Arenes

• Starting material for all arenes.
• Benzene is carcinogenic
• Colourless liquid
• Sweet odour
• Highly Flammable

Kekule's structure:   

• six membered carbon ring with alternating single and double bonds
• Fits with molecular formula C6H6 

Problems with Kekule's structure:

• Benzene does not decolourise bromine water, if double bonds were present it would.
• Does not take part in other electrophilic addition reactions
• Benzene is more stable than most alkenes, less reactive
• The bonds were all the same length in benzene, C-C bonds and C=C bonds are known to have different lengths
• The enthalpy change in benzene is less than the predicted enthalpy change for kekule's structure of benzene

• Cyclic hydrocarbon, 6 carbon atoms and 6 hydrogen atoms
• Planar hexagonal ring
• Each carbon is bonded to two other carbons and one hydrogen
• 4 electrons in outer shell of each carbon, three are involved in sigma bonds and one electron is in orbital above and below the plane of atoms.
• P-orbital for each carbon overlap
• Delocolised ring of pi bonds shared over all six carbon atoms, above and below the plane of carbon atoms. 

Chemical reactivity

• does not decolourise bromine water
• does not react with strong acids (HCl)
• does not react with halogens (Cl, Br, I)

This is because addition reactions would disrupt the delocalisation of the ring structure.

Instead, Benzene undergoes electrophillic substitution reactions. 

Conditions:

• H2SO4                                             General Equation
• 50 degrees      C6H6 + HNO3   --->    C6H5NO2 + H2O

Generation of the electrophile

HNO3 + H2SO4  --> NO2+  +  HSO4- + H2O

Forming the intermediate

(print and draw)

Regenration of the catalyst

H+ + HSO4- -->  H2SO4

Conditions:                                               general eqn 

• FeCl3 or AlCl3 (halogen carrier)                           C6H6 + Br2 --> C6H5Br + HBr 

Generating the electrophile

Br2 + FeBr3 -->  Br+   +  FeBr4-

Smiley intermediate

Regenerating the catalyst

H+ + FeBr4- --> FeBr3 + HBr

Why benzene doesn't react with bromine: 

• As benzene's electrons ae spread out over all 6 carbon atoms, there is a lower electron density in the ring. 
• There is insufficient electron density to polarise the Br-Br bond
• Halogen carrier is needed is polarise bond and attract pi-electrons from ring

Why cyclohexene does react with bromine:

• the pi-bond (double bond) in cyclohexane contains localised electrons, shared between only two carbon atoms. This produces a region of high electron density.
• The electron density is sufficient to polarise the Br-Br bond and create an induced dipole.
• The pi-electron pair are attracted to the slightly polar bromine, causing the double bond to break. 
• Bromide ions are covalently bonded to the two carbon atoms

• Benzene ring with -OH group attatched.

Phenols in water

C6H5OH (dissolved in water) -->  C6H5O-  + H+

Phenols, when dissolved in water, lose a H+ ion fom the OH group.

Phenol to salt

C6H5OH  + NaOH(aq)  --> C6H5O-Na+  + H2O (neutralisation in aqueous NaOH)

or 

2C6H5OH  +  2Na  -->  2C6H5O-Na+  + H2   (phenol + sodium = salt + hydrogen gas)

• electrophillic substitution
• room temperature and no halogen carrier 

C6H5OH + 3Br2   -->  C6H3OBR3 + 3HBr

Ease of bromination of phenol

• lone pair of electrons on oxygen atom is drawn to ring
• creating higher electron density, ring is activated
• increased electron density polarises bromine molecules, which are then attracted to ring.