Aromatics

C6H6

• Benzene consists of a flat, regular hexagon of carbon atoms, each of which is bonded to a single hydrogen atom. • The bond angles between all bonds is 120 degrees.

1) Despite being unsaturated, it does not regularly undergo addition reactions. 2) All of the carbon atoms are equivalent, which implies that all of the carbon-carbon bonds are the same

• The C—C bonds in benzene are intermediate between a single and double bond (the bond length is also between the expected length for a single and double bond). • This is electron delocalisation and makes benzene unusually stable.

• Each carbon atom has 3 covalent bonds: one to a hydrogen atom and the other two to carbon atoms • The fourth electron of each carbon atom is in a p-orbital. • There are 6 p-orbitals in total (one for each carbon) and each contains a single electron. • These p- orbitals overlap and the electrons in them are delocalised. • This forms a region of electron density above and below the carbon ring. • This means that overall, each C-C bond in benzene is in between a single and double bond.

• Deocalisation means that electrons are spread over more than two atoms. • In the case of benzene this means over the 6 carbon atoms that form the ring.

It makes benzene (and it’s derivative aromatics) an unusually stable molecule

The enthalpy change of hydrogenation reactions

• The enthalpy change for the hydrogenation of cyclohexene (1 double bond) is -120 KjMol-1 • So the enthalpy change for the hydrogenation of of a ring with 3 alternate double bonds (hypothetical non-delocalised benzene) should be -360 KJmol-1 • However the actual enthalpy change for the hydrogenation of benzene is -208 KJmol^-1. • Thus proves the delocalised ring theory and proves that benzene is more stable than the initially proposed ring structure.

1) The ring is an area of high electron density, because of the delocalised bonding, and is therefore attacked by electrophiles. 2) The aromatic ring is very stable. It needs energy to be put in to break the ring before the system can be destroyed which is called the delocalisation energy. This means that the ring almost always stays intact in the reactions of arenes.

Electrophilic substitution reactions

• Arenes are non-polar so they do not mix with water but do mix with other hydrocarbons and non-polar solvents. • benzene is a colourlesss liquid at room temperature

• Benzene boils at 353K and freezes at 297K. • It’s boiling point is comparable with that of hexane but it’s melting point is much higher than hexanes.

• This is because benzenes flat,hexagonal molecules pack very closely together in the solid state, making them hard to separate. • The separation of molecules is what must happen in order for melting to occur.

• Arenes burn in air with flames that are noticeably smoky. • This is because they have a high carbon:hydrogen ratio compared with alkanes. • There is usually unburnt carbon that remains when they burn in air- this produces soot. • A smoky foam suggests an aromatic compound.

• The delocalised system of the aromatic ring has a high electron density that attracts electrophiles. • The electrons in the delocalised ring are also attracted to the electrophile.

1) Nitration 2) Friedel- Crafts acylation

• The delocalised system of the aromatic ring has a high electron density and attracts electrophiles and is itself attracted to the electrophile. • A bond forms between one of the carbon atoms and the electrophile. • To do this, the carbon uses electrons from the delocalised system which destroys the system. • To get back to the stability of the aromatic system, the carbon loses a H+ ion with the electron in the C—H bond returning to the delocalised system. • The sum of these reactions is the substitution of H+ by the electrophile.

The substitution of a NO2 group for one of the hydrogen atoms on an arene ring.

NO2+

H2SO4 + NHO3 —> NO2+ + HSO4- + H2O

• A reaction mixture of concentrated sulphuric acid and concentrated nitric acid is needed. • Because sulfuric acid is a stronger acid than nitric acid and donates a proton H+ to HNO3 to form H2NO3 + • The H2NO3 + then loses a molecule of water to give NO2+ which is called the nitronium ion or nitryl cation

1) H2SO4 + HNO3 —> H2NO3+ + HSO4- 2) HNO3+ —> NO2+ + H2O

Because the HSO4- produced when the electrophile is formed then reacts with the H+ from the aromatic, therefore regenerating the catalyst.