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Chemistry > Chapter 25 - Aromatic Chemistry > Flashcards

Chapter 25 - Aromatic Chemistry Flashcards

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1
Q

What is the formula of benzene?

A

C6H6

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2
Q

Define aromatic compound

A

an organic substance that contains a benzene ring

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3
Q

List 3 things benzene rings can be found in

A

pharmaceuticals, dyes and explosives

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4
Q

What is a -Cl attachment called in an aromatic compound?

A

chloro

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5
Q

What is a -Br attachment called in an aromatic compound?

A

bromo

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6
Q

What is a -CH3 attachment called in an aromatic compound?

A

methyl

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7
Q

What is a -NO2 attachment called in an aromatic compound?

A

nitro

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8
Q

What are numbers used to give the lowest possible combination of numbers called?

A

Locant numbers

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9
Q

Define locant numbers

A

numbers used to give the lowest possible combination of numbers

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10
Q

What is a benzene group present in a molecule named as? What is this similar to?

A

Phenyl group

Methyl group for CH3

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11
Q

Who suggested the first structure of benzene?

A

Kekulé

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12
Q

What were the main 2 principles of his structure?

A

That there were 2 forms of benzene and that the position of the double bonds constantly alternated from one to the other

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13
Q

What is more reactive?

c=c or c-c

A

c=c because the pi bond is easier to break than the sigma bond

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14
Q

Based on the length of C bonds, explain why Kekulés structure must be incorrect

A

If there were both C=C and C-C the bonds would have different lengths, but in Benzene each bond is the same length, suggesting there are no C=C

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15
Q

Based on reaction types explain why Kekulés structure must be incorrect

A

Kekulés structure makes benzene look like cyclic ALKENE so we would expect it to undergo addition reactions to break the double bonds, however benzene undergoes substitution reactions which suggests there are no C=C bonds present

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16
Q

Based on energy profiles explain why Kekulés structure must be incorrect

A

Kekulés model suggests that the total enthalpy change of benzene should be the addition of the hydrogenation of each C=C bond, totalling -360kJmol-1, however experimentally it is actually around -200kJmol-1 suggesting that the suggested structure of benzene is not correct

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17
Q

State 3 reasons why Kekulé’s structure of benzene is incorrect

A
  1. The type of reactions benzene undergoes is not equivalent to the model
  2. The bond lengths of benzene is not equivalent to the model
  3. The enthalpy change of hydrogenation is not equivalent to the one suggested by the model.
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18
Q

State 10 key features of the delocalised model of benzene

A
  1. 6C and 6H atoms form a cyclic hydrocarbon (not alkene)
  2. These carbon atoms are arranged in a planar (which means flat) hexagonal ring
  3. The arrangement of atoms around each C atom is trigonal planar with bond angles of 120
  4. Each C atom has 4e-, 3 of which are bonded to 2C and 1H
  5. The 4th e- is located in a P- orbital which sticks up above and below the ring
  6. The P orbitals on each C atom overlap sideways with those on the 2 adjacent C atoms
  7. This forms a Pi cloud of electron density above and below the ring
  8. The pi cloud contains 6e-
  9. The density is spread out within the ring
  10. The electrons in the cloud are said to be delocalised
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19
Q

Which model of benzene is currently accepted to be accurate?

A

The delocalised model of benzene

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20
Q

Define delocalised

A

Not belonging to a specific atom

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21
Q

How many delocalised electrons are spread out within the pi cloud?

A

6e-

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22
Q

What shape does each C atom have in the delocalised model of benzene?

A

trigonal planar

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23
Q

What does each of the bonding electrons in a carbon atom bond to in the delocalised model of benzene?

A
  1. adjacent C atom
  2. adjacent C atom
  3. adjacent H atom
  4. not bonded - located in the p orbital above or below the c atom
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24
Q

What shape do the 6 C atoms have in the delocalised model of benzene?

A

Planar hexagonal ring

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25
Q

Is benzene more or less reactive than predicted by Kekulés model?

A

Less reactive -more stable

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26
Q

What 3 features show that benzene is less reactive than predicted by Kekulé?

A

Benzene does not react with bromine water, strong acids or halogens

27
Q

What are the 3 types of causes for mechanisms?

A

Electrophilic
Nucleophilic
Free radical

28
Q

What are the 3 types of effects for mechanisms?

A

Elimination
Addition
Substitution

29
Q

What type of mechanism is benzene susceptible to and why?

A

Electrophilic substitution
Does not contain C=C so no addition
Has an electron cloud with a strong electron density meaning electrophiles are attracted to it

30
Q

Explain the general mechanism of an electrophilic substitution reaction of benzene

A
  1. The electrophile is attracted to the benzene ring which has a high electron density
  2. The electrophile accepts a pair of electrons from the delocalised pi cloud
  3. A covalent bond is formed between the electrophile and C atom on benzene
  4. The delocalised pi cloud is disrupted
  5. Because of this the intermediate is highly reactive
  6. An electron pair from a covalent bond between the C atom and a H atom goes to restoring the benzene ring
  7. the H+ ion is rapidly lost
  8. The delocalised pi cloud is restored
31
Q

what conditions are required for the nitration of benzene?

A

Conc H2SO4, 50 degrees, reflux

32
Q

What group is substituted on in the nitration of benzene?

A

NO2+

33
Q

What reagent is required in the nitration of benzene? Why?

A

HNO3

It supplies the NO2+ ion

34
Q

Write the equation for the formation of a nitrate electrophile from its reagent

A

HNO3 + H2SO4 –> HSO4- + H2O

35
Q

What conditions are required for the electrophilic substitution of benzene

A

Conc H2SO4
50 degrees
Reflux

36
Q

What reagents can be used for the halogenation of benzene?

A

Br2 or Cl2

37
Q

What catalysts can be used for the halogenation of benzene?

A

AlCl3 or AlBr3

38
Q

Write the equation for the formation of a halogenation electrophile from its reagent

A

Br2 + FeBr3 –> Br+ + FeBr4-

39
Q

What reagents can be used for the alkylation of benzene?

A

haloalkane

40
Q

What catalysts can be used for the alkylation of benzene?

A

A halogen carrier such as AlCl3

41
Q

Write the equation for the formation of a alkylation electrophile from its reagent

A

Haloalkane + AlCl3 –> carbocation+ + AlCl4-

42
Q

What reagents can be used for the acylation of benzene?

A

Acyl chloride

43
Q

What catalysts can be used for the acylation of benzene?

A

AlCl3

44
Q

Write the equation for the formation of a acylation electrophile from its reagent

A

Acyl chloride + AlCl3 –> carbocation+ + AlCl4-

45
Q

Which of cylohexene and benzene are resistant to bromination?

A

benzene

46
Q

Explain why benzene is resistant to bromination

A

It has a lower pi electron density than cyclohexene which is not strong enough to polarise Br2

47
Q

List the directing groups

A

NH2, NO2 and OH

48
Q

how do electron donating groups act as a directing group?

A

It donates a lone pair of electrons to the benzene ring when bonded, which causes its electron density to increase and is therefore more reactive, and the benzene more readily attacks an electrophile

49
Q

Does a directing group make benzene more or less reactive?

A

More

50
Q

What does a directing group prevent the need of for benzene to react?

A

A cataylst

51
Q

Electron donating groups direct new groups on where in the benzene?

A

C 2 and 4

52
Q

Electron withdrawing groups direct new groups on where in the benzene?

A

C 3 and 5

53
Q

give examples of an electron donating group

A

NH2 OH

54
Q

give examples of an electron withdawing group

A

NO2

55
Q

Explain how electron withdrawing groups act as directing groups

A

They draw electrons away from the centre of benzene, making it less reactive and the benzene less readily attacks nitrophiles

56
Q

define phenols

A

A group of organic chemicals that contains an OH group attached directly to a benzene ring

57
Q

is phenol soluble

A

Yes slightly because it contains an OH group

58
Q

Is phenol acidic

A

Yes it can form weak acidic solutions due to its H+

59
Q

How can you establish between a phenol and carboxylic acid?

A

react with sodium carbonate - CA will react, phenol will not

60
Q

Phenol + NaOH –>

A

Phenoxide salt and water

61
Q

Is phenol more reactive than benzene? why/why not?

A

Yes because it has an OH group which increases the electron density and hence the reactivity

62
Q

Can benzene react with Br2 without a carrier?

A

No

63
Q

Can phenol react with Br2 without a carrier?

A

yes

64
Q

Phenol + dilute HNO3 –>

A

2 nitrophenol or 4 nitrophenol

Chemistry (21 decks)

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