Organic chemistry is the study of compounds that have carbon as the principal element. Such compounds are called organic compounds, and all the rest are inorganic compounds. There are millions of organic compounds because a carbon atom can link with other carbon atoms as well as atoms of other elements.

A hydrocarbon is an organic compound consisting of hydrogen and carbon atoms. The simplest hydrocarbon is one carbon atom and four hydrogen atoms, or CH₄. All hydrocarbons larger than CH₄ have one or more carbon atoms bonded to another carbon atom. The bond can be single, double, or triple, and may be found as chains or rings of carbon. The alkanes are hydrocarbons with single carbon-to-carbon bonds, the alkenes have a double carbon-to-carbon bond, and the alkynes have a triple carbon-to-carbon bond. The alkanes, alkenes, and alkynes can have straight- or branched-chain molecules. When the number of carbon atoms is greater than three, there are different arrangements that can occur for a particular number of carbon atoms. The different arrangements with the same molecular formula are called isomers. Isomers have different physical properties, so each isomer is given its own name.

The alkanes have all the hydrogen atoms possible, so they are saturated hydrocarbons. The alkenes and the alkynes can add more hydrogens to the molecule, so they are unsaturated hydrocarbons. Unsaturated hydrocarbons are more chemically reactive than saturated molecules.

Hydrocarbons that occur in a ring or cycle structure are cyclohydrocarbons. A six-carbon cyclohydrocarbon with three double bonds has different properties than the other cyclohydrocarbons because the double bonds are not localized. This six-carbon molecule is benzene, the basic unit of the aromatic hydrocarbons.

Petroleumis a mixture of alkanes, cycloalkanes, and a few aromatic hydrocarbons that formed from the slow decomposition of buried marine plankton and algae. Petroleum from the ground, or crude oil, is distilled into petroleum products of natural gas, LPG, petroleum ether,gasoline, kerosene, diesel fuel, and motor oils. Each group contains a range of hydrocarbons and is processed according to use.

In addition to oxidation, hydrocarbons react by substitution, addition, and polymerization reactions. Reactions take place at sites of multiple bonds or lone pairs of electrons on the functional groups. The functional group determines the chemical properties of organic compounds. Functional group results in the hydrocarbon derivatives of alcohols,ethers, aldehydes, ketones, organic acids, esters, and amines.

Living organisms have an incredible number of highly organized chemical reactions that are catalyzed by enzymes, using food and energy to grow and reproduce. These biochemical processes involve building large macromolecules such as proteins, carbohydrates, and lipids.

Carbohydrates are a class of organic molecules composed of CHO. The monosaccharides are simple sugars such as glucose and fructose. Glucose is blood sugar, a source of energy. The disaccharides are sucrose (table sugar), lactose (milk sugar), and maltose (malt sugar). The polysaccharides are polymers or glucose in straight or branched chains used as a near-term source of stored energy. Plants store the energy in the form of starch, and animals store it in the form of glycogen. Cellulose is a polymer similar to starch that humans cannot digest.

Proteins are macromolecular polymers of amino acids held together by peptide bonds. There are twenty amino acids that are used in various polymer combinations to build structural, carrier, and functional proteins.

Nucleic acids are polymers composed of nucleotide units. Two forms are recognized,DNA and RNA. In most organisms DNA serves as the genetic material,RNA plays crucial roles in the synthesis of proteins.

Lipids are a fourth important class of biochemicals. True or neutralfats and oils belong in the category known as lipids and are esters formed from three fatty acids and glycerol into a triglyceride. True fats are usually solid triglycerides associated with animals, and oils are liquid triglycerides associated with plant life, but both represent a high-energy storage material. The other two subgroups are the phospholipids used in cell membranes and steroids, which primarily serve as hormones.

Polymers occur naturally in plants and animals, and synthetic polymers are made today from variations of the ethylene-derived monomers. Among the more widely used synthetic polymers derived from ethylene are polyethylene, polyvinyl chloride, polystyrene, and Teflon. Problems with the synthetic polymers include that (1) they are manufactured from fossil fuels that are also used as the primary energy supply, and (2) they do not readily decompose and tend to accumulate in the environment.