Definition
The best-known representative of the aromatics is benzene (benzenes), which consists of six carbon atoms arranged in a ring with angles of 120°. Benzene is usually drawn like a cylcoalkene, each with three alternating single and double bonds. However, benzene and the other aromatics do not belong to the alkenes and behave chemically differently. All bonds have approximately the same length between a single and a double bond. The 6 pi-electrons are not located between the individual atoms, they are delocalized evenly over the whole ring system. This is also called resonance structures. The Hückel rule can be used to determine whether a ring system is an aromatic. The following properties apply to aromatics:
- They contain 4n + 2 electrons in a conjugated system of p orbitals (n = 1, 2, 3, etc.).
- Aromatics are planar (flat).
- The molecules are cyclic (ring-shaped).
- Aromatics contain a continuous ring of p orbitals.
The electrons can also come from heteroatoms such as nitrogen, oxygen or sulfur. Therefore, heteroaromatics exist in which carbon atoms are replaced by other atoms. These include, for example, pyridine, purine, imidazole, furan and thiophene. Aromatics without heteroatoms belong to the hydrocarbons. Aromatics can also consist of two or more rings. They are then called polycyclic. Well-known examples are naphthalene, anthracene and phenanthrene.
Nomenclature
For naming purposes, the basic skeleton, for example the benzene ring, is used as a suffix and numbered. The names of the substituents are listed alphabetically as prefixes. Relative designations for trivial names:
- 1,2-substituted: ortho (-).
- 1,3-substituted: meta (-)
- 1,4-substituted: para (-)
The benzene ring as a substituent is called a phenyl group. If it carries a methylene group, the group is called benzyl group.
Representative
A small selection of aromatic compounds:
- Acetophenone
- Aniline
- Anisole
- Benzene
- Benzaldehyde
- Benzoic acid
- Chlorobenzene
- Heterocycles (heteroaromatics)
- Naphthalene
- Nitrobenzene
- Phenol, phenols
- Salicylic acid
- Styrene
- Toluene
- Xylene
Properties
- Despite their name, only a minority of aromatics are flavoring agents.
- Aromatics without heteroatoms are apolar, lipophilic and hardly soluble in water.
- The boiling point is comparatively low, because no hydrogen bonds are possible.
- Aromatics are less reactive than alkenes because the electrons are delocalized.
Reactions
Due to the different chemical properties of aromatics, they have a different reactivity than alkenes. For example, addition reactions with halogens or hydrochloric acid do not occur. Instead, aromatics can be substituted electrophilically.
In pharmaceuticals
Countless active pharmaceutical ingredients and excipients contain aromatic structural elements.
