Hydrocarbons

Definition and Classification

Hydrocarbons are organic compounds consisting solely of hydrogen and carbon atoms.
They are commonly classified into Alkanes, Alkenes, and Alkynes, based on the bond types present.
Alkanes (saturated hydrocarbons) have only single carbon-carbon bonds.
Alkenes (unsaturated hydrocarbons) have at least one carbon-carbon double bond.
Alkynes (also unsaturated hydrocarbons) have at least one carbon-carbon triple bond.

Properties and Reactions

Hydrocarbons can be gaseous, liquid or solid at room temperature, depending on their molecular weight.
The simplest hydrocarbon, methane (CH4), is a gas, while higher members like pentane (C5H12) and hexane (C6H14) are liquids, and still higher members are solids.
Hydrocarbons are generally non-polar and have weak intermolecular forces, leading to relatively low melting and boiling points.
They are mostly insoluble in water but dissolve in organic solvents.
Hydrocarbons undergo reactions like combustion, substitution, and addition. Combustion of hydrocarbons produces carbon dioxide and water vapour.
In substitution reactions, a hydrogen atom in hydrocarbon is replaced by other elements or groups, most commonly in halogenation.
Addition reactions are characteristic of unsaturated hydrocarbons where a pi bond breaks to form two new sigma bonds.

Alkanes

Alkanes exhibit chain isomerism due to different possible arrangements of carbon atoms.
They undergo free radical substitution reactions, such as halogenation.
In the presence of UV light, the chlorine molecule dissociates into chlorine radicals which then react with methane in a three-step process: Initiation, Propagation, and Termination.

Alkenes

Alkenes show both chain and positional isomerism. Positional isomerism results from the different possible locations of the double bond.
They react readily in addition reactions due to the high electron density of the double bond.
A common reaction is the addition of hydrogen in the presence of a catalyst, or addition of halogens, water, and hydrogen halides.

Alkynes

Alkynes also exhibit both chain and positional isomerism due to variation in the location of the triple bond.
They undergo addition reactions similar to alkenes but require two equivalents of the reacting substance due to the presence of two pi bonds.

Significance of Hydrocarbons

Hydrocarbons serve as fuels and are key constituents of many organic compounds used in daily life, including plastics, rubbers, and pharmaceuticals.
However, incomplete combustion of hydrocarbons leads to pollution and is a major environmental concern.