Chapter 25 Aromatic Chemistry Flashcards
What is the molecular formula of Benzene?
C6H6
What is the empirical formula of Benzene?
CH
Describe Kekulés model of Benzene
The structure of Benzene was based on six membered ring of carbon atoms joined by alternating single and double bonds.
Explain the evidence that led to the development of the delocalised model of Benzene
Expectations of Kekule model:
-Reactive and be able to undergo Electrophilic addition—> eg, decolourise orange Bromine water
-Alternating carbon to carbon bond lengths as C—C are longer than C==C, resulting in a irregular hexagon
-The enthalpy change of Hydrogenation to be three times that of cyclohexene. The enthalpy change of hydrogenation for cyclohexene is -120KJmol-, therefore the enthalpy change for Benzene was expected to be -360KJmol-
Actual
-Benzene was unreactive and could only undergo substitution reactions in the presence of a catalyst.
Eg, reacts with Br in presence of halogen carrier catalyst
-X-Ray diffraction found that the Carbon to carbon bonds were all the same length resulting in a regular hexagon, therefore the Carbon- carbon bonds are intermediate between
C—C and C==C.
-The enthalpy change of hydrogenation was much lower than expected —> -208KJmol-
Therefore the enthalpy change of hydrogenation of Benzene was less exothermic than expected
Describe the delocalised structure of Benzene
Each Carbon atom has an electron in a p orbital
The adjacent p orbitals overlap sideways forming π bonds
The electrons in the π bonds become delocalised leading to a delocalised π bond system above and below the plane of the ring containing 6 delocalised electrons.
How are aromatic compounds with one substituent group named?
The Benzene ring is considered to be the parent chain.
Alkyl, nitro and halogen groups are all considered to be the prefixes.
How are aromatic compounds named when benzene is the substituent?
When the Benzene ring is attached to an alkyl chain with a functional group or to an alkyl chain with seven or more carbon atoms, the prefix phenyl is used.
How are aromatic compounds with two substituent groups named?
The ring is numbered, starting with one of the substituent groups.
The substituent groups are listed in alphabetical order using the smallest numbers possible.
What mechanism does Benzene undertake?
Electrophilic substitution
What are the different Electrophilic substitution reactions of Benzene?
-Nitration
-Halogenation
-Alkylation
-Acylation
Describe how Benzene undergoes Electrophilic substitution
A pair of electrons leaves the delocalised system to form a bond to the electrophile
This disrupts the stable delocalised system and therefore forms an unstable intermediate
To restore stability, the pair of electrons in the C-H moves back into the ring
There is an overall substitution of hydrogen
What are the conditions required for the nitration of Benzene?
Concentrated sulfuric acid
Heated to 50 degrees
What are the reagents required for the nitration of Benzene?
Benzene reacts with concentrated nitric acid
What is the overall equation for the nitration of Benzene?
C6H6 + HNO3 —> C6H5NO2 + H2O
What is mononitration?
When a single nitro group is substituted onto Benzene
What is polynitration?
When multiple nitro groups are substituted onto the benzene ring following a temperature rise that exceeds 50 degrees
What is the problem of polynitration?
Can create potentially explosive products
What is an electrophile?
Electron pair acceptor
What is the equation for the production of the Nitronium electrophile used in nitration?
HNO3 + H2SO4 —> NO2^+ + HSO4^- + H2O
What is the equation for the regeneration of the catalyst used in nitration?
H^+ + HSO4^- —> H2SO4
What are the conditions required for the halogenation of Benzene?
Reflux in the presence of a halogen carrier
(AlCl3/ AlBr3 / FeCl3/ FeBr3- the halogen carrier used must be the same as the halogen in the mechanism)
What are the reagents required for the halogenation of Benzene?
Benzene is reacted with a halogen in the presence of a halogen carrier catalyst
What is the overall equation for the chlorination of Benzene?
C6H6 + Cl2 ——> C6H5Cl + HCl
What is the equation for the production of the chloronium electrophile used in the chlorination of benzene?
AlCl3 + Cl2 —> AlCl4^- + Cl^+
Or
FeCl3 + Cl2 —> FeCl4^- + Cl^+
What is the equation for the regeneration of the catalyst used in halogenation?
FeCl4^- + H^+ —> FeCl3 + HCl
AlCl4^- + H^+ —> AlCl3 + HCl
FeBr4^- + H^+ —> FeBr3 + HBr
AlBr4^- + H^+ —> AlBr3 + HBr
What is the Alkylation of Benzene?
Substitution of a Hydrogen atom in the Benzene ring with an alkyl group- eg, methyl CH3, ethyl C2H5 to produce an alkyl Arene
What are the reagents required for the alkylation of benzene?
Benzene is reacted with a chloroalkane
What are the conditions required for the alkylation of Benzene?
Reflux in the presence of halogen carrier catalysts AlCl3 or FeCl3
What is the overall equation for the production of ethyl benzene?
C6H6 + CH3CH2Cl —–> C6H5CH2CH3 + HCl
What is the equation for the formation of the electrophile required for the production of ethyl benzene via alkylation?
CH3CH2Cl + AlCl3 —> CH3CH2^+ + AlCl4^-
What is the equation for the regeneration of the catalyst used in alkylation?
AlCl4^- + H^+ —> AlCl3 + HCl
Or
FeCl4^- + H^+ —> FeCl3 + HCl
What is the Acylation of Benzene?
Substitution of Hydrogen atom in a Benzene ring with an acyl group eg, ethanoyl CH3CO or propanoyl CH3CH2CO to produce a phenylketone.
What are the reagents required for the acylation of benzene?
React benzene with acyl chloride
What are the conditions required for the acylation of benzene?
Reflux in the presence of a halogen carrier catalyst AlCl3 or FeCl3
What is the overall equation for the production of phenylethanone via the acylation of Benzene?
C6H6 + CH3COCl —–> C6H5COCH3 + HCl
What is the equation for the formation of the electrophile required for the production of phenylethanone via acylation ?
CH3COCl + AlCl3 —> CH3CO^+ + AlCl4^-
What is the equation for the regeneration of the catalyst used in acylation?
AlCl4^- + H^+ —> AlCl3 + HCl
Or
FeCl4^- + H^+ —> FeCl3 + HCl
Compare the reactivity of alkenes with Benzene
Alkenes contain a π bond with localised electrons shared between two atoms, this forms an area of high electron density.
Therefore, allowing alkenes to induce a dipole in non polar molecules so they act as electrophiles during the mechanism of Electrophilic addition.
Benzene has π bond electrons which are delocalised and shared between more than two atoms, this forms an area of low electron density.
Therefore, the benzene ring is unable to induce a dipole in non-polar molecules such as Bromine.
In this case, a catalyst is required to generate the electrophile in order to undergo Electrophilic substitution.
What is the molecular formula of Phenol?
C6H5OH
What is the difference between a phenol and alcohol?
Phenol contains the -OH group attached directly to the Benzene ring
Alcohols mean the -OH group is bonded to a carbon side chain rather than the aromatic ring.
What is the acidic nature of Phenol?
Weak acid
Why is Phenol a weak acid?
Slightly soluble in water, therefore partially dissociates in water to produce H^+ ions and a Phenoxide ion.
Compare the acidity of Phenols with alcohols and carboxylic acids
Phenol is more acidic than alcohols but less acidic than carboxylic acids
Ethanol does not react with strong or weak bases
Phenol is able to react with strong bases but not weak bases (Na2CO3 - no effervescence)
Carboxylic acids are able to react with strong and weak bases (Na2CO3- effervescence)
Give an example of a strong base
NaOH
Give an example of a weak base
Na2CO3
What are the reactions of Phenol?
Sodium hydroxide
Bromination
Nitration
How does Phenol react with sodium hydroxide?
Phenol reacts with NaOH to form a salt and water
The salt is called Sodium Phenoxide
What is the equation for the neutralisation reaction of Phenol with sodium hydroxide?
C6H5OH + NaOH —> C6H5O^-Na^+ + H2O
How does Phenol react with Bromine?
Phenol reacts with aqueous Bromine using Electrophilic substitution to form 2,4,6-tribromophenol and HBr.
Describe the observations that take place during the bromination of Phenol?
Orange Bromine water is decolourised
A white precipitate of 2,4,6-tribromophenol is formed.
Describe the conditions used for the bromination of Phenol
Room temperature
No halogen carrier catalyst is required
How does Phenol react with Nitric acid?
Phenol reacts with dilute nitric acid at room temperature to form a mixture of 2-nitrophenol and 4-nitrophenol and water.
Describe the conditions required for the nitration of Phenol
Dilute nitric acid at room temperature
No catalyst of concentrated H2SO4 required
Why is Phenol more reactive than Benzene?
Lone pair of electrons in the Oxygen p orbital of the hydroxy group becomes delocalised in the delocalised ring of pi electrons in Benzene ring.
This activates the ring by increasing the electron density of the delocalised system
Therefore, phenol is able to polarise and attract electrophiles more strongly than Benzene.
Compare the reactivities of Benzene and Phenol
When Benzene undergoes nitration, Benzene needs concentrated HNO3 and concentrated H2SO4 which acts as a catalyst at 50 degrees to generate the NO2^+ electrophile.
When Benzene undergoes bromination, Benzene requires a halogen carrier catalyst (AlBr3/ FeBr3) under reflux to generate the Br^+ electrophile.
When Phenol undergoes nitration, Phenol reacts with dilute nitric acid at room temperature without the need of a catalyst
When Phenol undergoes bromination, Phenol reacts with Bromine at room temperature without the need of a catalyst.
Explain why the delocalised model of benzene accounts for the observed stability of benzene better than Kekules model
The delocalised model has π bond electron density spread out rather than having concentrated areas of electron density from separate localised bonds as suggested by Kekules model. Compounds containing delocalised electrons are more stable than those that do not have delocalised electrons.
What are directing groups?
Groups which have a directing effect on the position of any second substituent group on the Benzene ring.
What are the ortho positions?
2 or 6
What are the meta positions?
3 or 5
What is the para position?
4
What are electron donating groups?
Donate electron density to the benzene ring, this increases electron density at positions 2,4 and 6 so the electrophiles are then directed to these positions.
What are electron withdrawing groups?
Withdraw electron density from the benzene ring which lowers the electron density at positions 2,4 and 6. Therefore, the electrophile is then directed to positions 3 or 5.
What is the directing effect of -OH and -NH2 groups?
Electron donating groups meaning they direct substitution at positions 2 and 4.
What is the directing effect of the -NO2 group?
Electron withdrawing group which means it directs substitution at position 3.
Explain what is meant by delocalised π bond electrons?
Electrons shared between more than two atoms formed by the sideways overlap of adjacent p orbitals
Compare the structures of Benzene and the unstable intermediate of a mechanism
Benzene has a π bond delocalised system containing 6 delocalised electrons across 6 carbon atoms. This is because each carbon atom has an electron in a p orbital and adjacent p orbitals overlap sideways. The electrons in the π bond become delocalised leading to a delocalised π bond system above and below the plane of the benzene ring.
However, the intermediate has 4 delocalised electrons across 5 carbon atoms because a pair of electrons from the delocalised π bond system leaves the ring to form a bond with the electrophile. Therefore, creating an unstable intermediate.
