Alkanes

The organic compound that consists of single-bonded carbon and hydrogen atoms is known as Alkane. They’re the simplest and least reactive hydrocarbon species. Alkane is an saturated hydrocarbon. Alkanes chemical formula= CnH2n+2. Alkanes are subdivided into 3 groups:

• Straight-chain Alkanes
  
• Cycloalkanes   Alkanes
• The Branched Alkanes

The distinctive feature of Alkane is its lack of unsaturation which makes them uninteresting under laboratory conditions. The two main commercial sources of alkane are petroleum (crude oil) and natural gas. The molecules of Alkane can be drawn by displayed formulae where each atom is revealed as its symbol (C or H) and the chemical bond by a straight line. With the range of variation in alkane, the simplest is methane (CH4) and the complex is pentacontane (C50H102) or an isomer of tetradecane (C14H30). Methane carries one carbon atom and four hydrogen atoms and the rest can be made by substituting carbon atoms for hydrogens.

Examples of Alkane

• CH4: Methane
• C2H6: Ethane
• C3H8: Propane
• C4H10: Butane
• C5H12: Pentane
• C6H14: Hexane
• C7H16: Heptane
• C8H18: Octane
• C9H20: Nonane
• C10H22: Decane

Physical Properties of Alkanes

• Alkanes are colourless.
• Alkanes with:
  Lowest molecular weight (CH4 to C4H10)=gases
  Intermediate molecular weight (C5H12 to C17H36)=liquids
  Heaviest molecular weight (above C18H38)=waxy solids 
• The boiling and melting point of an Alkane is determined by weight and has a linear relationship with the molecular weight.
• Alkanes are insoluble in water and are hence termed hydrophobic. It only has single bonds of C-H and C-C.
• Alkanes dissolve in organic solvents but are insoluble in water. If they dissolve in water, then:
  • London dispersion forces- breaking down of intermolecular forces within the substance.
  • Hydrogen bonds- the primary intermolecular attraction in water.

Types of Alkanes

1. Straight-Chain Alkanes

They’ve got the carbons bonded together in a continuous chain, similar to snakes.

They’re represented as n-alkanes.

Examples,

CH3-CH2-CH2-CH3: n-Butane

CH3-CH2-CH2-CH2-CH3: n-Pentane

The structures can be drawn in two ways:

a) All the carbon and hydrogen atoms are shown.

b) It can be drawn as a chain, where carbon atoms are represented by edges.

2.Cycloalkanes

They’re monocyclic saturated hydrocarbons.

Chemical formula without rings=CnH2(n).

Hydrogen and carbon atoms are bonded together in a single loop.

Its structure contains a single ring.

Chemical Formula with rings=CnH2(n+1-r).

The carbon-carbon bonds are single.

Examples,

Cyclobutane, cyclohexane etc.

3.Branched Alkanes

It is derived from straight-chain alkanes but has branches with the alkyl group.

Alkyl group: A group of carbon and hydrogen atoms attached to an alkane molecule.

Examples-

2-methylpropane. 2-methyl heptane, 2,3- dimethyl hexane etc.Chemical Properties of Alkanes

• With most chemical compounds alkanes are only weakly reactive.
• The reaction of alkanes with oxygen is known as a combustion reaction.
• In the absence of oxygen, carbon monoxide or soot is formed.
• The reaction of alkanes with halogens is named a radical halogenation reaction. It is an exothermic reaction.
• In the presence of a nickel catalyst, alkanes react with steam to give hydrogen.
• The Alkyl groups can be transported from one compound to another by transmetalation reactions.
• In chemical reactions, Alkanes are inert.
• Alkanes burst into flames in the presence of a spark to form CO2 and H2O
  •          CH4+2O2→CO2+2H20
• The mixture of butane and isobutane is used in cigarette lighters
  •          2C4H10+13O2→8CO2+10H2O
• In charcoal lighter fluid, the mixture of C5 to C6 hydrocarbons
  •          C5H12+8O2→5CO2+6H2O
• In gasoline, the mixture of C6 to C8
  •         2C8H18+2502→16CO2+18H2O
• The chemical reaction with bromine gives alkyl bromide
  •          CH4+Br2→CH3Br+HBr

Cracking Alkanes

The breaking down of huge alkanes into smaller and more useful bits. i.e., alkanes and alkenes using high heat. These reactions are known as cracking reactions.

Usually, the source of huge alkanes is Naptha or the gas oil fraction from the fractional distillation of crude oil (petroleum). They are obtained as liquids but re-vaporised to the gaseous phase before cracking. An example involving C15H32,

C15H32→2C2H4+C3H6+C8H18

Reaction of Alkanes

Halogenated Alkanes

The halogenated alkanes are named haloalkanes or halogenoalkanes. The reaction of halogen with an alkane in the presence of heat forms haloalkanes. If not exposed to heat, the reaction will not take place but once the reaction starts, the heat source can be removed and the reaction will continue. It replaces one or more hydrogen atoms with halogen(fluorine, iodine, bromine or chlorine). General formula=RX (R= alkyl and X=halogen).

A halogenation reaction is a simple substitution reaction wherein the C-H bond is broken and replaced by a C-X bond.

For example, chlorination of methane:

CH4+Cl2 + energy → CH3Cl+HCl

Kinetics and Rate

The additional energy is needed for most reactions. The required energy is needed for molecules to travel through the energy barriers that separates them from becoming reaction products. These energy fences are known as the activation energy/enthalpy of activation of the reactions.

Oxidation

Through a free-radical mechanism, Alkanes can be oxidized to CO2 and H2O. The released energy after oxidation of Alkane is known as the heat of combustion. E.g., the heat of combustion in oxidized propane is 688 kilo-calories/mole.

Often, the heat of combustion is used to assess the relative stability of isomeric hydrocarbons. The difference in heats of combustion of two alkanes converts to a difference in their potential energies. Lower Potential energy=More stable product. In alkanes,the branched isomers are more stable.

In a homologous series, the liberated oxidized energy increases by 157 kilo-calories (approx.) for each additional methylene (CH2) unit.

Isomerism

• Alkanes with more than three carbon atoms arranged in various forms are structural isomers.
• The carbon atoms arranged in a single chain with no branches are the simplest isomers.
• There is an increase in the number of carbon atoms with the increase in the number of isomers.

Uses of Alkane

• Used for heating and cooking=Propane
• Used in lighters and aerosol cans=Butane
• Used to produce glue for shoes, leather products and many more=Hexane
• A major component of gasoline=Heptane
• Used in gasoline that reduces knock=Octane.
• A component of gasoline=Decane
• Major components of lubricant oils etc.
• CH3Cl: methyl chloride-chloromethane, used as refrigerant & to manufacture silicones, methylcellulose, and synthetic rubber.
• CH3Cl2: methyl chloride- dichloromethane, used as laboratory & industrial solvent.
• CH3Cl3: chloroform- trichloromethane, used as an industrial solvent.

Effects of Alkanes

• Suspected carcinogens (cancer-causing substances).
• Severe liver damage.
• Once used as a dry-cleaning solvent & in a fire extinguisher, Carbon tetrachloride (CCl4) is no longer recommended to be used. Even used in less amounts can cause serious illness.
• Bromine containing compounds have adverse effects on the environment.
• Chlorofluorocarbons & methane causes the greenhouse effect.
• Ozone depletion.
• Methane & Ethane causes suffocation.

• Short-chain alkanes are found in plant tissues.
• Occurs in various ways in nature- Acyclic alkanes.
• Methanogenic archaea- produce large quantities of methane; the end of the carbon cycle
• CO2+4H2→CH4+2H20
• Methanogens- producer of marsh gas.
• Candida Tropicale, Pichia etc., use alkanes as a source of carbon or energy.
• Alkanes protect plants like plant cuticle and epicuticular wax against water loss, bacteria, fungi and harmful insects.
• Edible vegetable oil contains a small number of biogenic alkanes.
• Alkanes are also found in animal products like shark liver oil.

Conclusion

The distinctive feature of Alkane is its lack of unsaturation which makes them uninteresting under laboratory conditions. The two main commercial sources of alkane are petroleum (crude oil) and natural gas. The additional energy is needed for most reactions. The required energy is needed for molecules to travel through the energy barriers that separates them from becoming reaction products. Usually, the source of huge alkanes is Naptha or the gas oil fraction from the fractional distillation of crude oil (petroleum). They are obtained as liquids but re-vaporised to the gaseous phase before cracking.