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OCR Chemistry > Chapter 25 - Aromatic Compounds > Flashcards

Chapter 25 - Aromatic Compounds Flashcards

Introducing benzene, Electrophilic substitution reactions of benzene, The chemistry of phenol, Directing groups.

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1
Q

What is benzene?

A

A cyclic hydrocarbon compound C6H6

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2
Q

Define arene

A

An aromatic hydrocarbon containing one or more benzene rings

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3
Q

Define aromatic

A

Any compound containing one or more benzene rings

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4
Q

What is the electrophilic addition argument against benzene having the Kekule structure?

A

Three C=C bonds is expected, but not there, which is why bromine molecule cannot be polarised to undergo addition reaction.

This only undergoes electrophilic substitution reaction

No reaction with bromine water at room temperature

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5
Q

Outline the stability argument against benzene having the Kekule structure

A

One C=C bond in 6 carbon ring, enthalpy -120kJ/mol
Two C=C bonds in 6 carbon ring, enthalpy -240kJ/mol
Expect 3 C=C bonds to be -360kJ/mol but is actually -208kJ/mol
Delocalisation energy
More stable than expected

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6
Q

What causes benzene to have its unexpected properties?

A

Localised p-orbitals overlap and form delocalised π orbitals above and below the carbon structure (rings)

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7
Q

Define electrophile

A

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

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8
Q

What are the conditions for nitration of benzene?

A

Benzene + HNO3

with H2SO4 at 50C

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9
Q

As halogens cannot react on their own with benzene, what is required?

A

Halogen carrier

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10
Q

Give two examples of chlorine carriers

A

AlCl3

FeCl3

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11
Q

What is formed as a byproduct of halogenation of benzene?

A

HX

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12
Q

What are the two types of Friedel Crafts reactions of benzene?

A

Alkylation

Acylation

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13
Q

Outline alkylation of benzene

A

Benzene + RCl with FeCl3

Forms alkylbenzene + HCl

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14
Q

Outline acylation of benzene

A

Benzene + RCOCl with FeCl3

Forms Phenylalkanone + HCl

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15
Q

Name the mechanism for bromination of benzene

A

Electrophilic aromatic substitution

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16
Q

Outline electrophilic aromatic substitution

A 
Electrophile near benzene
Arrow from benzene to electrophile
Intermediate ion of benzene
Arrow from H-C bond to ring
Product and H+
17
Q

How does a nitrating mixture produce an electrophile?

A

H2SO4 + HNO3 -> NO2+ + HSO4- + H2O

Nitronium ion is electrophile

18
Q

How is the H2SO4 catalyst regenerated?

A

HSO4- + H+ -> H2SO4

H+ from benzene

19
Q

How is an electrophile formed in reaction with a halogen carrier?

A

X2 + AlX3 -> X+ + AlX4-

20
Q

How is the halogen carrier catalyst regenerated?

A

AlX4- + H+ -> AlX3 + HX

21
Q

What is the structure of phenol?

A

C6H6O

OH attached to benzene ring

22
Q

What are the products of phenol reacting with NaOH?

A

Sodium phenoxide: C6H5O-Na+

Water

23
Q

Why does phenol not react with Na2CO3?

A

Too weak of an acid to react with weak base

24
Q

What is formed on reaction of phenol with excess Br2?

A

C6H3OBr3 - OH in position 1 and Br in positions 2,4 and 6

3HBr

25
Q

What are the reaction conditions of bromination of phenol?

A

Room temperature

No catalyst

26
Q

Why does phenol react more readily with bromine/chlorine than benzene reacts with chlorine/bromine?

A
  • In phenol, lone pair of electrons on Oxygen is partially delocalised into the ring
  • This increases the electron density in the ring
  • Therefore Electrophile (Cl+/Br+) is more polarised
  • The aromatic ring in phenol is therefore more susceptible to attack from electrophiles than benzene
27
Q

What is the term for OH increasing benzene’s reactivity?

A

Activating group

28
Q

Define activation

A

An activating group increases the rate of electrophilic aromatic substitution compared to that of benzene

29
Q

Define deactivation

A

A deactivating group decreases the rate of electrophilic aromatic substitution compared to that of benzene

30
Q

Outline differences between phenol and nitrobenzene reacting with bromine

A

Phenol is rapid reaction, nitro is slower
Nitro needs halogen carrier
Phenol at room temp, nitro needs high temp
Phenol can have multiple substitutions, nitro can have only one

31
Q

What do activating groups do with their electrons in benzene?

A

Donate electrons

32
Q

What do deactivating groups do with their electrons in benzene?

A

Withdraw electrons

33
Q

Where do the NH2 and OH groups direct on benzene?

A

2,4 and 6 positions

34
Q

Where does the NO2 group direct on benzene?

A

3 and 5 positions

35
Q

Describe the delocalised model of benzene

A
  • Benzene is a planar, cyclic, hexagonal hydrocarbon, C6H6.
  • Each C atom uses 3 of its available 4 electrons in bonding to two other C atoms and to one H atom.
  • Each C atom has one electron in a p-orbital at right angles to the plane of the bonded C and H atoms.
  • Adjacent p-orbital electron overlap sideways, in both directions, above and below the plane of C atoms to from a ring of electron density.
  • This overlapping of p-orbitals creates a system of π-bonds which spread over all 6 C atoms in the ring structure.
  • The 6 electrons occupying this system of π-bonds are said to be delocalised.
36
Q

Explain the evidence that led scientists to doubt the model proposed by kekule.

A
  • Bond length intermediate is different from C=C (short) and C-C (long)
  • Enthalpy change of hydrogenation is less exothermic than expected when compared to enthalpy change of hydrogenation of cyclohexene.
  • Benzene can only react with Br2 at high temp or in presence of a halogen carrier
37
Q

Where do Halogens direct on Benzene?

A

2,4,6 positions

38
Q

What do Halogens, NH2 and OH do with electrons?

A

Donate Electrons

39
Q

What do COOH, NO2, CN do with electrons?

A

Withdraw Electrons

OCR Chemistry (28 decks)

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