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A2 Chemistry > 6A - Aromatic compounds > Flashcards

6A - Aromatic compounds Flashcards

1
Q

What is a aromatic compound?

A

Contains benzene

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2
Q

Formula of benzene = ?

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

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
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3
Q

What is a carcinogen?

A

Something that has the ability to cause cancer

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4
Q

What did kekulé think benzene looked like?

A
  • 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
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5
Q

3 pieces of evidence against kekulé’s structure of benzene

A
  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.
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6
Q

How do the delocalised rings of electrons form in benzene?

A
  • 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
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7
Q

3 pieces of evidence to support delocalised structure of benzene

A

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
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8
Q

Common question!

A
  • 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
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9
Q

What type of reactions do benzene undergo?

A

Electrophilic substitution

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10
Q

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

A
  • Nitration
  • Halogenation
  • Alkylation
  • Acylation
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11
Q

Nitration of benzene:

  • Reagents
  • Conditions
  • Mechanism
A
  • R = Benzene, conc HNO3(aq), conc H2SO4(aq) catalyst
  • C = < 50ºC (for mono substitution) > 50ºC gives a di substitution

Mechanism:

  1. Form NO2+ electrophile:
    • HNO3(aq) + H2SO4(aq) → NO2+(aq) + HSO4-(aq) + H2O(l)
  2. Reaction mechanism - diagram
  3. Reform catalyst:
    • H+(aq) + HSO4-(aq) → H2SO4(aq)
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12
Q

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

What is different about the conditions?

A

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

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13
Q

What is TNT?

A

An explosive - it’s an abbreviation of trinitroluene

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14
Q

Halogenation of benzene - chlorination:

  • Reagents
  • Conditions
  • Mechanism
A
  • 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
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15
Q

Halogenation of benzene - bromination:

  • Reagents
  • Conditions
  • Mechanism
A
  • 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
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16
Q

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

How can iodobenzene be made?

A
  • I2 is too unreactive to sunstitue on benzene - even in the presence of a catalyst
  • BUT iodobenze can be made by reacting benzene + iodochloride
17
Q

Alkylation of benzene:

  • What is this reaction useful for?
  • Reagents
  • Conditions
  • Mechanism
A
  • 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
18
Q

Acylation of benzene

  • What does it form?
  • Reagents
  • Conditions
  • Mechanism
A
  • 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
19
Q

Define Electrophile

A

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

20
Q

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
21
Q

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

A

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

22
Q

What type of reactions do phenol undergo?

A

Electrophilic substitution reactions

23
Q

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

A
  • Nitration
  • Bromination
24
Q

Bromination of phenol:

  • Reagents
  • Conditions
  • Products
A
  • R = Phenol, Br2(aq)
  • C = RTP

Always forms 2,4,6 - tribromophenol

25
Q

Nitration of phenol:

  • Reagents
  • Conditions
  • Products
A
  • R = HNO3(aq) (Doesn’t need to be conc), phenol
  • C = RTP

Could form 2-nitrophenol or 4-nitrophenol

26
Q

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
27
Q

What do activating & deactivating groups do?

A

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

28
Q

Compare reactions of benzene and cyclohexene:

  • Type of attacking speciesType of reaction undergone
  • Condtitions required for bromination
  • Mechanism
  • Explanation for the difference in reactivity
29
Q

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

A2 Chemistry (17 decks)

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