Most organic compounds contain elements other than carbon and hydrogen. Most of these compounds are considered hydrocarbon derivatives. That is, they are essentially hydrocarbons, but instead of one or more hydrogen atoms, they have an additional atom or groups of atoms called functional groups. In many simple molecules, the functional group is attached to an alkyl group.
An atom or groups of atoms that give a family of organic compounds its characteristic chemical and physical properties is called a functional group.
What is the difference in the following compounds?
Research in organic chemistry is organized around functional groups. Each functional group defines a family of organic compounds. Although there are millions of organic compounds, there are only a few functional groups. Functional groups, therefore, facilitate the study of millions of organic compounds. Each functional group determines the characteristic properties of a homologous series. The properties of organic compounds are predominantly determined by the properties of the functional group present in a compound. The concept of functional group is important in organic chemistry for three reasons:
| Name of Class | Functional Group | General Formula |
|---|---|---|
| Alkane | None | R-H |
| Alkyne | -C≡C- | R-C≡C-R |
| Alcohol | -OH | R-OH |
| Ether | -O- | R-O-R' |
| Aldehyde | -CHO | R-CHO |
| Ketone | -CO- | R-CO-R' |
| Carboxylic Acid | -COOH | R-COOH |
| Ester | -COO- | R-COO-R' |
| Amide | -CONH₂ | R-CONH₂ |
Groups containing Carbon, Hydrogen, and Halogens: Haloalkanes are characterized by the presence of the halogen atom. The haloalkane is a compound in which one hydrogen atom of an alkane is substituted by one halogen atom. Which of the following molecules are haloalkanes?
Alcohols are characterized by the presence of the hydroxyl group (-OH) attached to a hydrocarbon chain.
When an -OH group is attached to a benzene ring, the compound is called a phenol.
Phenol was the first antiseptic used in an operation theatre.
Organic compounds that have two alkyl groups attached to the same oxygen atom are called ethers. These compounds have a C-O-C linkage in their molecules.
The general formula for ethers is R-O-R', where R and R' are alkyl groups which may be the same or different.
Aldehydes and ketones contain the carbonyl group. An aldehyde has at least one hydrogen atom attached to the carbonyl carbon atom. A ketone has two hydrocarbon groups (alkyl) bonded to the carbonyl carbon atom.
The functional group of organic acid is called the carboxyl group.
The general formula for carboxylic acids is R-COOH, where R is H or an alkyl group.
Compounds having the general formula R-COO-R' are called esters. R and R' are alkyl groups which may be the same or different.
Compounds having the general formula R-CONH₂ are called amides. R is an alkyl group.
Classify the following compounds as an alcohol, ether, or phenol:
Solution:
Identify the following compounds as an aldehyde, ketone, or carboxylic acid:
Solution:
Almost all synthesis involves the interconversion of at least one functional group to another. A functional group is the active portion of the molecule. It plays a key role in the synthesis of new compounds. The key to designing most organic synthesis is the functional group in the target molecules.