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Chemistry ALEVEL - Ocr A > 25 - Aromatic Compounds > Flashcards

25 - Aromatic Compounds Flashcards

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1
Q

What is an aromatic hydrocarbon/ arene

A

Hydrocarbon within alternating double and single bonds between C atoms, forming rings

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2
Q

What is kekules structure of benzene

A

Consists of a hexagonal planar ring of 6 carbons
Each C atom is joined to 2 other C’s and 1 hydrogen
With alternating double and single bonds

Adapted to say constantly flipping between 2 isomers

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3
Q

What evidence is there to disprove kekules model

3

A

1- lack of reactivity of benzene
(Doesn’t decoularise benzene)

2- the lengths of carbon bonds
(Using x Ray diffraction)

3- unexpected hydrogenation enthalpies

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4
Q

What are some features of benzene

A 
It is colourless 
Sweet smelling 
Naturally found in crude oil
Found in cigarette smoke and petrol
Classified as a carcinogen
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5
Q

What are the 2 possible structures of benzene

A

Kekulé model

Delocalised model

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6
Q

What does the lack of reactivity of benzene suggest

A

Unlike alkenes
Benzene doesn’t undergo electrophilic addition reactions
Bromine can not be added across the double bonds
Doesn’t decolourise bromine water

Supports evidence to delocalised model

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7
Q

Why does a difference in length of carbon carbon bonds disprove kekule model

X Ray diffraction

A

If it was right you would expect 3 bonds with length of a carbon single bond and 3 the length of carbon double bond
However

All bond lengths are the same in benzene
Length of 140pm

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8
Q

How does benzenes hydrogenation enthalpies disprove kekule

A

Would be expected to have enthalpy of hydrogenation 3 x that of cyclohexene (3x-120=360)
But it is actually -208kjmol-1

This is less exothermic than expected
Meaning more energy was inputted to break bonds of benezene than would be expected to break kekules bonds

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9
Q

What does benzene breaking bonds being far less exothermic than expected suggest

A

It’s resistance to reaction suggests it’s more stable

Stability brought by delocalised ring of electron density

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10
Q

What is the delocalised model of benzene

A
  • Benzene is a hexagonal planar made up of 6 carbons.
  • Each carbon uses 3 of its 4 valence electrons in bonding to 2 other carbons and 1 hydrogen
  • Each C has 1 electron in a P orbital at RA to benzene planar
  • Adjacent P orbitals overlap sideways in both directions, above and below the planar
  • Creates pi bonds that spread over all 6 carbons = pi bonds are delocalised
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11
Q

Substituent définition

A

Atom/group of atoms which replaces 1 or more H atoms n the parent chain of a hydocarbon

Parent chain = benzene

1 = monosubstituted 
2= disubstituted
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12
Q

Prefixes to benzene (parent chain)

A

Alkyl
Halogens = bromo iodo chloro
Nitro = NO2

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13
Q

When is the prefix phenyl used instead of benzene

A

When a benzene ring is attached to

An alkyl chain with 7 or more Cs
Alkyl chain with functional group

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14
Q

Exceptions in naming compounds

A

Benzoic acid = carboxylic acid functional group on benzene

Phenylamine = NH2 on benzene

Benzaldehyde = CHO on benzene

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15
Q

Naming disubstituted compounds

A

Numbers

Add to use lowest numbers

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16
Q

What makes benzene stable

A

The delocalised model has pi bond electron density spread out over the 6 carbons

Compounds containing delocalised electrons are more stable than those that don’t

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17
Q

What happens in benzenes electrophilic substitutions

A

A hydrogen atom on benzene is replaced by another atom/group of

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18
Q

What is an electrophile

A

An electron pair acceptor

that is attracted to areas of rich electron density, where it accepts a pair of electrons to form new covalent bonds

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19
Q

What conditions are needed in the nitration of benzene

H is replaced by NO2 group

A

Reacts slowly with HNO3= nitric acid

In presence of H2SO4 = sulfuric acid

Heated to 50 in water bath

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20
Q

What happens it temperature rises over 50* in nitration of benzene

A

Further substitution reactions will happen

At 70* dinitrobenzene is formed

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21
Q

What is nitrobenzene useful for

A

Dyes
Pharmaceuticals
Pesticides
Paracetamol

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22
Q

What is the electrophile in the nitration of benzene

A

NO2+

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23
Q

How is the electrophile produced in the nitration of benzene

First step

A

Reaction of conc. nitric acid and conc. sulfuric acid

HNO3 + H2SO4 —> HSO4- + H2O + NO2+

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24
Q

What happens in step 2 of the nitration of benzene

A
  • NO2+ accepts a pair of electrons from the benzene ring to form a dative covalent bond
  • the organic intermediate formed is unstable, and breaks down to form the organic product
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25
Q

What is the organic product made at the end of step 2 in the nitration of benzene

A

Nitrobenzene + H+ ion

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26
Q

What happens in step 3 of the reaction mechanism of nitration of benzene

A

The catalyst is regenerated at the end

H+ reacts with HSO4- to reform H2SO4

27
Q

Why do halogens need a halogen carrier to react with benzene

A

An electrophile is not polarised enough by the benzene ring, because there is insufficient electron density around any 2 carbons due to the delocalised ring

The halogen carrier makes the electrophile strong enough to attack the stable benzene ring

28
Q

How does a halogen carrier help to make good electrophiles

A

They accept lone pair of electrons from a halogen atom on the electrophile
As the lone pair is pulled away, the polarisation increases, and can form a carbocation

This makes the electrophile strong enough to react with the stable ring

29
Q

What is a halogen carrier

And examples

A

A halogen compound used during the halogénation of benzene

AlCl3
FeBr3
Fe

30
Q

What conditions are needed for the bromination of benzene

A

Room temp and pressure

Presence of halogen carrier

31
Q

What is the electrophile in bromination of benzene

A

Br+

32
Q

What is step 1 of the bromination of benzene

A

Br+ is generated by halogen carrier catalyst reacting with Bromine

Because benzene is too stable to react with non polar Bromine

Br2 + FeBr3 —> FeBr4- + Br+

33
Q

What is step 2 in the bromination of benzene

A

Bromonium ion accepts a pair of electrons from the benzene ring to form a dative covalent bond

This forms an unstable intermediate, which breaks down to produce the organic product and H+

34
Q

What is the organic product made in step 2 of bromination of benzene

A

Bromobenzene + H+

35
Q

What is step 3 of the bromination of benzene

A

The halogen carrier reforms by reacting with H+

FeBr4- + H+ —> FeBr3 + HBr

36
Q

What is an alkylation reaction

Friedel - crafts

A

The substitution of a hydrogen atom in the benzene ring by an alkyl group

It increases the number of carbon atoms in compound, by forming c-c bonds

37
Q

Conditions needed for the alkylation of benzene

A

Reaction of a halo alkane in the presence of AlCl3

Under reflux

38
Q

What is an acylation reaction

A

Hydrogen is replaced with an acyl chloride in presence of catalyst AlCl3

39
Q

What does an acylation reaction form

A

An aromatic ketone

Named phenyl

40
Q

Electrophilic addition in alkenes

A
  • Pi bond in alkene is localised above and below plane = high electron density
  • localised electrons induce dipole in non polar bromine, making one slightly positive
  • slightly positive bromine enables the bromine molecule to act like an electrophile
41
Q

What can’t benzene undergo electrophilic addition

A
  • Benzene has delocalised electrons spread above and below plane of carbon atoms in ring structure
  • The electron density is less than that in double bond of alkene
  • there is insufficient electron density around any 2 Cs to polarisé non polar bromine
  • prevents react taking place
42
Q

What are phenols

A

Type of organic chemical contains a hydroxy functional group

That is directly bonded to an aromatic ring

43
Q

When is a compound named an alcohol rather than phenol

A

Aromatic Alcohol =

OH group attached to a alkyl chain

2-phenylethanol

44
Q

What is phenol used in

A 
Paints 
Detergents 
Plastics 
Aspirin 
Disinfectants
45
Q

How was phenol original manufactured

A

Benzene + H2SO4 + NaOH —> C6H5OH + Na2SO3 + 2H2O

46
Q

How is phenol made nowadays

A

C6H6 + C3H6 +O2 —> CH3COCH3 + C6H5OH

47
Q

Why is phenol a weak acid

A

Is less soluble in water than alcohols
Because or non polar benzene ring

Only partially dissociates into a phenoxide ion and a proton when dissolved in water

48
Q

What is phenol more acidic than

A

Ethanol

Because does not react with strong base = NaOH
Or weak base = Na2CO3

49
Q

What is phenol less acidic than

A

Carboxylic acids

Strong enough to react with weak base = sodium carbonate Na2CO3

This reaction can be used to distinguish a phenol and carboxylic acid = carboxylic produces CO2

50
Q

Phenol in neutralisation reactions

A

Room temp
Phenol reacts with NaOH solution

Forming sodium phenoxide (salt) and water

51
Q

Bromination of phenol

A

Phenol reacts with bromine water - 3Br2 - aqueous solution of bromine)

Forms a white precipitate = decolourises bromine water
Of 246-Tribromophenol
And 3HBr

52
Q

Conditions needed for bromination of phenol

A

No halogen carrier catalyst

Room temp

53
Q

What happens in Nitration of phenol

A

Phenol readily reacts with DILUTE nitric acid
No sulfuric acid needed

Produces 2 isomers and water
2 nitrophenol
4 nitrophenol

54
Q

What are the differences in reactivity of phenol and benzene

A

Bromine and nitric acid react more readily

Phenol does not require a halogen carrier catalyst - or need CONCENTRATED nitric acid with a sulphuric acid catalyst

55
Q

Why does phenol have and increased reactivity

Compared to benzene

A

Lone pair of electrons in p orbital of oxygen on hydroxy overlaps with delocalised ring of electron in benzene ring

The lone pair is partially delocalised into the pisystem (donated)

Increases the electron density in benzene ring

Attracts more electrophiles

56
Q

Why does phenol not need a halogen carrier in bromination

A

The electron density in the pisystem has increased

Making it more susceptible to attack from electrophiles than in benzene

The electron density is sufficient enough to polarise bromine

So no need for halogen carrier

57
Q

What is the difference between unsubstituted benzene ring and substituted benzene ring

A

Unsubstituted
- electrophiles can react with any of the C atoms

Substituted
- the functional group can change the electron density at certain C atoms, making them more or less likely to react

58
Q

Conditions for bromination with phenylamine NH2

A

+ Br2

Tribromophenylamine + 3HBr

Reacts readily

59
Q

Bromination with nitrobenzene

A

Requires halogen catalyst

High temperature

60
Q

Electron donating groups

activates the ring

A

Activates the ring as the aromatic ring reacts more readily with electrophiles

Have electrons in orbitals that overlap with delocalised ring
Increase the density

At 2,4,6

61
Q

Example of electron donating groups

2,4,6 directing

A

NH2

OH

62
Q

Electron withdrawing groups

Deactivate

A

Deactivates the aroma tingling, as the aromatic ring reacts less readily with electrophiles

Are electronegative
So withdraw electron density at carbons 246
So becomes unlikely to react in these positisions

63
Q

Examples of electron withdrawing groups

3,5 directing

A

NO2

Chemistry ALEVEL - Ocr A (8 decks)

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