10. Aromatic Chemistry Flashcards
What structure are arenes based on?
A benzene ring structure
How many carbons and hydrogens does benzene contain?
6 of each
How was the bonding in a benzene ring originally thought to be?
A ring of six carbon atoms containing alternate single and double bonds
What is the IUPAC name of a cyclic structure with alternate single and double bonds?
Cyclohexa - 1, 3, 5 - triene
What is the molecular formula of cyclohexa - 1, 3, 5 - triene?
C6H6
What is the Kekule structure?
What the structure of benzene was originally thought to be; six carbon atoms with alternate single and double bonds
Do double and single bonds have the same length?
No
Why are double bonds shorter than single bonds?
The extra overlap of the π orbitals brings the atoms closer together and the bond is shorter
What was the evidence that benzene could not have the Kekule structure?
Analysis of benzene shows all bonds have the same length; but single and double bonds are different lengths
What are the length of the bonds in benzene?
0.142 nm - intermediate between single and double
What is the length of a C-C bond?
0.154 nm
What is the length of a C=C bond?
0.134 nm
What was the alternate suggestion to the Kekule structure for benzene?
Contained delocalised electrons in six overlapping p orbitals at right angles to the plane of atoms
Describe the structure of benzene
- 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
How can the delocalised electrons in benzene be represented?
As a circle within the hexagonal ring
What effect do the p-electrons in benzene have on stability?
They have a stabilising effect, meaning it is hard for an addition reaction to take place
What are arenes?
Hydrocarbons based on benzene
Why is it hard for an addition reaction to take place with benzene?
The 6 delocalised p electrons have a stabilising effect
What shape is benzene?
Planar
What are all the bond angles in benzene?
120°
What happens when benzene is added to water?
It doesn’t decolourise
Why doesn’t benzene decolourise when added to water?
There are no double bonds so it is difficult for an addition reaction to take place
What is more stable: benzene or cyclohexa - 1, 3, 5 - triene?
Benzene due to delocalised electrons
What are the two ways aromatic compounds can be named?
- based on benzene
* based on phenyl
What do aromatic compounds all contain?
Benzene
What does it mean, if aromatic compounds are named based on benzene?
The benzene ring is classed as the main functional group
How are aromatic compounds named if they are named based on benzene?
With the suffix -benzene
How would a benzene ring with one chlorine atom instead of a hydrogen be named?
Chlorobenzene (C6H5Cl)
How would a benzene ring with one NO2 group instead of a hydrogen be named?
Nitrobenzene (C6H5NO2)
How would a benzene ring with one CH3 group instead of a hydrogen be named?
Methylbenzene (C6H5CH3)
How would a benzene ring with one CH2Cl group instead of a hydrogen be named?
Chloromethylbenzene (C6H5CH2Cl)
What does it mean, if aromatic compounds are named based on phenyl?
When the benzene is not the main functional group
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
How would a benzene ring with one NH2 group instead of a hydrogen be named?
Phenylamine (C6H5NH2)
How would a benzene ring with one COCH3 group instead of a hydrogen be named?
Phenylethanone (C6H5COCH3)
How would a benzene ring with one CH=CH2 group instead of a hydrogen be named?
Phenylethene (C6H5CH2CH2)
What are common examples of aromatic compounds which do not use benzyl or phenyl in full in the name?
- benzoic acid (C6H5COOH)
* phenol (C6H5OH)
Structural formula for benzoic acid?
C6H5COOH
Structural formula for phenol?
C6H5OH
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
When should numbers be used in the names of aromatic compounds?
When there is more than one functional group bonded to the benzene ring
How can the unexpected stability of benzene be illustrated?
By considering thermochemical data from hydrogenation of benzene in comparison to that of cyclohexene
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
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
What do the delocalised electrons in benzene constitute?
An electron rich area
What does it mean, that delocalised electrons constitute an electron rich area in benzene?
They are susceptible to attack by electrophiles
Why does benzene undergo electrophilic substitution?
The electron rich area of delocalised electrons makes them susceptible to attack by electrophiles
What do the delocalised electrons prevent benzene reacting as?
An alkene
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
What reactions do benzene undergo?
Electrophilic substitution
What does benzene undergo, as opposed to electrophilic addition?
Electrophilic substitution
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
In electrophilic substitution of benzene, why is the positive intermediate species more stable than a normal cation?
Due to the delocalised electrons
In electrophilic substitution of benzene, how is the stability of the ring restored?
A proton is eliminated
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
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’
What generates the electrophile in the electrophilic substitution of benzene?
A catalyst
What is nitration?
The electrophilic substitution of benzene where the electrophile is a nitronium ion
What is the formula for a nitronium ion?
NO₂⁺
In nitration, how is the nitronium ion generated?
By the reaction between nitric and sulphuric acid
What is the process of generating nitronium ions?
- Nitric acid (the weaker acid) acts as a base and accepts a proton
H₂SO₄ + HNO₃ → H₂NO₃⁺ + HSO₄⁻
- Water is lost
H₂NO₃⁺ → H₂O + NO₂⁺
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
What happens with benzene, once a nitronium ion is generated?
Electrophilic substitution takes place
What is the overall equation for benzene and nitric acid?
C6H6 + HNO3 → C6H5NO2 + H2O
What mechanism is nitration?
Electrophilic substitution
What are the reagents for nitration?
Concentrated nitric acid and concentrated sulphuric acid
What are the conditions for nitration?
Warm to 50°C
In nitration, what must be ensured about the temperature?
That it doesn’t rise above 55°C
Why in nitration is the temperature ‘warm to 50°C’?
Otherwise further substitution will occur
What does nitration produce?
Compounds which can be turned into other molecules ie they are useful in the synthesis of other organic compounds
What are the uses of nitration?
- synthesis of amines and dyes
* manufacture of explosives
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
How can nitrobenzene be reduced to phenyl amine?
By treating it with tin and concentrated HCl and heating under reflux
Why must nitrobenzene be heated under reflux to form phenylamine?
Otherwise it is impossible to introduce NH2 into a benzene ring
Why should LiAlH₄ and NaBH₄ not be used in the reduction of nitrobenzene to phenyl amine?
Since they give other products
Equation for the reduction of nitrobenzene to form phenyl amine?
C6H5NO2 + 6[H] → C6H5NH2 + 2H2O
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
What type of reaction is that of nitrobenzene to form phenyl amine?
Reduction
What are phenyl amines used to manufacture?
Dyes
What happens to nitro compounds when heated?
Decompose violently
Why are nitro compounds used as explosives?
They decompose violently when heated
What is the IUPAC name for TNT?
2, 4, 6 - trinitromethylbenzene
How is TNT made?
By introducing 3 nitro groups into methyl benzene by reacting with concentrated nitric acid and sulphuric acids
What are Friedel-Crafts acylation reactions?
Electrophilic substitution where a hydrogen atom on the benzene ring is substituted for an acylium ion
What is an acylium ion?
R - C = O⁻
In Friedel-Crafts acylation reactions, how is the acylium ion generated?
Using an acyl chloride and an anhydrous AlCl₃ catalyst
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
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⁻
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
What is the equation for the regeneration of the AlCl₃ catalyst in the Friedel-Crafts acylation reactions?
AlCl₄⁻ + H⁺ → AlCl₃ + HCl
What 3 reactions are covered in the aromatic chemistry topic?
- nitration (benzene → nitrobenzene)
- acylation (benzene → phenylketone)
- reduction (nitrobenzene → phenyl amine)
