6A - Aromatic compounds

Question 1

What is a aromatic compound?

Answer 1

Contains benzene

Question 2

Formula of benzene = ?

• Look and smell?
• Where is it found?
• What is it an example of?

Answer 2

Benzene = C₆H₆

• Colourless, sweet smelling and highly flammable liquid
• Found:
  
  • Naturally in crude oil - is a component of petrol
  • In cigarette smoke
• It’s a carcinogen = capable of causing cancer

Question 3

What is a carcinogen?

Answer 3

Something that has the ability to cause cancer

Question 4

What did kekulé think benzene looked like?

Answer 4

• 6C’s and 6H’s arranged in a ring - C₆H₆
• 3 x C-C and 3 x C=C
• C-C bonds are slightly longer than C=C bonds

Question 5

3 pieces of evidence against kekulé’s structure of benzene

Answer 5

1. Benzene has equal bond lengths forming a regular hexagonal structure
   
   • Evidence from x-ray crystallography and tunnelling electron microscopy
   • Kekulé’s struture would’ve lead to the compound being skewered
2. Benzene doesn’t undergo electrophilic addition reactions with bromine
   
   • Doesn’t decolourise with bromine water
3. Benzene has a less exotheric enthalpy change of hydrogenation than expected
   4.

Question 6

How do the delocalised rings of electrons form in benzene?

Answer 6

• P-orbitals are present above and below the plane of the carbon ring
• Sideways overlap of p-orbitals create a delocalised rings of electrons above and below the plane of the benzene ring
• ∴ creates a system of Π bonds spread over the 6Cs in the ring

Question 7

3 pieces of evidence to support delocalised structure of benzene

Answer 7

Bond lengths are all the same:

• Kekulé’s structure would lead to 3 long + 3 short bonds ∴ it would be skewered
• In reality bond lengths are half way between C-C and C=C

Less exothermic enthalpy change of hydrogenation than expected:

• Need more energy to break bonds ∴ benzene is more stable than expected

Doesn’t undergo electrophilic addition reactions with Br_(aq):

• Kekulé expected it to delcolourise Br_(aq) but it didn’t
• If it did react with benzene it would lose the ring structure
• Undergoes electrophilic substitution reactions instead

Question 8

Common question!

Answer 8

• Benzene has a formula C₆H₆. It’s a planar and is a regular hexagon
• Main framework of molecules is helf together by 6 C-H σ bonds and 6 C-H σ bonds, each containing 2 electrons
• This leaves 1 outer electron on each carbon atom in a p-orbital orientated perpendicular to the plane of the molecule
• DIAGRAM:
  
  • P-orbitals overlap sideways in both directions to form a delocalised system of π bonds above and below the plane of the molecule
  • This delocalised π system contains 6 electrons that are spread across 6 C atoms
• All carbon - carbon bonds are equal in length. They’re an intermediate length to a C-C and a C=C bond

Question 9

What type of reactions do benzene undergo?

Answer 9

Electrophilic substitution

Question 10

What are the 4 types of reactions that you need to know for benzene?

Answer 10

• Nitration
• Halogenation
• Alkylation
• Acylation

Question 11

Nitration of benzene:

• Reagents
• Conditions
• Mechanism

Answer 11

• R = Benzene, conc HNO_3(aq), conc H₂SO_4(aq) catalyst
• C = < 50ºC (for mono substitution) > 50ºC gives a di substitution

Mechanism:

1. Form NO₂⁺ electrophile:
   
   • HNO_3(aq) + H₂SO_4(aq) → NO₂⁺_(aq) + HSO₄^-_(aq) + H₂O_(l)
2. Reaction mechanism - diagram
3. Reform catalyst:
   
   • H⁺_(aq) + HSO₄^-_{(aq) →} H₂SO_4(aq)

Question 12

What is different about nitration of methylbenzene to nitration of benzene?

What is different about the conditions?

Answer 12

CH₃ is 2,4 directing ∴ instead of substituting NO2⁺ on carbon 1, 3 and 5 it is substituted onto carbons 2 + 4.

The reagents remain the same as nitration fo benzene:

• Benzene, conc HNO_3(aq), conc H₂SO_4(aq) catalyst

The temperature is lowered to 30ºC

Question 13

What is TNT?

Answer 13

An explosive - it’s an abbreviation of trinitroluene

Question 14

Halogenation of benzene - chlorination:

• Reagents
• Conditions
• Mechanism

Answer 14

• R = Benzene, Cl₂
• C = Halogen carrier of AlCl₃ or FeCl₃ as a catalyst, RTP

Mechanism:

1. Form Cl⁺ electrophile from halogen carrier:
   
   • Cl₂ + AlCl₃ → Cl⁺ + AlCl₄^-
2. Reaction mechanism - diagram
3. Reform halogen carrier:
   
   • AlCl₄^- + H⁺ → AlCl₃ + HCl

Question 15

Halogenation of benzene - bromination:

• Reagents
• Conditions
• Mechanism

Answer 15

• R = Benzene, Cl₂
• C = Halogen carrier of AlCl₃ or FeCl₃ as a catalyst, RTP

Mechanism:

1. Form Br⁺ electrophile from halogen carrier:
   
   • Br₂ + AlCl₃ → Br⁺ + AlCl4^-
2. Reaction mechanism - diagram
3. Reform halogen carrier:
4. AlCl4^- + H⁺ → AlCl₃ + HCl

Question 16

Why does I₂ not substitute on benzene like Br₂ and Cl₂ does?

How can iodobenzene be made?

Answer 16

• I₂ is too unreactive to sunstitue on benzene - even in the presence of a catalyst
• BUT iodobenze can be made by reacting benzene + iodochloride

Question 17

Alkylation of benzene:

• What is this reaction useful for?
• Reagents
• Conditions
• Mechanism

Answer 17

• Useful as forms new C-C bonds to extend the carbon chain
• R = Haloalkane (Cl or Br), benzene
• C = Corresponding halogen carrier - AlCl₃ / Chloroalkanes are most common

Mechanism:

1. Form Cl⁺ electrophile from halogen carrier:
   
   • Cl₂ + AlCl₃ → Cl⁺ + AlCl4^-
2. Reaction mechanism - diagram
3. Reform halogen carrier:
   
   • AlCl4^- + H⁺ → AlCl₃ + HCl

Question 18

Acylation of benzene

• What does it form?
• Reagents
• Conditions
• Mechanism

Answer 18

• Forms aromatic ketones by substituting a acyl chloride onto benzene
• R = Acyl chloride, benzene
• C = AlCl₃ halogen carrier as catalyst, RTP

Mechanism:

1. Form Cl⁺ electrophile from halogen carrier:
   
   • Cl₂ + AlCl₃ → Cl⁺ + AlCl4^-
2. Reaction mechanism - diagram
3. Reform halogen carrier:
   
   • AlCl4^- + H⁺ → AlCl₃ + HCl

Question 19

Define Electrophile

Answer 19

Electrophile = An atom or group of atoms which is attracted to an electron rich centre or atom, where it accepts a pair of electrons to form a covalent bond

Question 20

Phenol:

• Appearence at RTP
• Reaction with water - solubility in water?
• Add NaOH_(aq) to phenol in water
• Add Na₂CO_3(s) to phenol inw ater
• Add bromine water to solid phenol

Answer 20

Question 21

Why does phenol react with bromine water but benzene doesn’t?

Answer 21

O^- of the OH^- increases negative charge when it mixes of of it’s electrons with the electron density, increased negative charge attracts Br₂ more

Question 22

What type of reactions do phenol undergo?

Answer 22

Electrophilic substitution reactions

Question 23

Name 2 types of electrophilic substitution reactions you need to know for phenol

Answer 23

• Nitration
• Bromination

Question 24

Bromination of phenol:

• Reagents
• Conditions
• Products

Answer 24

• R = Phenol, Br_2(aq)
• C = RTP

Always forms 2,4,6 - tribromophenol

Question 25

_Nitration of phenol:_ * Reagents * Conditions * Products

Answer 25

* R = HNO_3(aq) (Doesn't need to be conc), phenol * C = RTP Could form 2-nitrophenol or 4-nitrophenol

Question 26

_Activating and deactivating groups:_ * 3 examples of activating groups * 1 example of a de-activating group * Do the substitution reactions require a catalyst? * Directing effect * Extent of substitution

Answer 26

Question 27

What do activating & deactivating groups do?

Answer 27

**Activating group** = Donate lone pair, increases electron density of ring, increase reactivity **Deactivating group** = Accept's lone pair, decreases electron density of ring, decreases reactivity

Question 28

_Compare reactions of benzene and cyclohexene:_ * Type of attacking speciesType of reaction undergone * Condtitions required for bromination * Mechanism * Explanation for the difference in reactivity

Answer 28

Question 29

_Compare reactions of benzene and phenol:_ * Products of bromination * Products of nitration * Type of attacking species * Condtitions required for bromination * Conditions required for nitration * Type of reaction undergone and mechanism * Explanation for the difference in reactivity

Answer 29