aromatic chemistry

Question 1

What was wrong with Kekule’s benzene structure and why?

Answer 1

● Doesn’t react readily with halogens…
○ If it had 3 C=C bonds it’s expected to react rapidly
with Br2 by electrophilic addition.
○ It doesn’t react readily as its e- is too low - requires a
catalyst and does so by electrophilic substitution.
● Bond lengths…
○ C=C bonds are shorter than C-C bonds ∴ benzene should be irregular/deformed.
○ When bond length were measured. They were all the same, intermediate in length between single and double carbon-carbon bonds ∴ regular hexagonal shape.
● Enthalpy change of hydrogenation…
○ Scientists measured it to be -120kJmol-1 for
cyclohexene ∴ with benzene we’d expect -360kJmol-1.
○ Was measured to be -208kJmol-1 ∴ more stable than
expected.

Question 2

Describe the π-ring in benzene and its consequences

Answer 2

1. Sideways overlap of 6 p-orbitals (each containing 1 e-).
2. Electron density above and below the plane spread over
   6 12C’s ∴ low electron density ∴ unable to polarise many
   molecules ∴ reactions require catalysts.
3. Delocalised electrons meaning not attached to any single
   atom.

Question 3

how does the structure of cyclohextriene compare with the structure of benzene

Answer 3

• π-bond(s) are localised between 2 carbons in left structure (1)
• π-bond(s) are delocalised in right structure (1)

Question 4

Why are the carbon-carbon bonds in benzene all equal in
length?

Answer 4

As the π bonds in benzene are delocalised.

Question 5

Describe the nomenclature for polysubstituted arenes

Answer 5

The lowest number is given to the lowest position of the alphabet

1-bromo-4-chloro-2-ethylbenzene

When benzene is attached to an alkyl group with 7 or more
carbons OR an alkyl with a functional group on it, it is the substituent:
phenylethanoate

Question 6

Give the 3 non-systematic arenes you need to know

Answer 6

Benzoic acid

Phenylamine

Benzaldehyde

Question 7

What are the conditions, reactants, steps (with mechanism), and precautions of nitration?

Answer 7

● Conditions/reactants: 50 °C, conc. HNO3 (nitric acid), conc. H2SO4 (sulfuric acid).
1. Form the nitronium ion…
1.1. Stage 1: H2SO4 + HNO3 → HSO4- + H2NO3+
1.2. Stage 2: H2NO3+ → NO2+ + H2O
1.3. Overall: HNO3 + 2H2SO4 → NO2+ + 2HSO4- + H2O
2. React nitronium ion with benzene…
The catalyst H2SO4 has reformed.
● Perform within a water bath ∵ reaction is exothermic. Any temperature too high will lead to polynitration rather than mononitration

Question 8

What are the reactants and steps (with mechanism) of the halogenation of benzene?

Answer 8

● Reactants: halogen carrier (e.g., FeBr3/AlBr3 for bromination and FeCl3/AlCl3 for chlorination) as a catalyst.

  1. The halogen carrier polarises the halogen allowing it to react with the halogen carrier to form a positive electrophile. ○ Br2 + FeBr3 → Br+ + FeBr4- 2. React the electrophile with the benzene...

A positive electrophile has to be formed because benzene isn’t very polarising.

Question 9

What are the conditions, reactants, and mechanism for Friedel–Crafts alkylation of benzene?

Answer 9

● Conditions: heating under reflux.
● Reactants: haloalkane with its halogen carrier (e.g., AlCl3 for
CH3Cl).
● Mechanism: electrophilic substitution,

The CH3Cl + AlCl3 forms [CH3]+ + [AlCl4]-.

Question 10

What are the conditions, reactants, equation, and mechanism for Friedel– Crafts acylation of benzene?

Answer 10

● Conditions: heating under reflux, 50 °C.

● Reactants: acyl halide and halogen carrier (e.g., CH3COCl for AlCl3).
● Equation: CH3COCl + AlCl3 →[CH3CO]+ + [AlCl4]-. ● Mechanism: electrophilic substitution,

Question 11

How does benzene compare to other alkenes? (3)

Answer 11

1) Bezene reacts by electrophilic substitution WHEREAS other alkenes react by electrophilic addition.
2) Benzene has 6 delocalised e-‘s in π-ring, 12 localised σ e-‘s (2 in each C-C) WHEREAS other alkenes have 2 localised e-‘s in π-bond and 2
in σ-bond in C=C.

Benzene has 12 σ-bonds in total so 24 σ e-‘s.

3) Benzene requires a cataylst (eg, halogen carrier) ∵ its e- density is too low to polarise molecules WHEREAS other alkenes react readily ∵ high e- density.

Question 12

How does the reactivity of methylbenzene and benzene compare?

Answer 12

Methylbenzene is more reactive ∵ alkyl group releases electrons into π-ring ∴ increasing its electron density ∴ more polarising ∴ more reactive.

Question 13

What is phenol?

Answer 13

A benzene group with a -OH directly attached.

Question 14

What type of acid is phenol and how will it react with metals and bases?

Answer 14

1. A weak acid.
2. Reacts with metals and strong bases (eg, NaOH):
   THIS INCLUDES ANY OTHER AROMATIC COMPOUND WITH AN -OH.
3. Too weak to react with weak bases (eg, Na2CO3) unlike carboxylic acids.

Question 15

How do alcohols react with bases?

Answer 15

They don’t because they’re not acidic.

Question 16

How can you distinguish between phenol and a carboxylic acid?

Answer 16

Carboxylic acids will react with a weak base (e.g., NaCO3) leading to
effervescence HOWEVER phenol will not.

Question 17

Describe the bromination of phenol (product, conditions, reactants)

Answer 17

● Forms a white ppt (can be used to distinguish from alkene as the bromine decolourises).
● No halogen carrier required, reacts readily, room temperature.

Question 18

Describe the nitration of phenol (with equation / diagrams)

Answer 18

1. Reacts readily with dilute nitric acid to form a mixture:
2. Reacts readily conc. nitric acid to produce 2,4,6-trinitrophenol:

Question 19

Why is phenol more reactive than benzene? (3)

Answer 19

• The lone pair on the 16O partially delocalises into the π-ring (1)
• Electron density increases (1)
• Making it more polarising (1) as it incudes dipoles making
molecules polar.

Question 20

Give 2 uses of phenol

Answer 20

● Antiseptic (used by Lister, called carbolic acid). ● Detergents.
● Dyes

Question 21

Describe the 3 cases with ‘directing groups’ with examples

Answer 21

1. Unsubstituted ring - electron density constant so electrophiles are equally likely to react with any carbon.
2. Substituted with electron-donating group (-OH or -NH2) - e-’s partially delocalise into the ring increasing its density at carbon 2, 4, 6 making them more likely to react.
3. Substituted with electron-withdrawing group (-NO2) - no e-’s to delocalise, withdraws density from the ring, particularly carbons 2, 4, 6 making 3, 5 more likely to react.
   You can remember -NO2 as being an electron-withdrawing group since NO for “NOT OPEN”.