Carbon and Hydrocarbons: Properties, Uses & Reactions

Carbon and the Carbon Cycle

Carbon is a nonmetallic element, relatively scarce in the Earth's crust and atmosphere. It is a fundamental component of living matter, both plant and animal. The carbon cycle provides the atmosphere with carbon dioxide. Carbon dioxide allows plants, through photosynthesis, to synthesize their living matter. This plant material is assimilated by animals to form their own living matter. The carbon returns to the atmosphere as carbon dioxide through the respiration of living organisms, the decomposition of organic matter, and the combustion of fossil fuels formed from these materials.

Properties of Carbon Compounds

Carbon compounds tend to react relatively slowly compared with many ionic substances. Their melting and boiling points are often low for molecular (covalent) compounds, although thermal stability varies by structure. Many organic compounds are insoluble in water but soluble in organic solvents, and they generally do not conduct electricity.

Justification: The great stability of covalent bonds between atoms and the presence of weak intermolecular forces mean these compounds often require significant activation energy to decompose. They do not dissolve in water because they do not form ions; electrons are not free to move as in ionic or metallic substances.

Isomers and Hydrocarbons

Isomers are compounds that have the same molecular formula but differ in their properties and structure. Hydrocarbons are organic compounds whose molecules are composed only of carbon and hydrogen.

Open-Chain Hydrocarbons: Alkanes (Saturated)

Alkanes are open-chain (acyclic) hydrocarbons in which all the bonds between carbon atoms are single; they are saturated hydrocarbons. Application: Methane and natural gas are used as fuels. Butane burns, emitting large amounts of heat. Domestic gas cylinders often contain butane in a liquid state under high pressure. When an alkane loses a hydrogen atom in the chain, it is transformed into an alkyl radical. Radicals can form covalent bonds with other elements or groups.

Unsaturated Hydrocarbons: Alkenes

Alkenes are open-chain hydrocarbons that have at least one double bond. Application: Alkenes are important chemical feedstocks. From ethene (ethylene) polyethylene is obtained, a polymer widely used in the manufacture of bottles, containers and numerous everyday objects. Various synthetic rubbers are produced from different alkenes.

Unsaturated Hydrocarbons: Alkynes

Alkynes are open-chain hydrocarbons that contain at least one triple bond. Application: Acetylene is a gas that burns with a very high flame temperature and is used in welding and in oxy-acetylene torches for metal cutting. It is also used as a raw material for the synthesis of other organic compounds.

Cyclic Hydrocarbons and Aromatic Compounds

Closed-chain hydrocarbons (alicyclic hydrocarbons) are compounds in which the terminal carbon atom of the chain is linked to the first to form a cycle. Cycloalkanes possess only single bonds. Cycloalkenes have one or more double bonds, and cycloalkynes contain one or more triple bonds. Many of these cyclic compounds are used as intermediates in the chemical industries.

Aromatic hydrocarbons are characterized by a distinct aroma in many cases and include derivatives of benzene. Applications: Benzene is a good solvent for many organic compounds and is used as a raw material in the chemical industry.

Functional Groups and Halogenated Compounds

A functional group is an atom or group of atoms united in a molecular feature that identifies compounds of the same organic family and is responsible for the similarity of their chemical properties.

Halogenated compounds are those in which one or more hydrogens in a hydrocarbon have been replaced by halogen atoms. Application: Some halogenated compounds are useful solvents. Polyvinyl chloride (PVC) and other halogen-containing polymers are widely used, among other applications, as insulation in electrical cables, pipes, flooring and construction elements.