Aromatic Chemistry

Question 1

explain the structure of benzene

Answer 1

• molecular formula C6H6
• six C atoms in a hexagonal ring, one H bonded to each C
• each C atom bonded to two other C atoms and one H atom by single covalent sigma bonds
• one unused electron on each C in p orbital
• these six p electrons are delocalised and form a ring

Question 2

why is benzene far more stable than theoretical compound cyclohexa-1,3,5-triene?

Answer 2

• delocalised ring of electrons
• proven by the enthalpy change of cyclohexene
• one would expect 3 times as much energy for 3x the bonds

Question 3

explain the reactions of benzene

Answer 3

• benzene ring is a region of high electron density, attracts electrophiles
• doesn’t undergo addition as would destroy the stable ring
• undergoes substitution reactions, where one H is substituted for the electrophile

Question 4

give the R,C,R,E for nitration of benzene

Answer 4

R - benzene > nitrobenzene
C - temp below 55 degrees
R - conc nitric acid in presence of conc sulfuric acid
E - +NO2

Question 5

give the overall equation for production of nitronium ion

Answer 5

HNO3 + 2H2SO4 > +NO2 + 2HSO4- + H3O+

Question 6

what is the importance of nitration of benzene?

Answer 6

• nitro compounds can be reduced to form aromatic amines, used to manufacture dyes and pharmaceuticals
• used for explosives

Question 7

give the R,C,R,E for friedel-crafts acylation

Answer 7

R - benzene > phenyl ketone
C - heat under reflux
R - acyl chloride in presence of anhydrous AlCl3
E - RCO+

Question 8

how is the acyl chloride electrophile made stronger?

Answer 8

• AlCl3 accepts a lone pair of electrons from the acyl chloride
• as lone pair of electrons is pulled away, polarisation in acyl chloride increases, forming a carbocation
• making it a stronger electrophile, allowing it to react w benzene ring

Question 9

give the overall equation for formation of electrophile for FCA

Answer 9

AlCl3 + RCOCl > [RCO]+ + [AlCl4]-

Question 10

explain the FCA works?

Answer 10

1. electrons in benzene ring attracted to carbocation
2. two electrons from benzene bond with carbocation
3. partially breaks delocalised ring and gives it a positive charge
4. negative charge AlCl4- ion attracted to positively charged ring
5. one chloride ion breaks away from ion and bonds with hydrogen ion
6. removes hydrogen from ring, formimg HCl