Unit 6: Aromatic Chemistry

Arenes are hydrocarbons (made of carbon and hydrogen atoms) based on C6H6 (benzene) which is the simplest arene.Benzene is an unsaturated molecule (contains bonds bigger than single bonds) but it is very stable.

  This is benzene's skeletal formula. At each corner is a carbon and hydrogen atom. Arenes can have different functional groups at the corners (replacing hydrogen).

Benzene is planar and all it's bond angles are 120 degrees.

It's carbon-carbon bonds are all the same length, instead of alternating single bonds to double bonds. THus the bond lengths are an intermediate between single and double bonds. Essentially a one and half bond.

Some of the electrons are delocalised as carbon has only formed 3 of its 4 possible bonds. So the spare electron for each carbon atom is in a p-orbital. This causes an area of electron density above and below the ring.

Kekelue's structure for benzene is shown below. It has the alternating double and single bonds which are now accpeted as wrong.

However, with doulbe bonds you would expect benzen to undergo electrophilic aditition but in reality benzene does not.

This led to other scientists investiagting benzene's structure which was a bit of a mystery.  

The enthalpy change when a c=c bond is hydrogenated (a reduction reaction whihc adds hydrogen atoms and usually leads to a molecule being more saturated) is 119 KJmol-1 so when hydrogenating kekule's sturucture of benzene you would expect the enthaply change to be          3 x 119 = 357 KJmol-1 as it has 3 double bonds.

But in fact the ACTUAL enthalpy is less, 208 kJmol-1  Meaning benzene is more stable than thought (by 152 kJmol-1).

C6H5 CH3     Methylbenzene

C6H5 Br        Bromobenzene

C6H4 Br2     1,2-dibromobenzene or any positional isomers

C6H5 Cl       Chlorobenzene

C6H4 Cl2     1,2-dichlorobenzene or any positional isomers

C6H5 C2H5 ethylbenzene

C6H5 C3H7 propylbenzene

C6H5 C4H9 Butylbenzene

C6H5 NO2 nitrobenzene

Adding functional groups onto benzene decreases it's electron density (from the rings of delocalised electrons) and thus makes futher reactions for the molecule difficult.

e.g.

          electron density of benzene > electron density of methylbenzene

electron density of methylbenzene > electron density of ethylbenzene

   electron density of ethylbenzene > electron density of propylbenzene

          electron density of benzene > electron density of bromobenzene

electron density of bromobenzene > electron density of 1,2-dibromobenzene

The more functional groups, the lower the electron density and the harder for further reactions to occur.

Arenes burn with a smokey flame because they have a high carbon:hydrogen ratio. For benzene this ratio is 6:6 or 1:1 ...very smokey.

There is usual unbunred carbon remaining when they burn in the air and this produces soot.

A smokey flame may suggest an arene.

Make sure you are familiar with electrophilic addition from AS before moving onto electrophilic substitution, so not to confuse you.

The delocalised system of the benzene ring has a high electron density that can attract an electrophilie (an electron pair acceptor/ positively charged ion). The electrons are also attracted to the electrophile and a bond forms, but this destabilises the aromatic system (as electrons have been used from the system to form the bond). To go back to stability of the aromatic system, the carbon that now has a bond to the electrophile loses a H+ ion (giving the electrons lost from the H atom to the aromatic system). Overall a H+ is replaced by an El+ (electrophile).

This is the same process overall as a nitration and Friedel-Crafts acylation reactions. 

Nitration is the substitution of a NO2 group for one of the hydrogen atoms in the arene ring.

The electrophile, NO2 + , is produced in the following reactions in a mixture of conc nitric and conc sulfric acids.

H2SO4 + HNO3 --> H2NO3 +    +  HSO4 -

H2NO3 +           --> NO2  +      +   H2O 

This is very similar to electrophilic substitution. The nitronium ion, which is an electrophile, is attracted to the high electron density of the aromatic ring where it forms a co-ordinative bond. To return stability to the aromatic system, a H+ ion is released so the electrons can replace those in the system that formed the co-ordinative bond.

Nitration is an important step in the making of explosives, such as TNT. TNT is useful because it is an explosive, a solid and has low-melting point.

TNT is made by nitrating methylbenzene, which is also known as toluene. 

  When TNT explodes:   TNT + 10.5 O2 --> 14 CO2 + 3N2 + 5H2O

The reaction is very strongly exothermic. The rapid formation of a lot of gas and heat is what causes the destruction.

Other compounds containing nitrogen atoms are also explosive, such as 1,3,5-trinitophenol which is used as a detonator when exploding to set off other explosives.

These reaction use aluminium chloride as a catalyst (AlCl3).

The mechanism is a substitution, with RCO substituting for a hydrogen on the aromatic ring.

Acyl chlorides provide the RCO group. They react with AlCl3.

RCOCl + AlCl3 --> RCO+     +     AlCl4-

The Al atom has six electrons in it's outer shell (3 of it's own and 3 from the three Cl's) and can accept 2 more from the Cl atom of RCOCl.

RCO+ is a good electrophile that attacks the benzene ring in the substitution mechanism.

AlCl3 is a catalyst and so is reformed in the reaction:

AlCl4-   +  H+  -->  AlCl3   +    HCl