Alkanes contain one or more carbon atoms linked in a chain, sometimes with side branches. These carbon atoms have electrons that can form bonds. Some of these electron bonds are used up when the carbons link with one another. All the other available electrons form bonds with hydrogen atoms. Ethane, a typical alkane, has the chemical formula C2H6. Alkenes and alkynes have two or more carbon atoms similarly linked in a chain. However, alkenes and alkynes are not completely saturated with hydrogen atoms. In an alkene, at least two carbon atoms form a double bond with each other instead of using this extra bond to unite with hydrogen. As a result, the alkene called ethylene has the formula C2H4 instead of C2H6. In an alkyne, at least two carbon atoms form a triple bond with each other, so that the alkyne called acetylene is even less saturated than ethylene. It has the formula C2H2.
Three or more carbon atoms can unite with one another to form a ring. For example, the six-carbon ring cyclohexane has as many hydrogen atoms as it can hold, resulting in the formula C6H12. Benzene also is a six-carbon ring, but its formula is C6H6. In this case, the charges of the extra electrons distribute themselves equally among the six carbon atoms of the ring instead of forming local bonds between adjacent carbons. This phenomenon is called resonance.
When an --OH group replaces a hydrogen atom in an alkane, the result is an alcohol, such as ethyl alcohol (C2H5OH). Aldehydes contain a --COH group, with the oxygen atom attached to the carbon by a double bond and the hydrogen attached directly to the carbon. Acetaldehyde is CH3COH. Organic acids have a --COOH group. Acetic acid is CH3COOH. Ethers are compounds in which an oxygen atom sits in the midst of a carbon chain. Ketones have an oxygen atom that is attached by a double bond to some carbon that lies somewhere in the middle of a carbon chain. Esters form when an acid and an alcohol unite with one another. Simple sugars, such as glucose, also contain oxygen. Many glucose molecules can unite with one another in long chains to form large molecules, such as starch or cellulose. Such large molecules with repeated units are called polymers.
In addition to its carbon, hydrogen and oxygen atoms, amino acids contain nitrogen. Twenty of these important nitrogen-containing molecules unite in different combinations to form larger molecules called proteins. The nucleic acids DNA and RNA contain both nitrogen and phosphorus, as do the energy-rich molecules ATP and NADPH.
The sulfur-containing --SH group forms organic molecules called thiols or mercaptans, just as the --OH group forms alcohols. C2H5SH is a mercaptan analogous to ethyl alcohol. Some amino acids, such as cysteine, also contain sulfur. Porphyrins are complex organic molecules in which a metal atom is an important constituent. A porphyrin called chlorophyll has a magnesium atom in the center of its molecule. An iron atom occupies a central position in the hemoglobin molecule.