Background for Aromatic Compounds
Kekule proposed the structure of benzene in 1865. The carbons in benzene are sp2 hybridized, have a bond angle of 120o, and have a trigonal planar shape. Benzene can be drawn with all carbons and hydrogens, with just lines, or using a circle in the ring.
Aromaticity, Huckel=s rule
Pi electrons are counted as follows: each double bond counts as two pi electrons, each + charge counts as zero pi electrons, each B charge counts as two pi electrons, each radical counts as one pi electron. Set the total number of pi electrons as equal to 4n + 2; solve for n. If the compound is aromatic, then it is cyclic with a continuous ring of overlapping p orbitals and n equals a whole number. If the compound is antiaromatic, then it is cyclic with a continuous ring of overlapping p orbitals, but n does not equal a whole number. If the compound is not aromatic, then it is noncyclic or is cyclic without a continuous ring of overlapping p orbitals.
Nomenclature of Aromatic Compounds
C6H5CH3 is named toluene. C6H5OH is named phenol. C6H5NH2 is named aniline. C6H5OCH3 is named anisole. C6H5CH=CH2 is named styrene. A benzene with an aldehyde group is named benzaldehyde. A benzene with a methyl ketone group is called acetophenone. A benzene with a carboxylic acid group is named benzoic acid. The benzene is the parent compound. Number the ring so that the groups have the lowest possible numbers. If a special name is used for a benzene derivative, then that group is in position one; then number the other groups. Ortho means that the two groups are on adjacent carbons or on the 1,2 positions. Meta means that the two groups are separated by one carbon or on the 1,3-positions. Para means that the two groups are on opposite ends or on the 1,4-positions. The fused ring structures of anthracene, naphthalene, and phenanthrene are drawn out.
Reactions of Aromatic Compounds
Benzene is halogenated with bromine and iron (III) bromide; chlorine and aluminum chloride; or iodine and nitric acid to give a halobenzene. Nitration of benzene with nitric acid, in the presence of sulfuric acid, produces nitrobenzene. Benzene is sulfonated with sulfur trioxide, in the presence of sulfuric acid, to yield benzenesulfonic acid. In Friedel-Craft alkylation, benzene reacts with aluminum chloride and alkyl halide to form an alkylbenzene. In Friedel-Craft acylation, benzene reacts with aluminum chloride and an acid halide to form a ketone.