Home > Aromatic hydrocarbon
An aromatic hydrocarbon (abbreviated as AH), or arene is a hydrocarbon, the molecular structure of which incorporates one or more planar sets of six carbon atoms that are connected by delocalised electrons numbering the same as if they consisted of alternating single and double covalent bonds.After the simplest possible aromatic hydrocarbon, benzene, such a configuration of six carbon atoms is known as a benzene ring.
Aromatic hydrocarbons were so named due to their generally intense smell, long before their molecular structure was understood.
1 Models of benzene ring electron configurations
thumb Naphthalene
Each carbon atom in the hexagonal cycle has four electrons to share. One goes to the hydrogen atom, and one each to the two neighboring carbons.
This leaves one to share with one of its two neighboring carbon atoms, which is why the benzene molecule is drawn with alternating single and double bonds around the hexagon.
thumb Toluene
Many chemists just draw a circle around the inside of the ring to show that there are six electrons floating around.
The electrons float above and below the ring, and the electromagnetic fields they generate keep the ring flat.
thumb Benzene
In modern terminology, benzene rings can be described as compounds in which a continuous, closed system of rings contains separate sets of sigma and pi electrons.
The atomic orbitals forming the sigma system are sp2 hybridized, and those forming the pi system are pure p orbitals.
Erich Hückel's "4n+2" rule can be used to predict aromaticity by the count of delocalized (pi) electrons—if it equals (4n+2), where n is a non-negative integer, then the molecule is likely aromatic.
2 Properties of aromatic hydrocarbons
- They have closed conjugation.
- The Carbon atoms are sp2 hybridized, and therefore have a trigonal planar structure.
- The Carbon-Hydrogen ratio is very small.
- They burn with a sooty yellow flame because of the low carbon-hydrogen ratio.
- They undergo electrophilic substitution reactions, unlike aliphatic compounds which undergo nucleophilic substitution reactionIn chemistry, nucleophilic subsitution is a type of chemical reaction in which one nucleophile (electron donor) replaces another as a covalent substituent of some atom. In the examples given here, this is a carbon atom, but this is far from the only possis.
3 Members of this group of substances
Aromatic hydrocarbons can be monocyclic or polycyclic.
Benzene, C6H6, is the simplest AH and was recognized as the first aromatic hydrocarbon, with the nature of its bonding first being recognized by Friedrich August Kekulé von StradonitzFriedrich August Kekule von Stradonitz ( September 7, 1829 July 13, 1896) was a German organic chemist. He was born in Darmstadt. He was professor at Ghent ( 1858- 1865) and at Bonn. He studied various carbon compounds, especially benzene, proposing a car in the 19th centuryAlternative meaning: Nineteenth Century (periodical ( 18th century — 19th century — 20th century — more centuries) As a means of recording the passage of time, the 19th century was that century which lasted from 1801- 1900. Events The Little Ice Age ended. He envisioned the delocalisedIn chemical compounds, delocalised electrons do not belong strictly to a covalent bond or single atom, but move more freely between the parts of the compound. This stablizes the structure. One particular example is the benzene ring, where the central ring electons rapidly shifting configuration between one of two forms, or resonance structures, in which double bonds rapidly move about the hexagonal ring. However, the total strength of the aromatic bonds involved in aromaticity is stronger than the total strength of the bonds when formulated as a combination of single and double bonds. Aromatic bonding, therefore, must be recognized as a type of bonding distinct from other types of multiple bonding, such as double or triple bonds. This can be described more quantitatively with molecular orbitalIn quantum chemistry, molecular orbitals are the statistical states electrons can have within molecules. Molecular orbitals are formed by the combination of atomic orbitals. It's next to impossible to find out what the orbitals of a molecule are directly. theory.
