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OCR A-Level 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 so it is expected
No reaction with bromine water at room temperature
No other electrophilic addition reactions
Only substitution reactions

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5
Q

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

A

Two forms in rapid equilibrium
Approaching Br-Br not attracted to bonds
Bonds change from C=C to C-C before reaction occurs
Bromine doesn’t react

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6
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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7
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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8
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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9
Q

What are the conditions for nitration of benzene?

A

Benzene + HNO3

with H2SO4 at 50C

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10
Q

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

A

Halogen carrier

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11
Q

Give two examples of chlorine carriers

A

AlCl3

FeCl3

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12
Q

What is formed as a byproduct of halogenation of benzene?

A

HX

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13
Q

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

A

Alkylation

Acylation

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14
Q

Outline alkylation of benzene

A

Benzene + RCl with FeCl3

Forms alkylbenzene + HCl

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15
Q

Outline acylation of benzene

A

Benzene + RCOCl with FeCl3

Forms Phenylalkanone + HCl

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16
Q

Name the mechanism for bromination of benzene

A

Electrophilic aromatic substitution

17
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+
18
Q

How does a nitrating mixture produce an electrophile?

A

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

Nitronium ion is electrophile

19
Q

How is the H2SO4 catalyst regenerated?

A

HSO4- + H+ -> H2SO4

H+ from benzene

20
Q

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

A

X2 + AlX3 -> X+ + AlX4-

21
Q

How is the halogen carrier catalyst regenerated?

A

AlX4- + H+ -> AlX3 + HX

22
Q

What is the structure of phenol?

A

C6H6O

OH attached to benzene ring

23
Q

What are the products of phenol reacting with NaOH?

A

Sodium phenoxide: C6H5O-Na+

Water

24
Q

Why does phenol not react with Na2CO3?

A

Too weak of an acid to react with weak base

25
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

26
Q

What are the reaction conditions of bromination of phenol?

A

Room temperature

No catalyst

27
Q

Why does phenol react so readily with bromine?

A

OH group acts to increase reactivity of benzene ring

28
Q

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

A

Activating group

29
Q

Define activation

A

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

30
Q

Define deactivation

A

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

31
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

32
Q

What do activating groups do with their electrons in benzene?

A

Donate electrons

33
Q

What do deactivating groups do with their electrons in benzene?

A

Withdraw electrons

34
Q

Where do the NH2 and OH groups direct on benzene?

A

2,4 and 6 positions

35
Q

Where does the NO2 group direct on benzene?

A

3 and 5 positions

OCR A-Level Chemistry (34 decks)

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