives of these simple hydrocarbons. Alkanes which have long carbon chains are often called paraffins in chemical industry. The simplest alkane is methane with the formula CH₄. The second alkane is ethane with the formula C₂H₆. The general formula of alkanes is C_nH_2n+2.

Methane

The simplest alkane is the gas methane, whose molecular formula is CH4. Methane exists as a tetrahedral shape, but it is often represented by a flattened structure as are most organic compounds.

Flattened structures for the three simplest alkanes are given in the adjacent figure. In many cases the structures can be further simplified without loss of information by omitting all single bonds and writing the letter symbol of the element close to the letter symbol of the element to which it is attached.

Thus the representation of methane as CH4, ethane as H3CCH3 (rather than as C2H6), and propane as H3CCH2CH3 (rather than as C3H8) is a representation of structure as well as of molecular composition.

The alkanes above propane are named by giving the number of carbons (in Greek) with the ending -ane added. If an alkane is not a straight chain, then the longest straight chain in it is used as the basis of the name and the shorter side chains are considered to be substituents; thus names such as methylpropane and methylbutane are derived.

Saturated hydrocarbons occur in three forms: straight-chain forms , branched chain forms and cyclic forms which are known as cycloalkanes. The cycloalkanes contain only single bonds, and have the general formula C_nH_2n. Cyclomethane and cycloethane obviously cannot exist, but cyclopropane can; it is a triangular stable structure, though somewhat reactive because the bond angles are somewhat strained to form the triangular structure. The bond angles in cyclopropane are those of an equilateral triangle, 60 degrees, as compared to the tetrahedral bond angle of 109.5 degrees.

Cyclobutane is a square structure; it is less reactive than cyclopropane because the bond angle strain is less, 90 degrees compared to the tetrahedral bond angle of 109.5 degrees. Cyclopentane and larger cycloalkanes are, like the normal alkanes, quite unreactive; there is no significant bond angle strain in these molecules.

Notable Points about Alkanes

• Alkanes occur in nature in various ways; biologically they are not among the essential materials.
• Musk of the Musk dear contains Cycloalkanes with 14 to 18 carbon atoms.
• Methanogens are the bacteria that are found in the guts of cows, produce large quantities of methane by the metabolism of carbon dioxide or other oxidized organic compounds. The energy is released by the oxidation of hydrogen as follows:
• CO2 + 4H2 → CH4 + 2H2O
• 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 can be liquefied at fairly low pressures, and are well known as liquefied petroleum gas (LPG). Propane, for example, is used in the propane gas burner and as a fuel for cars, butane in disposable cigarette lighters. The two alkanes are used as propellants in aerosol sprays.
• From pentane to octane the alkanes are reasonably volatile liquids. They are used as fuels in internal combustion engines, as they vaporise easily on entry into the combustion chamber without forming droplets, which would impair the uniformity of the combustion. Branched-chain alkanes are preferred as they are much less prone to premature ignition, which causes knocking, than their straight-chain homologues. This propensity to premature ignition is measured by the octane rating of the fuel, where 2,2,4-trimethylpentane (isooctane) has an arbitrary value of 100, and heptane has a value of zero.
• Apart from their use as fuels, the middle alkanes are also good solvents for nonpolar substances.
• Alkanes from nonane to hexadecane (C₁₆) are liquids of higher viscosity, less and less suitable for use in gasoline. They form instead the major part of diesel and aviation fuel. Diesel fuels are characterized by their cetane number, cetane being an old name for hexadecane. However, the higher melting points of these alkanes can cause problems at low temperatures and in Polar Regions, where the fuel becomes too thick to flow correctly.
• Alkanes from hexadecane upwards form the most important components of fuel oil and lubricating oil. In the latter function, they work at the same time as anti-corrosive agents, as their hydrophobic nature means that water cannot reach the metal surface. Many solid alkanes find use as paraffin wax, for example, in candles. This should not be confused however with true wax, which consists primarily of esters.
• Alkanes with a chain length of approximately 35 or more carbon atoms are found in bitumen, used, for example, in road surfacing. However, the higher alkanes have little value and are usually split into lower alkanes by cracking.
• Some synthetic polymers such as polyethylene and polypropylene are alkanes with chains containing hundreds of thousands of carbon atoms. These materials are used in innumerable applications, and billions of kilograms of these materials are made and used each year.