Functional groups are a group of atoms with characteristic properties, which when linked to the carbon skeleton of a molecule, alter the properties of that molecule. For example, the presence of certain functional groups on a molecule will make them hydrophilic, whereas others will make them hydrophobic. These functional groups are an indispensable part of organic chemistry and important components of biological molecules, such as carbohydrates, proteins, lipids, and nucleic acids. Each functional group is a unique arrangement of atoms and is assigned a name for the combination of atoms that make up the group.

Hydrocarbons
Strong, stable bonds between carbon atoms produce complex molecules containing chains, branches, and rings. The chemistry of these compounds is called organic chemistry. Hydrocarbons are organic compounds composed of only carbon and hydrogen. The alkanes are saturated hydrocarbons—that is, hydrocarbons that contain only single bonds. Alkenes contain one or more carbon-carbon double bonds. Alkynes contain one or more carbon-carbon triple bonds. Aromatic hydrocarbons contain ring structures. They are also known as aryl groups.

Alcohols and Ethers
Many organic compounds that are not hydrocarbons can be thought of as derivatives of hydrocarbons. A hydrocarbon derivative can be formed by replacing one or more hydrogen atoms of a hydrocarbon with a functional group, which contains at least one atom of an element other than carbon or hydrogen. The properties of hydrocarbon derivatives are determined largely by the functional group. The –OH group, known as a hydroxyl group, is the functional group of an alcohol. The –R–O–R– group is the functional group of an ether.

Aldehydes, Ketones, Carboxylic Acids, and Esters
Functional groups related to the carbonyl group include the –CHO group of an aldehyde, the –CO– group of a ketone, the –COOH group of a carboxylic acid, and the –COOR group of an ester. The carbonyl group, a carbon-oxygen double bond, is the key structure in these classes of organic molecules: Aldehydes contain at least one hydrogen atom attached to the carbonyl carbon atom, ketones contain two carbon groups attached to the carbonyl carbon atom, carboxylic acids contain a hydroxyl group attached to the carbonyl carbon atom, and esters contain an oxygen atom attached to another carbon group connected to the carbonyl carbon atom. All of these compounds contain oxidized carbon atoms relative to the carbon atom of an alcohol group.

Amines
The addition of nitrogen into an organic framework leads to two families of molecules. Compounds containing a nitrogen atom bonded in a hydrocarbon framework are classified as amines.

This text is adapted from Openstax, Chemistry 2e, Section 20.1: Hydrocarbons, Openstax, Chemistry 2e, Section 20.2: Alcohols and Ethers, Openstax, Chemistry 2e, Section 20.3: Aldehydes, Ketones, Carboxylic Acids, and Esters, and Openstax, Chemistry 2e, Section 20.4: Amines and Amides.