Aromatic chemistry

Question 1

The nature of the bonding in a benzene ring, limited to planar structure and bond length intermediate between single and double.

Answer 1

Each carbon atom in the ring forms three σ bonds using the sp2 orbitals

The remaining p orbitals overlap laterally with p orbitals of neighbouring carbon atoms to form a π system

This extensive sideways overlap of p orbitals results in the electrons being delocalised and able to freely spread over the entire ring causing a π system

The π system is made up of two ring shaped clouds of electron density - one above the plane and one below it

Benzene and other aromatic compounds are regular and planar compounds with bond angles of 120 o

The delocalisation of electrons means that all of the carbon-carbon bonds in these compounds are identical and have both single and double bond character

The bonds all being the same length is evidence for the delocalised ring structure of benzene

Question 2

Why is benzene more stable than the theoretical molecule cyclohexa-1,3,5-triene?

Answer 2

Delocalisation of p electrons

Question 3

3 pieces of evidence that disprove Kekule’s model

Answer 3

Problem with reactivity:
-benzene undergoes substiution reactions, whereas other alkenes undergo addition reactions

Problem with shape:
-benzene is a planar molecule
-C-C single and double bonds should be different lengths
C-C 0.154 nm
C=C 0.134 nm
-in benzene all the bonds are exactly the same - intermediate in length between C-C and C=C at 0.139 nm
-hence it’s a perfectly regular hexagon.

Problems with stability:
-benzene is stable due to its delocalised ring of electrons
-forms as a result of the carbon-carbon bonds being neither single nor double bonds
-instead an intermediate length with electrons in the p-orbitals sticking out above and below the carbon ring
-means that, when compared to cyclohexa-1,3,5-triene, benzene has a much less exothermic enthalpy of hydrogenation - -208kJmol-1
-predicted: 360kJmol-1
-more energy is needed to break benzene’s bonds than cyclohexa-1,3,5-triene.
-hence more stable compound

use thermochemical evidence from enthalpies of hydrogenation to account for this extra stability

Question 4

explain why substitution reactions occur in preference to addition reactions.

Question 5

Electrophilic attack on benzene rings results in substitution, limited to monosubstitutions.

Question 6

Nitration is an important step in synthesis, including the manufacture of explosives and formation of amines.

Question 7

Friedel–Crafts acylation reactions are also important steps in synthesis.

Question 8

Students should be able to outline the electrophilic substitution mechanisms of:

nitration, including the generation of the nitronium ion
acylation using AlCl3 as a catalyst.