6A - Aromatic compounds Flashcards
What is a aromatic compound?
Contains benzene
Formula of benzene = ?
- Look and smell?
- Where is it found?
- What is it an example of?
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
What is a carcinogen?
Something that has the ability to cause cancer
What did kekulé think benzene looked like?
- 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
3 pieces of evidence against kekulé’s structure of benzene
-
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
-
Benzene doesn’t undergo electrophilic addition reactions with bromine
- Doesn’t decolourise with bromine water
-
Benzene has a less exotheric enthalpy change of hydrogenation than expected
4.
How do the delocalised rings of electrons form in benzene?
- 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
3 pieces of evidence to support delocalised structure of benzene
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
Common question!
- 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
What type of reactions do benzene undergo?
Electrophilic substitution
What are the 4 types of reactions that you need to know for benzene?
- Nitration
- Halogenation
- Alkylation
- Acylation
Nitration of benzene:
- Reagents
- Conditions
- Mechanism
- R = Benzene, conc HNO3(aq), conc H2SO4(aq) catalyst
- C = < 50ºC (for mono substitution) > 50ºC gives a di substitution
Mechanism:
- Form NO2+ electrophile:
- HNO3(aq) + H2SO4(aq) → NO2+(aq) + HSO4-(aq) + H2O(l)
- Reaction mechanism - diagram
- Reform catalyst:
- H+(aq) + HSO4-(aq) → H2SO4(aq)
What is different about nitration of methylbenzene to nitration of benzene?
What is different about the conditions?
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 HNO3(aq), conc H2SO4(aq) catalyst
The temperature is lowered to 30ºC
What is TNT?
An explosive - it’s an abbreviation of trinitroluene
Halogenation of benzene - chlorination:
- Reagents
- Conditions
- Mechanism
- R = Benzene, Cl2
- C = Halogen carrier of AlCl3 or FeCl3 as a catalyst, RTP
Mechanism:
- Form Cl+ electrophile from halogen carrier:
- Cl2 + AlCl3 → Cl+ + AlCl4-
- Reaction mechanism - diagram
- Reform halogen carrier:
- AlCl4- + H+ → AlCl3 + HCl
Halogenation of benzene - bromination:
- Reagents
- Conditions
- Mechanism
- R = Benzene, Cl2
- C = Halogen carrier of AlCl3 or FeCl3 as a catalyst, RTP
Mechanism:
- Form Br+ electrophile from halogen carrier:
- Br2 + AlCl3 → Br+ + AlCl4-
- Reaction mechanism - diagram
- Reform halogen carrier:
- AlCl4- + H+ → AlCl3 + HCl
Why does I2 not substitute on benzene like Br2 and Cl2 does?
How can iodobenzene be made?
- I2 is too unreactive to sunstitue on benzene - even in the presence of a catalyst
- BUT iodobenze can be made by reacting benzene + iodochloride
Alkylation of benzene:
- What is this reaction useful for?
- Reagents
- Conditions
- Mechanism
- 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:
- Form Cl+ electrophile from halogen carrier:
- Cl2 + AlCl3 → Cl+ + AlCl4-
- Reaction mechanism - diagram
- Reform halogen carrier:
- AlCl4- + H+ → AlCl3 + HCl
Phenol:
- Appearence at RTP
- Reaction with water - solubility in water?
- Add NaOH(aq) to phenol in water
- Add Na2CO3(s) to phenol inw ater
- Add bromine water to solid phenol
Bromination of phenol:
- Reagents
- Conditions
- Products
- R = Phenol, Br2(aq)
- C = RTP
Always forms 2,4,6 - tribromophenol
Nitration of phenol:
- Reagents
- Conditions
- Products
- R = HNO3(aq) (Doesn’t need to be conc), phenol
- C = RTP
Could form 2-nitrophenol or 4-nitrophenol
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
Compare reactions of benzene and cyclohexene:
- Type of attacking speciesType of reaction undergone
- Condtitions required for bromination
- Mechanism
- Explanation for the difference in reactivity
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
