4.2 aromaticity

study of compounds containing carbon rings

• a planar and cyclic structure with more stability than other geometric arrangements of atoms with the same molecular formula

• a hydrocarbon with sigma bonds and delocalised pi electrons between carbon atoms forming a circle

C6H6

• hexagon made of 6 carbon atoms in a planar ring • each carbon covalently bonded to two other carbons and a hydrogen • (this means each carbon atom has one unpaired electron in the outer p-orbital. these unpaired electrons combine to form a delocalised ring of electrons) • each bond angle in the structure is 120°/trigonal planar arrangement • 4th electron (2p) forms a 𝛑 bond by overlap with the 2p electron from adjacent carbons. this results in a ‘ring’ of delocalised 𝛑 electrons above and below the plane of the benzene molecule • this results in double bond character between C atoms • each C-C in the benzene ring had a length between that of a double and single bond which suggests that the carbon-carbon bonds are an intermediate between single and double bonds • ⏣

• Kekule structure (proposed by August Kekule in 1865) • the structure suggests that benzene is cyclic with 3 x C=C bonds • consists of alternate single and double carbon bonds • the structure is still used today to represent benzene but is not the official accepted structure of benzene as this model had some flaws • ⌬

• kekule proposed the structures were not static and alternated between each structure (double bonds differed but still separated as usual) • we call these the ‘resonance’ versions • ⌬

• if benzene is unsaturated, it should undergo electrophilic addition reactions readily, but it doesnt • when benzene is chlorinated in the presence of an aluminium chloride catalyst to form 1,2-dichlorobenzene, only 1 structure can be formed; which indicates that all the hydrogens in benzene are equivalent • measurements from xray diffraction studies show that the electron density around carbon atoms is equal and that all the C-C bond lengths are equal and measure 0.140nm - between that of a double and single bond

by comparing the enthalpies of hydrogenation of similar compounds to benzene

• if the enthalpy of hydrogenation of cyclohexane is -120KJmol^-1 for hydrogenating 1 double bonds • cyclohexadiene should be -249KJmol^-1 for 2 double bonds • so cyclohextriene (kekule’s structure) should be -360KJm^-1 for 3 double bonds • however when experimentally performed on benzene, it showed a hydrogenation value of -208KJmol^-

more (151KJmol^-1 more) • which is why benzene is described as being aromatic and has stability

• stabilisation energy/resonance energy/delocalisation energy of benzene

• any compound gains stability if its able to move electrons around (delocalised electrons) • all of the double bonds in benzene overlap and therefore lots of delocalisation occurs - which enhances the stability • this is sometimes described as conjugation which confers stability to the benzene ring • more conjugation (delocalisation) = more stability

more

• a term used to describe an organic compound which contains a ring but is not aromatic (benzene ring)

• mostly the same but with benzene as the suffix • number the carbons in the ring from the carbon with a substituent on • always number using the lowest number combination • e.g 1-2, dichlorobenzene • benzaldehyde • benzoic acid/benzene carboxylic acid

• electrophilic substitution reactions

• benzene is resistant to reacting due to its enhanced stability • it will therefore not behave like a normal alkene and undergo electrophilic addition reactions • to keep its stability, benzene undergoes electrophilic substitution reactions • addition reactions to benzene would destroy the stability - which is energetically unfavourable • the region of high electron density above and below the plane of the molecule results in benzene being attacked by electrophiles

• halogenation - adding a halogen • nitration - adding a nitro, NO2 group • friedel-crafts alkylation (all electrophilic substitution)

• as the chloride is negatively charged and would be repelled by the high electron density in the benzene ring

e.g - cant use a reagent like HCl as its Cl- - so instead need to make a ‘positive chloride ion’ - this is done by reacting chlorine gas with anhydrous AlCl3 (catalyst) AlCl3 + Cl2 —> AlCl4- + Cl+ (electrophile) - eventually produces H+ and chlorobenzene AlCl4- + H+ —> AlCl3 (regenerated catalyst) + HCl • an electrophilic substitution reaction

• same mechanism of electrophilic substitution • higher temp causes multi substitution on the benzene ring (so not higher than 55°C) • adding conc nitric and conc sulfuric acids generate the NO2+ ion which is known as the nitronium ion cH2SO4 + cHNO3 —> HSO4- + NO2+ (nitronium ion) + H2O

• room temperature • chlorine gas, Cl2 • anhydrous AlCl3 (catalyst)

• cH2SO4 AND cHNO3 • 55°C • reflux