aromatics

Question 1

describe the reactivity of a cyclic alkene vs an aromatic

Answer 1

alkene: the electrons are localised, the electron density is higher, so more susceptible to electrophilic attack/ accepts electrophile more

aromatic: pi bond system is delocalised, the electron density is lower, so less susceptible to electrophilic attack

Question 2

describe the kekule structure of benzene

Answer 2

1.
- electrons are localised in the pi bond
- 6 p-orbitals overlap sideways adjacent to the carbon atom above and below plane of molecule
- 3 separate pi bonds, each have 2e- in them
2.
- pi bond above and below plane of c atoms

Question 3

describe the delocalised structure of benzene

Answer 3

1.
- e- in pi bond system
- p orbitals on adjacent c atoms overlap above and below plane of 6 atoms
2.
- pi bond system above and below plane (ring of 6 c atoms)

Question 4

similarities of kekule and delocalised model

Answer 4

• sideways overlap of p orbitals
• pi bond above and below plane of c atoms

Question 5

difference in kekule and delocalised

Answer 5

• kekule has alternating pi bonds and delocalised has pi bond system
• kekule has 2 e- in pi bond, delocalised has 6
• kekule has localised e-, delocalised pi bond electrons spread around ring

Question 6

explain why phenol is more readily nitrated than benzene

Answer 6

• phenol has a lone pair of e- on the O atom which are partially delocalised into the pi bond system
• so phenol has a higher electron density than benzene
• and is more susceptible to electrophilic attack/ accepts electrophile more easily

Question 7

4.97g of benzoic acid is used to make an impure solid of 3-nitrobenzoic acid, which is purified and found to be 4.85g. Describe a method to obtain a pure sample of 3-nitrobenzoic acid from the impure solid, determine the percentage yield and check its purity.

Answer 7

1. purification
   - do recrystallisation by dissolving the impure solid 3-nitrobenzoic acid in a mininum volume of hot water/solvent, then cool the solution and filter the solid.
   - wash it with cold water and let it dry
2. % yield
   actual n / theoretical n x100 = 0.0290/0.0407 x100 = 71.3%
3. check purity
   - find the melting point of 3-nitrobenzoic acid
   - compare it to know values from a database
   - pure sample will have a mp very close to database value

Question 8

apply your knowledge of bonding in arenes to explain the difference in the relative ease of nitration of phenol, benzene and benzoic acid.

Answer 8

1. phenol has a lone pair of e- on the O atom that is partially delocalised into the pi bond system
2. so the electron density is higher than benzoic acid so it is more susceptible to electrophilic attack
3. the carboxylic acid group on benzoic acid is an electron withdrawing group, so it decreases the electron density of the ring and attracts NO2+ less

Question 9

why are the carbon to carbon bond lengths in benzene the same?

Answer 9

the pi bonds are delocalised

Question 10

explain why chlorine reacts more readily with C6H5N(CH3)2 than benzene

Answer 10

lone pair of electrons on N atom is partially delocalised into the ring increases electron density higher than benzene, so the Cl2 electrophile is more polarised