Alkanes are the most simple organic substances made up of carbon and hydrogen only. They have a simple formula of CnH2n +2. In these substances, the 4 valencies of carbon atoms are completed by single bonds to either other carbon atoms or a hydrogen atom.
They are, for that reason known as Saturated Hydrocarbons. Methane (CH4) is the most simple member of this family. Each carbon atom in an alkane is sp3 hybridized and has a tetrahedral geometry.
Properties of Alkanes
1.Alkanes containing up to 4 carbon atoms are colorless, odorless gases while pentane to heptadecane (C5 to C17) are colorless, odorless liquids. The greater members from C18 onwards are waxy solids which are also colorless and odorless.
- Alkanes are non-polar or very weakly polar and are insoluble in polar solvents like water, however soluble in non-polar solvents like benzene, ether, carbon tetrachloride, and so on.
- Their physical constants like boiling points, melting points, density, etc. increase with the increase in the number of carbon atoms, whereas solubility reduces with an increase in molecular mass. The boiling point increases by 20 to 30 ° C for the addition of each CH2 group to the molecule. The boiling points of alkanes having branched-chain structures are lower than their isomeric typical chain alkanes, e.g., n-butane has a higher boiling point-0.50 C than isobutane (-1 1.7 ° C).
- The melting points of alkanes also increase with the increase in molecular mass but this increase is not so regular.
Structure of Alkanes
Alkanes have the general formula CnH2n +2. For instance, an alkane with 2 (n) carbon atoms, will have 6 (2n + 2) hydrogen atoms. Their surrounding atoms are connected with sigma bonds and form tetrahedral centers around the carbon atoms. As these bonds are all single bonds, there is free rotation around all connections.
Each carbon atom has 4 bonds (either C-H or C-C bonds), and each hydrogen atom is signed up with a carbon atom (H-C bonds). A series of linked carbon atoms is referred to as the carbon skeleton or carbon backbone. The number of carbon atoms is used to specify the size of the alkane (e.g., C2-alkane).
An alkyl group, normally abbreviated with the symbol R, is a functional group or side-chain that, like an alkane, consists solely of single-bonded carbon and hydrogen atoms; for example, R might represent a methyl or ethyl group. An alkyl group is a piece of a particle with the general formula (CH3) n, where n is an integer. For example, a methyl group (CH3) is a fragment of a methane molecule (CH4). In this example, n= 1.
Reactivity of Alkanes
The alkanes are also called paraffins (Latin: parum = little, affins = affinity) under regular conditions are inert towards acids, alkalis, oxidizing, and decreasing agents. Nevertheless, under ideal conditions, alkanes do undergo two kinds of reactions.
- Substitution Reactions
- Thermal and Catalytic Reactions
These reactions happen at high temperatures or on the absorption of light energy through the development of extremely reactive complimentary radicals. The unreactivity of alkanes under typical conditions might be discussed on the basis of the non-polarity of the bonds forming them.
The electronegativity values of carbon (2.5) and hydrogen (2.1) do not differ significantly and the bonding electrons in between C-H and C-C are equally shared making them practically nonpolar. In view of this, the ionic reagents such as acids, alkalies, oxidizing agents, etc. find no reaction site in the alkane particles to which they could be connected.
Inertness of σ-bond
The unreactivity of alkanes can also be discussed on the basis of the inertness of a σ-bond. In a σ-bond, the electrons are really firmly held between the nuclei which makes it a really stable bond. A great deal of energy is needed to break it. Furthermore, the electrons present in a σ-bond can neither attack on any electrophile nor a nucleophile can attack them. Both these facts make alkanes less reactive.
Uses of Alkanes
Alkanes are really versatile and are being used as solvents, heating oils, fuels, in fat synthesis, in the synthesis of fats by air oxidation, in the manufacture of albumen, in the change to olefins, etc.
- Methane is used:
- (i) as a fuel and as illuminating gas.
- (ii) for the preparation of methyl chloride, dichloromethane, chloroform, and carbon tetrachloride.
- (iii) for the industrial preparation of methyl alcohol, formaldehyde, and hydrogen cyanide.
- (iv) for the preparation of carbon black utilized in paints, printing inks, and auto tires.
- (v) is utilized to produce urea fertilizer.
Ethane is being used as heating fuel. In addition, ethane is made use of to prepare ethylene by pyrolysis or acetaldehyde and acetic acid by catalytical oxidation.
Propane is an essential and basic raw material in petrochemistry.
It finds use as liquid gas (LPG) in laboratories and households for heating and combustion functions.
In addition to being used as a solvent, n-pentane is utilized as a lathering agent in the preparation of phenolic resins and polystyrene, as a propellant for aerosol sprays, as a filling of low-temperature thermometers, and as a reference compound in gas chromatography.
Hexane is used for the extraction of oils and fats, as solvent and response medium in the preparation of plastics and synthetic rubber during the polymerization procedure, and as a dilutant for fast-drying lacquers, printing inks, and glues.
Heptane functions as a solvent in the laboratory and for fast-drying lacquers and glues.
Octane is utilized for the aromatization of xylenes and ethylbenzene but mainly as a solvent and in azeotropic distillations.
Nonane is used for the preparation of tensides (detergents) and as a carrier in distillation processes.