In this topic, we are going to go over the fundamental functional groups in organic chemistry, how to name them, and how to recognize them.
Alkanes are, while not a strictly speaking a functional group, is the basic building block of organic molecules. Alkanes are a part of a bigger family of compounds called carbohydrogens, molecules containing only carbons and hydrogens. The distinctive feature of alkanes is that alkanes only contain single bonds between carbons. Alkanes can be linear, branched, and cyclic.
Alkenes are carbohydrogens that contain at least one carbon-carbon double bond (C=C). They as well can be linear, branched, and cyclic.
Alkyne are carbohydrogens that contain at least one carbon-carbon triple bond (C≡C). While they can be linear, branched, and cyclic, no stable cyclic alkynes with less than eight carbons in the ring have been isolated.
It is also important to note, that although we often talk about cyclic vs acyclic molecules, the mere presence of a ring in a molecule does not count as a separate functional group.
While alkyl halides are not counted as a functional group per se, they have a distinct chemistry and properties that would put them into their own category. Alkyl halides are the compounds that contain a carbon-halogen bond.
Alcohols contain a bond between a carbon atom and the hydroxyl group (-OH). Depending on the nature of the carbon atom, alcohols are classified as primary (1°), secondary (2°), and tertiary (3°). A primary alcohol will have a carbon with the hydroxyl group attached to one other carbon, the secondary—to two, and tertiary—to three. Obviously, quaternary alcohols do not exist as it would require a carbon with the hydroxyl to have five bonds, which is impossible.
Ethers contain a carbon-oxygen-carbon bond. Ethers are common organic solvents as they are reasonably polar while have non-polar “tails” which allows them to dissolve a wide range of organic molecules. Yet ethers are relatively chemically inert and only react with strong acids.
Epoxides are, strictly speaking, cyclic ethers with only three atoms in a ring including the oxygen. However, due to the strain in the three-membered ring, they have a distinctly different reactivity from regular ethers. The strict IUPAC name for the epoxide is oxirane.
Thiols are structurally like alcohols; however, they contain a sulfur atom instead of oxygen.
Like ethers, sulfides contain a carbon-sulfur-carbon bond. They have a broader range of reactivity than ethers and are easily susceptible towards oxidation.
Amines contain at least one carbon-nitrogen bond, where the nitrogen atom is sp3-hybridized. Amines are also classified as primary, secondary, tertiary, and quaternary ammonium salts. The major difference here is that we classify amines by the number of carbons attached to the nitrogen atom.
Carbonyls contain a carbon-oxygen double bond and are classified into two distinct groups of compounds: aldehydes and ketones. In organic chemistry, we assume that the “R” group is a carbon-containing substituent. This way we can easily show what is relevant for our purposes and what is the “rest” that we don’t care about.
Note that an aldehyde contains a hydrogen atom and will be always on the end of a chain, while a ketone can be in the middle of a chain. The “R” groups in ketone do not have to be the same.
Carboxylic Acids and Carboxylic Acid Derivatives
Carboxylic acids and their derivatives represent a large family of organic molecules. While they all are different functional groups, they are related through their chemistry and are traditionally put together into the same family. We’ll use the same approach with the “R” groups as before to illustrate what is a part of the functional group and what is the “rest” of the molecule.
While nitrile does look a bit different from the rest of the family, it is still a derivative of a carboxylic acid based on the oxidation state of the carbon atom in the molecule. Also, carboxylic acid derivatives are complex functional groups which might look like a combination of other functional groups. Thus, it is important to remember that a more complex functional group will take precedence over the simpler functional group. For instance, a carboxylic acid is not a combination of a carbonyl and an alcohol.
Aromatic compounds or arenes are a huge class of organic molecules. We have a whole tutorial devoted to those compounds where we discuss the properties of arenes, the heteroaromatic versions of the molecules, and anti-aromatic molecules. The most common aromatic compound or a functional group that we are going to encounter in organic chemistry is some sort of derivative of benzene.
These are the fundamental functional groups that you are going to frequently encounter in your organic chemistry course. Thus, it is important to learn their names and to identify them in the molecules.
Here’s a handy summary table of different functional groups in organic chemistry and some of their important properties: