Introduction to aromatic chemistry
- Aromatic hydrocarbons containing one or more benzene ring. Benzene molecule is a ring of six carbon atoms which is bonded to one hydrogen atom (C6H6).
- Arenes occur naturally in materials such as crude oil and coal.
Naming compounds based on benzene
When one of the hydrogen atoms on a benzene ring is replaced by an atom or group of atoms, a benzen derivative is formed. Groups commonly attached to a benzene ring include -Cl (chloro), -Br (bromo), -NO2 (nitro) and alkyl groups such as -CH3 (methyl) and -C2H5 (ethyl).
Benzene is a starting material for a synthesis of many other aromatic materials. Including ethylbenzene, phenol and styrene. Use benzene to make detergents, explosives, pharmaceuticals and dyestuffs.
The structure of benzene
Problems with the Kekule structure
Kekule's proposed structure of benzene fitted with molecular formula of C6H6. However it failed to explain the chemical and physical properties of benzene fully.
Benzene's low reactivity
Kekule's model failed to explain benzene's low chemical reactivity:
- if C=C double bond were present then benzene would react in similar way to alkenes.
- each C=C double bond would be expected to react with bromine water, decolourising it.
This doesn't happen, nor does benzne partake in other electrophillic addition reactions expected from C=C bond in alkenes.
Kekule's equilibrium model of benzene
Suggested benzene had two different forms, differing by the positions of the double bonds. Two forms in rapid equilibrium so Br2 molecule couldn't be attracted to a double bond before structure changed.
The structure of benzene
The carbon-carbon bond lengths in benzene
C-C single bonds and C=C double bonds have different lengths. Years later revealed that all six of carbob-carbon bond lengths in benzene are same length. Evidence Kekule incorrect.
Hydrogenation of benzene
Kekule structure contained three C=C double bonds. Cyclohexa-1,3,5-triene.
- When an alkene reacts with hydrogen enthalpy change called enthalpy of hydrogenation. When cyclohexane, with one C=C bond reacts with hydrogen, enthalpy of hydrogenation is -120 KJmol.
- Kekule's structure, with three C=C bonds, would be expected to have an enthalpy change of hydrogenation of -360 KJmol.
When benzene is hydrogenated enthalpy change is -208 KJmol, 152 KJmol more than expected.
- real structure 152 KJmol more stable than Kekule's structure.
- energy known as delocalisation energy, or resonance energy of benzene.
Helps to explain why benzene is less reactive than alkenes.
The delocalised model of benzene
The delocalised model has the following features;
- Benzene is a cyclic hydrocarbon with six carbon atoms and six hydrogen atoms.
- The six carbon atoms are arranged in a planar hexagonal ring. Each C atom is bonded to two other carbon atoms and one H atom.
- The shape around each C atom is trigonal planar with a bond angle of 120°.
- Each C atom has four outer shell electrons. Three of these bond to two other carbon atoms and one H atom. The three bonds in this plane are called sigma bonds. This leaves a fourth outer shell in a 2p orbital above