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
-remaining p orbitals overlap laterally with p orbitals of neighbouring carbon atoms
-forms a π system
-results in the electrons being delocalised
-can freely spread over the entire ring

-π system is made up of two ring shaped clouds of electron density
-one above the plane and one below it
-aromatic compounds are regular and planar compounds with bond angles of 120 o
-delocalisation of electrons means all of the C-C bonds are identical
-have both single and double bond character
-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 substitution reactions, whereas other alkenes undergo addition reactions

Problem with shape:
-benzene = 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 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 due to C-C 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, 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.

Answer 4

-structure is too stable
-ring of delocalised electrons is electron-rich
-attracts electron-deficient species

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.