Arene Chemistry

Aliphatic and aromatic/arene

A straight or branched chain of organic substances

Includes one or more rings of six carbon atoms with delocalised bonding

The simplest arene is benzene. It has the molecular formula C6H6. Its basic structure is six C atoms in a hexagonal ring, with one H atom bonded to each C atom. Each C atom is bonded to two other C atoms and one H atom by single covalent σ-bonds. This leaves one unused electron on each C atom in a p orbital, perpendicular to the plane of the ring. The six p electrons are delocalised in a ring structure above and below the plane of carbon atoms.

Benzene is a planar molecule. The evidence suggests all the C-C bonds re the same and have a length and bond energy between a C-C single and C=C bond. The H-C-C bond angle is 120 in benzene

The six electrons in the pi bonds are delocalised and spread out over the whole ring. Delocalised means not attached to a particular atom.

Theoretically because there are 3 double nonds in the theoretical cyclohexa-1,3,5-triene one might expect the amount of energy to be three times as much as cyclohexene. However in the actual benszene the amount of energy is less. The 6 pi electrons are delocalised and not arranged in 3 double bond. The increase in stability connected to delocalisation is called the delocalisation energy.

Naming aromatic compounds can be complicated. The simplest molecules are derivatives of benzene and have benzene at the root of the name. 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 did, tri prefixes should be used.

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

Benzene is a carcinogen (cancers causing molecule) and is banned for use in schools. 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.

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

Nitration of benzene and other arenes is an important step in synthesising useful compounds e.g. explosive manufacture (like TNT, trinitrotoluene/ 2-methyl-1,3,5- trinitrobenzene) and formation of amines from which dyestuffs are manufactured. (The reaction for this is covered in the amines section.)

Overall equation for formation of the electrophile: HNO3 + 2H2SO4  +NO2 + 2HSO4 - + H3O+ This is an acid-base reaction In the first step the HNO3 acts as a base and accepts a proton (hydrogen ion) from the sulfuric acid. The intermediate then loses a water molecule leaving the electrophile +NO2

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

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

You should say that each carbon atom forms three covalent bonds. You should say that the remaining p orbitals overlap above and below the ring to form a pi system. You should say that the electrons in these p orbitals are delocalised.

benzene is planar. the bond angles in benzene are 120°. the C-C bonds are equal in length.