Introduction to Organic Chemistry: Alkanes, Alkenes, and Alkynes

Chapter 11: Introduction to Organic Chemistry

Carbon Tetravalency and Molecular Diversity

Organic chemistry is based on the unique ability of carbon atoms to form four covalent bonds, a property known as tetravalency. This allows carbon to create a vast array of molecular structures, leading to the diversity of organic compounds.

• Tetravalency: Each carbon atom can form four single covalent bonds with other atoms (such as hydrogen, oxygen, nitrogen, or other carbons).

• Structural Diversity: Carbon atoms can bond in straight chains, branched chains, or rings, and can form single, double, or triple bonds.

• Isomerism: The ability to form different structural arrangements (isomers) with the same molecular formula increases molecular diversity.

Example: Methane ($CH_4$) is the simplest hydrocarbon, with carbon forming four single bonds to hydrogen.

Structures and Reactivities of Alkanes, Alkenes, and Alkynes

Hydrocarbons are classified based on the types of bonds between carbon atoms. The three main types are alkanes, alkenes, and alkynes.

Type	General Formula	Bonding	Structure	Reactivity	Industrial Use
Alkanes	$C_nH_{2n+2}$	Single bonds (sigma, $\sigma$)	Saturated, tetrahedral geometry	Least reactive (undergo substitution reactions)	Methane as a fuel (natural gas)
Alkenes	$C_nH_{2n}$	At least one double bond (one sigma, one pi, $\pi$)	Unsaturated, planar around double bond	More reactive than alkanes (undergo addition reactions)	Ethene (ethylene) for making plastics (polyethylene)
Alkynes	$C_nH_{2n-2}$	At least one triple bond (one sigma, two pi)	Unsaturated, linear around triple bond	Most reactive (undergo addition reactions)	Ethine (acetylene) for welding torches

Key Points

• Alkanes: Only single bonds; saturated hydrocarbons; relatively unreactive; used as fuels.

• Alkenes: Contain at least one double bond; unsaturated; more reactive; used in polymer production.

• Alkynes: Contain at least one triple bond; unsaturated; most reactive; used in industrial applications like welding.

Examples

• Alkane: Methane ($CH_4$) – main component of natural gas.

• Alkene: Ethene ($C_2H_4$) – used to produce polyethylene plastic.

• Alkyne: Ethyne ($C_2H_2$) – used as a fuel in oxyacetylene welding torches.

Additional info: The reactivity of hydrocarbons increases with the number of multiple bonds due to the presence of pi ($\pi$) bonds, which are more easily broken during chemical reactions than sigma ($\sigma$) bonds.