Aromatic

Question 1

What are Hückel’s 4 rules for aromatic compounds?

Answer 1

Aromatic compounds are:

1. Cyclic
2. Have a ring of uninterrupted p-orbitals
3. Planar
4. Have (4n+2) pi electrons where n is an integer (2, 6, 10, 14, 18)

Question 2

Can aromatics be charged?

Answer 2

Yes, as long as they obey Hückel’s rules, they normally have an odd number of carbons if they don’t have heteroatoms.

Question 3

What is the origin of benzenes aromatic stability?

Answer 3

It has ‘closed shell bonding’ because all three bonding MOs are filled with no antibonding MOs being filled.

Question 4

How are Frost circles used?

Answer 4

Point the aromatic shape with a vertice pointing down, draw a circle from this point only just touching the shape, where the circle intersects with the shape is the relative energy difference between MOs. Below the midpoint in bonding and vice versa.

Question 5

What way do aromatic compounds typically react? Describe the two steps and their relative speeds.

Answer 5

By electrophilic substitution.

1. Slower - electrophiles are attacked by benzene to form a non-aromatic but stabilised carbocation.
2. Faster - a proton is removed by a base, restoring the aromatic system.

Question 6

Name the five most common electrophilic substitution reactions.

Answer 6

Halogenation, nitration, sulfonation, alkylation and acylation.

Question 7

How is benzene alkylated and acylated?

Answer 7

Using an alkyl chloride and acid chloride with an AlCl3 catalyst.