Aromatic Chemistry

Question 1

Benzene formula

Answer 1

C6H6

Question 2

Describe benzene

Answer 2

1. Planar molecule with six c-c bonds being the same length (139nm?)
2. Each c atom is bonded to two other c atoms and an H atom by single covalent bonds.
3. This leaves one unused electron on each c atom which forms pie bonds as each p orbital overlaps with the neighbouring p orbitals
4. The electrons are delocalised and they repel each other less because of this.
5. The electrons are also more spread out so makes benzene a stable molecule

Question 3

Evidence Disproving Kekule’s theory

Answer 3

1. C-C bond length
   - benzene bond length the same so disproves that there are double bonds and single c bonds
   - if it was a triene then three longer c-c bonds and three shorter c=c bonds
2. Addition reactions
   - benzene does not readily undergo addition reactions
   - eg it cannot discolourise bromine
3. Enthalpy of hydrogenation
   - expect triene and 3H2 to form a cyclohexane releasing 360 Kj/mol
   - instead benzene releases 208 Kj/mol showing 152Kj/mol more stable - this is due to the delocalised electrons

Question 4

Mechanism name for benzene

Answer 4

Electrophilic substitution

Question 5

Different types of electrophilic substitution

Answer 5

Nitration using NO2
Friedel-Crafts acylation

Question 6

Reagents for electrophilic substitution for NO2

Answer 6

Conc HNO3 and Conc H2SO4 both needed

The H2SO4 acts as a catalyst

Question 7

Different reactants involved in electrophilic substitution

Answer 7

NO2
Acyl group - Acyl chloride/ anhydride

Question 8

Mechanism of electrophilic substitution using NO2

Answer 8

Can use
HNO3 + H2SO4 —> NO2+ + H2SO4- + H2O

Question 9

Conditions for electro sub with NO2

Answer 9

50 degrees Celsius

Question 10

Reagents for electro sub friedel crafts

Answer 10

Acyl chloride/anhydride and AlCl3

Question 11

Conditions of electro sub for Acyl groups

Answer 11

Anhydrous to prevent the reaction of AlCl3

Question 12

Mechanism of electro sub for Acyl chloride

Answer 12

Question 13

Mechanism for electro sub of Acyl anhydride

Answer 13

Question 14

Why and how does electro sub happen

Answer 14

1. Aromatic compounds are attacked by electrophiles
2. This is because the aromatic ring is very electron rich due to the cloud of electrons above and below the ring
3. They undergo substitution reactions where h atoms on the ring are replaced.
4. They do not readily undergo addition reactions as they would lose their delocalisation and so their extra stability in the process