Chapter 25 - Aromatic Chemistry Flashcards
What are the two models for benzene?
1) kekulè model
2) delocalised model
What is the kekulè model?
In 1865, Kekulè proposed that benzene was made up of a planar ring of carbon atoms with alternating single and double bonds between them. Each carbon atom was bonded to one hydrogen atom.
What is benzene’s molecular formula
C6H6
Why was Kekulè’s model disproved ?
1) the C=C bond should decolourise bromine water during an electrophillic addition reaction however benzene does not decolourise bromine water under normal conditions so it does not undergo electrophillic addition reactions, suggesting benzene does not have double bonds.
2) Bond lengths can be measured using x-ray diffraction. The bond length of a single bond is 0.153nm and for a double bond is 0.134nm however in benzene all bonds were the same length (0.139nm).
3) cyclohexene has a hydrogenation enthalpy of -120 kj/mol therefore you would expect benzene to be 3x this so -360 kj/mol however it was actually -208 kj/mol so it was less exothermic than expected. This suggests benzene was more stable than the kekulè model.
What is the delocalised model of benzene?
1) The delocalised model says that the p-orbitals of all six carbon atoms overlap to create a pi system.
2) the Pi system is made up of two ring-shaped clouds of electrons, one above and below the plane of the six carbon atoms.
3) all the bonds in the ring are the same length because all the bonds are the same.
4) the electrons in the rings are said to be delocalised because they don’t belong to a specific carbon atom. They are represented as a circle inside the ring of carbons rather than double or single bonds.
When is benzene the parent chain?
1) halogens attached eg chlorobenzene
2) alkyl group (hydrocarbon less than 7Cs)
3) Nitro groups - NO2
When is benzene considered the substituting chain?
1) an alkyl chain with a functional group - 3- phenylpropanoic acid
2) an alkyl group with seven or more carbon atoms
In this case, the prefix phenyl- is used eg phenylamine
What are the exceptions when naming aromatic compunds?
1) phenylamine
2) benzoic acid
3) benzaldehyde
What type of reaction does benzene undergo?
Electrophillic substitution
What is Electrophillic substitution?
This is when a hydrogen atom is being substituted by an electrophile.
How does Electrophillic substitution work?
1) arrow from delocalised ring to electrophile
2) incomplete ring covering 4 carbons with + in the middle and electrophile and hydrogen attached
3) arrow from bond on hydrogen to incomplete ring.
4) final product without hydrogen attached.
What are the conditions for the nitration of benzene?
1) Sulfuric acid catalyst
2) heat to 50 degrees Celsius (water bath)
3) concentrated nitric acid
What is the mechanism for nitration of benzene?
1) HNO3 + H2SO4 -> NO2+, + HSO4 - + H20
2) NO2 + is electrophile and reacts with benzene in regular Electrophillic substitution reaction.
3) H+ + HSO4- -> H2SO4 - catalyst reformed
What are the conditions for the halogenation of Benzene?
1) halogen carrier catalyst eg AlCl3, FeCl3, AlBr3 etc
2) room temperature and pressure
Mechanism for the bromination of benzene?
1) Br2 + FeBr3 -> FeBr4 - + Br+
2) the electrophile Br+ undergoes regular Electrophillic substitution with benzene.
3) H+ + FeBr4- -> FeBr3 + HBr
What is alkylation of benzene?
H atom replaced with an alkyl group, which increases the number of c-c atoms in a compound.
It is sometimes called friedel-crafts alkylation
What are the conditions for alkylation of benzene?
1) reacts with haloalkane
2) AlCl3 acts as halogen carrier catalyst
Mechanism for alkylation of benzene to form methylbenzene?
1) AlCl3 + CH3Cl -> CH3+ + AlCl4-
2) CH3+ undergoes regular Electrophillic substitution reaction with benzene.
3) H+ + AlCl4- -> HCl + AlCl3
What are the conditions for Acylation of Benzene?
Reacts benzene with an acyl chloride in the presence of a halogen carrier catalysts (AlCl3) to form an aromatic ketone.
Acyl groups contain a C=O bond
Mechanism for acylation of benzene to form phenylethanone?
1) CH3COCl + AlCl3 -> CH3CO+ + AlCl4-
2) CH3CO+ undergoes regular Electrophillic substitution with benzene .
3) H+ + AlCl4- -> HCL + AlCl3
What is phenol?
An -OH bonded directly to benzene.
Molecular formula is C6H5OH
Why is phenol less soluble in water than alcohols?
Due to the presence of the non-polar benzene ring
What type of acid is phenol?
A weak acid
What does phenol partially dissociate in water to form?
A phenoxide ion (O- bonded to benzene) and H+
Phenol reacts with NaOH to form?
A salt (sodium phenoxide - O-Na+ bonded to benzene) and water
What is the conditions for bromination of phenol?
Room temperature
No halogen carrier catalyst
Equation for the bromination of phenol?
Phenol + 3Bromine →2,4,6-tribromophenol + 3hydrogen bromide
What are the observations for the reaction of bromine water with phenol?
1) decolourise bromine water
2) white precipitate forms from tribromophenol
Conditions for the nitration of phenol?
1) dilute nitric acid
2) room temperature
3) no catalyst required
Why does phenol react more readily than benzene?
1) the lone pair of electrons from the oxygen p-orbital of the -OH group is donated into the delocalised Pi-system of phenol.
2) the electron density of benzene ring in phenol is increased
3) therefore the ring is more likely to attract and polarise elctrophiles than benzene.
What is an electron donating functional group?
1) -OH or -NH2 functional group attached
2) substitution occurs more readily than benzene.
Why do electron donating functional groups allow the molecule to react more readily?
1) functional group will donate electrons into the delocalised ring in benzene
2) this increases the electron density in the ring, allowing the molecule to react more readily with electrophile
Where do electron donating groups direct substitution?
-2 and -4 positions
What is an electron withdrawing functional group?
1) -NO2 group attached - nitrobenzene
2) substitution occurs less easily than benzene. It requires extreme conditions eg halogen carrier and high temperatures
Why do electron withdrawing functional groups result in the molecule reacting less readily?
1) the functional group will withdraw electrons from the delocalised ring in benzene
2) this decreases the electron density in the ring resulting the molecule reacting less readily with electrophiles
Where do electron withdrawing groups direct substitution?
At the 3- position
What are an exception to the electron withdrawing/donating groups?
Halogens are Not electron donating groups however still direct substitution to the 2- and 4- position
