10. Aromatic Chemistry Flashcards
1
Q
What structure are arenes based on?
A
A benzene ring structure
2
Q
How many carbons and hydrogens does benzene contain?
A
6 of each
3
Q
How was the bonding in a benzene ring originally thought to be?
A
A ring of six carbon atoms containing alternate single and double bonds
4
Q
What is the IUPAC name of a cyclic structure with alternate single and double bonds?
A
Cyclohexa - 1, 3, 5 - triene
5
Q
What is the molecular formula of cyclohexa - 1, 3, 5 - triene?
A
C6H6
6
Q
What is the Kekule structure?
A
What the structure of benzene was originally thought to be; six carbon atoms with alternate single and double bonds
7
Q
Do double and single bonds have the same length?
A
No
8
Q
Why are double bonds shorter than single bonds?
A
The extra overlap of the π orbitals brings the atoms closer together and the bond is shorter
9
Q
What was the evidence that benzene could not have the Kekule structure?
A
Analysis of benzene shows all bonds have the same length; but single and double bonds are different lengths
10
Q
What are the length of the bonds in benzene?
A
0.142 nm - intermediate between single and double
11
Q
What is the length of a C-C bond?
A
0.154 nm
12
Q
What is the length of a C=C bond?
A
0.134 nm
13
Q
What was the alternate suggestion to the Kekule structure for benzene?
A
Contained delocalised electrons in six overlapping p orbitals at right angles to the plane of atoms
14
Q
Describe the structure of benzene
A
- delocalised electrons in six overlapping p orbitals at right angles to the plane of atoms
- p-orbitals overlap sideways with both neighbours and the six electrons are delocalised above and below the ring
15
Q
How can the delocalised electrons in benzene be represented?
A
As a circle within the hexagonal ring
16
Q
What effect do the p-electrons in benzene have on stability?
A
They have a stabilising effect, meaning it is hard for an addition reaction to take place
17
Q
What are arenes?
A
Hydrocarbons based on benzene
18
Q
Why is it hard for an addition reaction to take place with benzene?
A
The 6 delocalised p electrons have a stabilising effect
19
Q
What shape is benzene?
A
Planar
20
Q
What are all the bond angles in benzene?
A
120°
21
Q
What happens when benzene is added to water?
A
It doesn’t decolourise
22
Q
Why doesn’t benzene decolourise when added to water?
A
There are no double bonds so it is difficult for an addition reaction to take place
23
Q
What is more stable: benzene or cyclohexa - 1, 3, 5 - triene?
A
Benzene due to delocalised electrons
24
Q
What are the two ways aromatic compounds can be named?
A
- based on benzene
* based on phenyl
25
What do aromatic compounds all contain?
Benzene
26
What does it mean, if aromatic compounds are named based on benzene?
The benzene ring is classed as the main functional group
27
How are aromatic compounds named if they are named based on benzene?
With the suffix -benzene
28
How would a benzene ring with one chlorine atom instead of a hydrogen be named?
Chlorobenzene (C6H5Cl)
29
How would a benzene ring with one NO2 group instead of a hydrogen be named?
Nitrobenzene (C6H5NO2)
30
How would a benzene ring with one CH3 group instead of a hydrogen be named?
Methylbenzene (C6H5CH3)
31
How would a benzene ring with one CH2Cl group instead of a hydrogen be named?
Chloromethylbenzene (C6H5CH2Cl)
32
What does it mean, if aromatic compounds are named based on phenyl?
When the benzene is not the main functional group
33
How are aromatic compounds named if they are named based on phenyl?
The benzene is named as a prefix using -phenyl, with the suffix coming from the other functional group
34
How would a benzene ring with one NH2 group instead of a hydrogen be named?
Phenylamine (C6H5NH2)
35
How would a benzene ring with one COCH3 group instead of a hydrogen be named?
Phenylethanone (C6H5COCH3)
36
How would a benzene ring with one CH=CH2 group instead of a hydrogen be named?
Phenylethene (C6H5CH2CH2)
37
What are common examples of aromatic compounds which do not use benzyl or phenyl in full in the name?
* benzoic acid (C6H5COOH)
| * phenol (C6H5OH)
38
Structural formula for benzoic acid?
C6H5COOH
39
Structural formula for phenol?
C6H5OH
40
How are you supposed to know when you should name a compound based on phenyl or benzene?
There is no easy way to know - just learn common examples and use these to base the name of other molecules on
41
When should numbers be used in the names of aromatic compounds?
When there is more than one functional group bonded to the benzene ring
42
How can the unexpected stability of benzene be illustrated?
By considering thermochemical data from hydrogenation of benzene in comparison to that of cyclohexene
43
How does the enthalpy change for cyclohexene and benzene show that benzene has delocalised electrons?
* cyclohexene = - 120 kJ mol-1 (breaking 1 double bond in process)
* enthalpy change for benzene should be 3 x -120 = -360 kJmol-1 (but it is only -208kJmol-1)
* benzene reaction is less exothermic than expected illustrating benzene is more stable than that of the Kekule structure
44
What type of reactions does benzene not favour?
Those which would involve breaking up the delocalised system - so benzene won't readily undergo addition reactions
45
What do the delocalised electrons in benzene constitute?
An electron rich area
46
What does it mean, that delocalised electrons constitute an electron rich area in benzene?
They are susceptible to attack by electrophiles
47
Why does benzene undergo electrophilic substitution?
The electron rich area of delocalised electrons makes them susceptible to attack by electrophiles
48
What do the delocalised electrons prevent benzene reacting as?
An alkene
49
Why is electrophilic addition unlikely to occur in benzene?
The energy required for the ring to break in an electrophilic addition reaction is too high
50
What reactions do benzene undergo?
Electrophilic substitution
51
What does benzene undergo, as opposed to electrophilic addition?
Electrophilic substitution
52
Why does electrophilic substitution occur in benzene?
* presence of delocalised electrons means the benzene ring is electron rich
* therefore an electrophile that is able to accept an electron pair donated by the ring is the most likely type of reagent to react with benzene
53
In electrophilic substitution of benzene, why is the positive intermediate species more stable than a normal cation?
Due to the delocalised electrons
54
In electrophilic substitution of benzene, how is the stability of the ring restored?
A proton is eliminated
55
What type of electrophile is needed for electrophilic substitution with benzene? Why is this?
A 'powerful' one (a polar molecule or induced dipole isn't enough), in order to react with such a stable molecule
56
Can polar molecules be electrophiles in the electrophilic substitution of benzene?
No - because benzene is too stable so the electrophile needs to be more 'powerful'
57
What generates the electrophile in the electrophilic substitution of benzene?
A catalyst
58
What is nitration?
The electrophilic substitution of benzene where the electrophile is a nitronium ion
59
What is the formula for a nitronium ion?
NO₂⁺
60
In nitration, how is the nitronium ion generated?
By the reaction between nitric and sulphuric acid
61
What is the process of generating nitronium ions?
1. Nitric acid (the weaker acid) acts as a base and accepts a proton
H₂SO₄ + HNO₃ → H₂NO₃⁺ + HSO₄⁻
2. Water is lost
H₂NO₃⁺ → H₂O + NO₂⁺
62
What are the two equations and overall equation for how nitronium ions are formed?
H₂SO₄ + HNO₃ → H₂NO₃⁺ + HSO₄⁻
H₂NO₃⁺ → H₂O + NO₂⁺
overall: H₂SO₄ + HNO₃ → NO₂⁺ + HSO₄⁻ + H₂O
63
What happens with benzene, once a nitronium ion is generated?
Electrophilic substitution takes place
64
What is the overall equation for benzene and nitric acid?
C6H6 + HNO3 → C6H5NO2 + H2O
65
What mechanism is nitration?
Electrophilic substitution
66
What are the reagents for nitration?
Concentrated nitric acid and concentrated sulphuric acid
67
What are the conditions for nitration?
Warm to 50°C
68
In nitration, what must be ensured about the temperature?
That it doesn't rise above 55°C
69
Why in nitration is the temperature 'warm to 50°C'?
Otherwise further substitution will occur
70
What does nitration produce?
Compounds which can be turned into other molecules ie they are useful in the synthesis of other organic compounds
71
What are the uses of nitration?
* synthesis of amines and dyes
| * manufacture of explosives
72
How is nitrobenzene used in the synthesis of amines and dyes?
It can be reduced to phenyl amine by treating it with tin and concentrated HCl and heating under reflux
73
How can nitrobenzene be reduced to phenyl amine?
By treating it with tin and concentrated HCl and heating under reflux
74
Why must nitrobenzene be heated under reflux to form phenylamine?
Otherwise it is impossible to introduce NH2 into a benzene ring
75
Why should LiAlH₄ and NaBH₄ not be used in the reduction of nitrobenzene to phenyl amine?
Since they give other products
76
Equation for the reduction of nitrobenzene to form phenyl amine?
C6H5NO2 + 6[H] → C6H5NH2 + 2H2O
77
What can be used as a reducing agent, as opposed to tin and concentrated HCl, for the reduction of nitrobenzene to phenyl amine?
Hydrogen gas in the presence of a suitable catalyst
78
What type of reaction is that of nitrobenzene to form phenyl amine?
Reduction
79
What are phenyl amines used to manufacture?
Dyes
80
What happens to nitro compounds when heated?
Decompose violently
81
Why are nitro compounds used as explosives?
They decompose violently when heated
82
What is the IUPAC name for TNT?
2, 4, 6 - trinitromethylbenzene
83
How is TNT made?
By introducing 3 nitro groups into methyl benzene by reacting with concentrated nitric acid and sulphuric acids
84
What are Friedel-Crafts acylation reactions?
Electrophilic substitution where a hydrogen atom on the benzene ring is substituted for an acylium ion
85
What is an acylium ion?
R - C = O⁻
86
In Friedel-Crafts acylation reactions, how is the acylium ion generated?
Using an acyl chloride and an anhydrous AlCl₃ catalyst
87
What are the main steps in Friedel-Crafts acylation reactions?
* acylium ion generated
* RCO⁺ acts as electrophile that attacks benzene ring
* AlCl₃ is then regenerated
88
What is the equation for the formation of an acylium ion using an acyl chloride and an anhydrous AlCl₃ catalyst?
R-COCl + AlCl₃ → R-CO⁺ + AlCl₄⁻
89
Why is the formation of an acylium ion possible?
As the Al atom can accept a lone pair from the chloride atom in the acyl chloride forming a coordinate bond
90
What is the equation for the regeneration of the AlCl₃ catalyst in the Friedel-Crafts acylation reactions?
AlCl₄⁻ + H⁺ → AlCl₃ + HCl
91
What 3 reactions are covered in the aromatic chemistry topic?
1. nitration (benzene → nitrobenzene)
2. acylation (benzene → phenylketone)
3. reduction (nitrobenzene → phenyl amine)
