Aromatic chemistry

NO phenylmethanoic acid its benznoic acid No phenyl methanal its benzaldehyde No Amino benzene its phenylamine

Theoretically because there are 3 double bonds in the theoretical cyclohexa-1,3,5-triene one might expect the amount of energy to be 3 times as much as cyclohexene. However, in actual benzene the amount of energy is less. The 6 pi electrons are delocalised and not arranged in 3 double bonds

The increase in stability connected to delocalisation is called the delocalisation energy.

If two or more substituents are present on the benzene ring, their positions must be indicated by the use of numbers. This should be done to give the lowest possible numbers to the substituents. When two or more different substituents are present, they are listed in alphabetical order and di, tri prefixes should be used.

Benzene is a carcinogen Methylbenzene is less toxic and also reacts more readily than benzene as the methyl side group releases electrons into the delocalised system making it more attractive to electrophiles.

Benzene does not generally undergo addition reactions because these would involve breaking up the delocalised system. Most of benzene’s reactions involve substituting one hydrogen for another atom or group of atoms. Benzene has a high electron density and so attracts electrophiles. It reactions are usually electrophilic substitutions.

Change in functional group: benzene –> nitrobenzene Reagents: concentrated nitric acid in the presence of concentrated sulfuric acid (catalyst) Mechanism: Electrophilic substitution Electrophile: +NO2

Used to synthesise useful compounds like TNT trinitrotoluene

H2SO4 + HNO3 draw lone pair attcking the H on H2s04 and then the H attacks the O Forms a conjugate base and acid –>H2NO3+ HSO4- H2NO3 arrow from N to the O forms +NO2 and H2O In the first step the HNO3 acts as a base and accepts a proton (hydrogen ion) from the sulfuric acid

This reaction with benzene is done at 60oC. On using higher temperatures a second nitro group can be substituted.

Change in functional group: benzene –> phenyl ketone Reagents: acyl chloride in the presence of anhydrous aluminium chloride catalyst Conditions: heat under reflux (50OC) Mechanism: Electrophilic substitution

AlCl3 + CH3COCl  [CH3CO]+ + [AlCl4]-

Reagent: Sn and HCl or Fe and HCl Conditions: Heating Mechanism: reduction This reduction reaction can also be done with catalytic hydrogenation (H2 using a Ni catalyst)

If a –OH group, a Cl atom or an NH2 group is directly attached to a benzene ring the delocalisation in the benzene ring will extend to include the lone pairs on the N,O and Cl. This changes the properties and reactions of the side group.

The C-Cl bond is made stronger. Typical halogenoalkane substitution and elimination reactions do not occur. Also the electron rich benzene ring will repel nucleophiles.

Delocalisation makes the C-O bond stronger and the O-H bond weaker. Phenol does not act like an alcohol- it is more acidic and does not oxidise.

Less basic than aliphatic amines as lone pair is delocalised and less available for accepting a proton