(O) Topic 27 - Aromatic Chemistry

6 carbons bonded in a ring, each bonded to 1 hydrogen. The carbons have overlapping p orbitals above and below ther ring creating electron dense molecular orbitals which electrons are delocalised in The molecule is planar

• C-C bond length • Addition Reactions • Enthalpy of Hydrogenation

All of the C-C bonds in Benzence are the same length, and are inbetween the length of C-C bonds and C=C bonds. If it was a cyclic triene, there would be three shorter C-C bonds, and three longer C=C double bonds.

Benzene does not readily undergo addition reactions, but if it was a triene it would

• As you increase the number of double bonds in cyclic molecules, the enthalpy of hydrogenation decreases proportionally. • However, benzene does not follow this trend and has a much higher value for enthalpy of hydrogenation. • It is a more stable molecule and so requires much more energy to break the benzene ring bonds.

Arenes are aromatic compounds that contain one or more benzene rings

Electrophilic Substitution of Benzene with a nitro group

Substitution reactions used to attach extra carbon atoms to a benzene ring. There are 2 types, alkylation - adding an alkyl group (-R), and acylation - adding an acyl group (-COR)

Electrophilic Substitution - Friedel-Crafts acylation

H2SO4 + HNO3 -> HSO4^(1-) + NO2^(1+) +H2O

• RCOCl + AlCl3 -> AlCl4 + RCO+ • AlCl4- + H+ -> AlCl3 + HCl

• AlCl3 +RCO-O-COR -> RCO+ + [Cl3Al-O-CO-R]- • [Cl3Al-O-CO-R]- + H+ -> AlCl3 + HO-CO-R