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Chemistry > 6.1.1 Aromatic compounds > Flashcards

6.1.1 Aromatic compounds Flashcards

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1
Q

What were the expectations of the Kekule’s model?

A
  • undergo addition reactions
  • react with halogens without catalyst
  • irregular hexagonal rings with alternating bond lengths
  • enthalpy change of hydrogenation to be 3x more exothermic than that for cyclohexene (-120*3 = 360)
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2
Q

What experimental evidence led to the updated model?

A
  • undergoes substitution reactions not addition
  • when it reacts it requires a catalyst
  • regular, intermediate in length, planar hexagonal ring - X-ray diffraction
  • enthalpy of hydrogenation found to be less exothermic than expected (-208kj mol-1 not-360) so more stable than kekule expected
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3
Q

What is the bonding in benzene?

A
  • each carbon atom forms 3 sigma bonds to 2 other carbon atoms and a H
  • also has an electron in the p orbital
  • these p orbitals overlap forming pi bonds
  • electrons in the pi bonds become delocalised leading to pi electron cloud above and below the ring
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4
Q

What is the overall reaction equation for the mononitration of benzene??

A

C6H6 + HNO3 -> C5H5NO2 + H2O

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5
Q

What are the conditions for the mononitration of benzene?

A

50-55 dc
conc HNO3
conc H2SO4 catalyst

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6
Q

What happens if the temperature in the mononitration of benzene is too high?

A

polynitration
multiple substitutions around the ring - explosive

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7
Q

What is the mechanism for mononitration of benzene?

A

electrophilic substitution

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8
Q

What is the equation for the formation of the electrophile in he mononitration of benzene?

A

HNO3 + H2SO4 -> NO2+ + HSO4- + H20
NO2+ is the electrophile
a pair of electrons from the cloud is attracted to the nitronium ion

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9
Q

What is the equation for the reformation of the catalyst?

A

H+ + HSO4- -> H2SO4

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10
Q

What is the overall reaction equation for the monobromination of benzene?

A

C6H6 + Br2 -> C6H5Br + HBr

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11
Q

What is the equation for the formation of the catalyst in the monobromination of benzene?

A

FeBr3 + Br2 -> Br+ + FeBr4-

AlBr3 + Br2 -> Br+ AlBr4-

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12
Q

What is the equation for the reformation of the catalyst in the monobromination of benzene?

A

H+ + FeBr4- -> HBr + FeBr3

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13
Q

What is the alkylation of benzene?

A

substitution of H atom in benzene ring with an alkyl group like a methyl, ethyl, propyl, ect.

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14
Q

What is acylation of benzene?

A

substitution of H atom in benzene ring with an acyl group like an ethanoyl, propnanoyl

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15
Q

What conditions does alkylation and acylation involve?

A
  • electrophillic substitiution
  • halogen carrier catalyst AlBr3
  • Friedal-crafts reactions
  • both increase the number of carbon atoms in the structure
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16
Q

What happens in the alkylation reaction?

A

React benzene with a haloalkane in the presence of AlCl3 catalyst

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17
Q

What is the reaction for the production of ethylbenzene?

A

benzene + CH3CH2CL -> ethyl benzene + HCl

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18
Q

What is the formation of the electrophile for the production of ethyl benzene?

A

CH3CH2Cl + AlCl3 -> CH3CH2+ + AlCl4 -

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19
Q

What is the reformation of the catalyst for the production of ethylbenzene?

A

H+ + AlCl4- -> AlCl3 + HCl

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20
Q

What happens in an acylation reaction?

A

react benzene with acyl chloride in presence of AlCl3 catalyst

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21
Q

What is the overall reaction equation for the production of phenylethanone?

A

benzene + CH3COCL -> phenyl ethanone + HCl

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22
Q

How is the electrophile formed in the mechanism for the production of phenylethanone?

A

CH3COCL + AlCl3 -> CH3CO+ + AlCl4-

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23
Q

What is the reaction for the reformation of the catalyst for the production of phenylethanone?

A

H+ + AlCl4- -> AlCl3 + HCl

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24
Q

Compare the reactivity of alkenes and arenes

A

Benzene
- subs.
- catalyst e.g AlCl3
- pi electrons delocalised into the ring
- lower electron dens.
- can’t polarise Br2

Alkene
- add.
- no catalyst
- pi electrons localised
- high electron density
- can polarise Br2

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25
What is a phenol?
aromatic compounds with a hydroxyl bonded directly to a benzene ring
26
What type of acid is phenol?
weak
27
How does phenol react in water?
slightly soluble partially dissociated
28
What is the reaction equation for phenols dissociation?
C6H5OH -> C6H5O- +H+
29
What type of organic compound is phenol less acidic than?
carboxylic acids
30
What is phenol more acidic than?
ethanol
31
What bases can phenol react with?
strong bases like NaOH but not weak bases like Na2CO3
32
What is the reaction of phenol and NaOH?
neutralisation reaction salt and water
33
What is the reaction equation for the reaction of phenol with NaOH?
Phenol + NaOH -> sodium phenoxide + H2O C6H5OH + NaOH -> C6H5ONa + H2O
34
What happens in the reaction of phenol with bromine?
- Bromine is decolourisewd and a white precipitate of 2,4,6 - tribromophenol is formed
35
What happens when phenol reacts with nitric acid?
forms a mixture of 2-nitro phenol and 4-nitrophenol
36
What are the conditions for the reaction of phenol with nitric acid?
reacts at room temperature nitric acid needs to be concentrated conc sulfuric acid catalyst needed temperature of 50-55 degrees
37
Why is their increased reactivity of phenol?
the presence of the lone pair of electrons in the oxygen p-orbital of the hydroxyl group becomes delocalised in the delocalised ring of pi electrons of benzene ring electron density of the ring is increased ableto polarise / attarct electrophiles more easily
38
Compare how benzene and phenol react with bromine?
In benzene, benzene needs a halogen carrier catalyst to generate a Br+ electrophile In phenol, reacts without bromine without the need for catalyst
39
Compare how benzene and phenol react with HNO3?
In benzene, benzene needs a mixture of concentrated nitric acid and concentrated sulfuric acid and concentrated sulfuric acid at 50 degrees to generate a NO2+ electrophile In phenol, reacts at room temperature
40
Compare the electrons in benzene and phenol?
In benzene, pi electrons are delocalused low electron densiuty In phenol lone pair of electrons from opxygen p orbital of the hydroxyl group become delocalised into the pi electron cloud of the benzene ring increases electron density so more electrophiles attracted to phenol
41
What are the 2 types of aromatic directing groups?
- electron donating - electron withdrawing
42
What are electron doanting groups?
- OH and NH2 substituent groups -increase electron density at positions to 2,4,6 - electrophiles directed to these positions
43
What are electron withdrawing groups?
- NO2 group is electron withdrawing - lowers electron density at positions 2,4,6 - electrophiles directed to 3 and 5
44
2 chemical tests are carried out on aqueous solution of an aromatic compound Y. Results of the test show that Y decolourised Br2 and effervesed when reacted with sodium carbonate. What is the lowest nuber of caron atoms?
- Benzene cannot react with bromine without a halogen carrier but phenol can react without a halogen carrier : phenol - the Na2CO3 reacts with acid to form salt and water and CO2, which is the effervescence seen so there is a COOH group at the bottom of the phenol. Ans = 7
45
When bromine is reacted with nitrobenzene and phenylamine, what are the products>?
3- bromonitrobenzene 2-bromophenylamine
46
Are amide links ketones?
No as the carbon that it is bonded to the oxygen is not bonded to an alkyl group
47
Can phenols reacts with bases to form salts?
yes phenols are weakly acidic so they can react with bases
48
Why can phenols react with bases?
L.P on oxygen partially delocalises into benzene ring
49
Is a student correct if they said FeCL3 is not needed for quinol as it is more reactive than benzene?
the lone pair of electrons on the oxygen atom is partially delocalsed into the pi system making quinol more susceptible to electrophilic attack
50
What is the NO2 group reduced to?
NH2
51
How would you obtain a pure sample of 3-nitrobenzoic acid from an impure solid?
1) Recyrstallisation 2) Dissolve solid in minimal amoiunt of hot solvent 3) cool solution and filter solid 4) Wash with cool solvent and pat dry
52
A student synthesises 3-bromophenylamine into nitrobenzene but reduction occurs before bromination, explain the result and the 2 structures of the 2 isomers found?>
NH2 has a 2,4 directing effect so bromine will be at position 2 on one isomer and position 4 on another isomer
53
What is phenyl?
C6H5
54
What is a CH2Cl attached to a benzene ring?
chloromethyl benzene
55
What is the overall reaction equation for the halogenation of benzene?
C6H6 + Cl2 -> C6H5Cl + HCl
56
What is the equation for reforming the catalyst in the halogenation of benzene?
H+ +AlBr4- -> AlBr3 + HBr
57
What is the equation for forming the catalyst in the halogenation of benzene?
H+ + AlCl4- -> AlCl3 + HCl
58
What are properties of phenol?
soluble in water solid at RTP
59
How does phenol react with alkali?
phenol + alkali = salt and water
60
What is the equation for phenol and NaOH?
C6H5OH + NaOH = C6H5O- Na+ + H2O
61
How does phenol react with metal?
phenol + metal = salt and H2
62
What is the equation for phenol and K?
2C6H5OH + 2K = 2C6H5OK + H2
63
What is the observation of metal and phenol reacting?
solid / metal dissolves effervesence / bubbles
64
What are observations for the reaction of phenol and bromine?
- white ppt - bromine is decolourised
65
What is the reaction of phenol and bromine?
C6H5OH + 3Br2 = 2,4,6 tribromophenol + 3HBr
66
What is the mechanism for the reaction of phenol and bromine?
electrophilic substitution
67
Do you need a catalyst for the reaction of phenol and bromine?
no
68
Why is phenol more easily nitrated by benzene?
in phenol the electron density is hgiher than benzene so phenol is more susceptible to electrophilic attack phenol polarise electrophiles and accepts electrophiles more
69
How is phenol reactive?
the lone pair of electrons on the hydroxyl atom is partially delocalised into the p system making phenol more susceptible to electrophilic attack
70
What is the test for phenol?
Indicator/pH paper turns red / orange OR pH < 7 OR pH meter < 7 AND No reaction with Na2CO3/CO3 2–/carbonate 
71

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