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LGS OCR A-Level Chemistry A > Chapter 25 - Aromatic Compounds > Flashcards

Chapter 25 - Aromatic Compounds Flashcards

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

1
Q

What is benzene?

A

A cyclic hydrocarbon compound C6H6

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2
Q

what are some properties of benzene?

A

benzene is a sweet smelling, colourless, highly flammable liquid and is a carcinogen (cancer causing),

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3
Q

Define arene

A

An aromatic hydrocarbon containing one or more benzene rings

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4
Q

Define aromatic

A

Any compound containing one or more benzene rings

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5
Q

what are the three arguments against the kekule structure?

A

1) benzenes low chemical reactivity - doesn’t decolourise bromine like an alkene
2) carbon lengths in benzene are all the same but c=c is different length than c-c
3) benzenes enthalpy of hydrogenation is lower than expected. so its more stable than kekule structure.

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6
Q

what does delocalised mean?

A

delocalised electrons are shared between more than two atoms.

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7
Q

What causes benzene to have its unexpected properties?

A

the neighbouring carbons p-orbitals overlap and form delocalised π orbitals above and below the carbon structure. this is an delocalised electron ring.

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8
Q

what are benzenes unusual properties?

A

under normal conditions benzene does not:

1) decolourise bromine water
2) react with strong acids like HCL
3) react with halogens chlorine, bromine or iodine

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9
Q

why does benzene undergo substitution reaction rather than addition reactions?

A

in an addition reaction the electrons in the pi cloud would have to bond with the atom/group being added but this would make it more unstable hence its not energetically favourable hence it doesn’t happen.

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10
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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11
Q

what does the general electrophilic substitution of benzene look like?

A
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12
Q

What are the conditions for nitration of benzene?

A

Benzene reacts with conc HNO3 (Nitric acid) with conc H2SO4 ( sulphuric acid) acting as a catalyst at 50C

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13
Q

what happens in nitration if it gets hotter than 50 degrees?

A

In nitration if it gets hotter than 50 degrees more than one nitro (NO2) group may be added to the ring

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14
Q

nitration of benzene equation?

A

Benzene reacts + conc HNO3 (Nitric acid) = nitrobenzene + H2O

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15
Q

nitration of benzene mechanism? there are 3 steps…

A
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16
Q

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

A

Halogen carrier which acts as a catalyst

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17
Q

Give two examples of chlorine carriers

A

AlCl3
FeCl3

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18
Q

What is formed as a byproduct of halogenation of benzene?

A

H-X

(X= a halogen)

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19
Q

bromination of benzene equation? temperature?

A

C6H6 + Br2 = C6H5Br + HBr

happens at room temprature and in the prescence of a halogen carrier

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20
Q

bromination of benzene mechanism?

A
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21
Q

chlorination of benzene reaction?

A

C6H6 + Cl2 = C6H5Cl + HCl

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22
Q

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

A

Alkylation
Acylation

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23
Q

what is alkylation?

A

In alkylation, a haloalkane is reacted with an aromatic compound to introduce an alkyl group to the benzene group. a halogen carrier is required.

C6H6 + C2H5Cl = C6H5C2H5 + HCl

24
Q

Outline alkylation of benzene

A

Benzene + RCl in presence FeCl3
Forms alkylbenzene + HCl

Alkylbenzenes are derivatives of benzene, in which one or more hydrogen atoms are replaced by alkyl groups of different sizes.

25
Q

what is acylation?

A

In acylation an acyl chloride. RCOCL is reacted with an aromatic compound to form an aromatic ketone. A halogen carrier is required.

26
Q

Outline acylation of benzene

A

Benzene + RCOCl with FeCl3
Forms Phenylalkanone + HCl

example
benzene + ethanoyl chloride = phenylethanone + Hcl

27
Q

Name the mechanism for bromination of benzene

A

Electrophilic aromatic substitution

28
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+

29
Q

How does a nitrating mixture produce an electrophile?

A

H2SO4 + HNO3 -> NO2+ + HSO4- + H2O
Nitronium ion is electrophile

30
Q

How is the H2SO4 catalyst regenerated?

A

HSO4- + H+ -> H2SO4

(H+ from benzene)

31
Q

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

A

X2 + AlX3 -> X+ + AlX4-

32
Q

How is the halogen carrier catalyst regenerated?

A

AlX4- + H+ -> AlX3 + HX

33
Q

what is the mechanism for electrophilic addition in alkenes?

A
34
Q

What is the structure of phenol?

A

C6H6O
OH attached to benzene ring

35
Q

are phenols are an acid or base?

A

phenols are a weak acid

36
Q

why are phenols less soluble in water than alcohols?

A

phenols are less soluble than alcohols in water due to the presence of a non polar benzene ring

37
Q

is phenol more or less acidic than carboxylic acids and alcohols?

A

Phenol is more acidic than alcohol but less acidic than a carboxylic acid.

38
Q

What are the products of phenol reacting with NaOH?

A

phenol + NaOH = Sodium phenoxide( C6H5O-Na+) + Water

39
Q

Why does phenol not react with Na2CO3?

A

Too weak of an acid to react with weak base

40
Q

What is the reaction of phenol with excess Br2? what is seen?

A

C6H5OH + 3Br2 = H3OBr3 + 3Hbr

phenol decolourises bromine and the 2,4,6-tribromophenol forms a white precipitate

41
Q

What are the reaction conditions of bromination of phenol?

A

Room temperature
No halogen carrier catalyst required

42
Q

what is the equation for the Nitration of phenol?

A

C6H5OH + HNO3 =C6H4OHNO2 (2-nitrophenol) + H2O

note: HNO3 must be dilute nitric acid

43
Q

What is HNO3?

A

HNO3 is nitric acid

44
Q

Why does phenol react so readily with bromine?

A

OH group acts to increase reactivity of benzene ring

45
Q

What is the term for OH increasing benzenes’ reactivity?

A

Activating group

46
Q

Define activation

A

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

47
Q

compare the reactivity of benzene vs phenol for bromination and Nitration?

A

bromine and nitric acid react more readily with phenol than benzene.

48
Q

name the electron withdrawing and electron donating groups?

A

electron withdrawing= 3 directing and electron donating and 2,4 directing.

49
Q

Define deactivation?

A

A deactivating group decreases the rate electrophilic aromatic substitution happens in comparison to benzene.

50
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

51
Q

where are electron donating groups directing?

A

electron donating group are 2 and 4 directing.

52
Q

where are electron withdrawing groups directing groups?

A

electron withdrawing groups excluding halogens are 3 directing

53
Q

What do activating groups do with their electrons in benzene?

A

Donate electrons

54
Q

What do deactivating groups do with their electrons in benzene?

A

Withdraw electrons

55
Q

Where do the NH2 and OH groups direct on benzene?

A

2,4 and 6 positions

56
Q

Where does the NO2 group direct on benzene?

A

3 and 5 positions

57
Q

why do bromine and nitric acid react more readily with phenol than benzene?

A

bromine and nitric acid react more readily with phenol than benzene. This is caused by the lone pair of electrons from the oxygen p-orbital of the OH group is being donated into the π system of phenol.this increases the electron density of phenol. This means electrophiles are more strongly attracted to phenol hence the aromatic ring is more susceptible to attack.

LGS OCR A-Level Chemistry A (33 decks)

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