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).

• Molecular Diversity: Carbon atoms can bond in straight chains, branched chains, or rings, and can form single, double, or triple bonds, resulting in a wide variety of organic molecules.

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

Structures and Reactivities of Alkanes, Alkenes, and Alkynes

Alkanes, alkenes, and alkynes are three major classes of hydrocarbons, distinguished by the types of bonds between carbon atoms.

Class	General Formula	Bonding	Structure	Reactivity	Industrial Use
Alkanes	$C_nH_{2n+2}$	Single bonds (C–C)	Saturated, tetrahedral geometry	Least reactive (undergo substitution reactions)	Methane as a fuel (natural gas)
Alkenes	$C_nH_{2n}$	At least one double bond (C=C)	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 (C≡C)	Unsaturated, linear around triple bond	Most reactive (undergo addition reactions)	Ethine (acetylene) for welding torches

Key Points

• Alkanes are saturated hydrocarbons with only single bonds, making them relatively unreactive. Example: Methane ($CH_4$) is used as a household fuel.

• Alkenes contain at least one carbon-carbon double bond, making them more reactive. Example: Ethene ($C_2H_4$) is used in the production of plastics.

• Alkynes have at least one carbon-carbon triple bond, making them the most reactive among the three. Example: Ethyne ($C_2H_2$), also known as acetylene, is used in welding torches.

Examples and Applications

• Methane (Alkane): $CH_4$ is the main component of natural gas, used for heating and cooking.

• Ethene (Alkene): $C_2H_4$ is used to produce polyethylene, a common plastic.

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

Additional info: The increased reactivity of alkenes and alkynes compared to alkanes is due to the presence of π (pi) bonds, which are more easily broken during chemical reactions.