6.1.1 Aromatic compounds Flashcards
What were the expectations of the Kekule’s model?
- undergo addition reactions
- react with halogens without catalyst
- irregular hexagonal rings with alternating bond lengths
- enthalpy change of hydrogenation to be 3x more exothermic than that for cyclohexene (-120*3 = 360)
What experimental evidence led to the updated model?
- undergoes substitution reactions not addition
- when it reacts it requires a catalyst
- regular, intermediate in length, planar hexagonal ring - X-ray diffraction
- enthalpy of hydrogenation found to be less exothermic than expected (-208kj mol-1 not-360) so more stable than kekule expected
What is the bonding in benzene?
- each carbon atom forms 3 sigma bonds to 2 other carbon atoms and a H
- also has an electron in the p orbital
- these p orbitals overlap forming pi bonds
- electrons in the pi bonds become delocalised leading to pi electron cloud above and below the ring
What is the overall reaction equation for the mononitration of benzene??
C6H6 + HNO3 -> C5H5NO2 + H2O
What are the conditions for the mononitration of benzene?
50-55 dc
conc HNO3
conc H2SO4 catalyst
What happens if the temperature in the mononitration of benzene is too high?
polynitration
multiple substitutions around the ring - explosive
What is the mechanism for mononitration of benzene?
electrophilic substitution
What is the equation for the formation of the electrophile in he mononitration of benzene?
HNO3 + H2SO4 -> NO2+ + HSO4- + H20
NO2+ is the electrophile
a pair of electrons from the cloud is attracted to the nitronium ion
What is the equation for the reformation of the catalyst?
H+ + HSO4- -> H2SO4
What is the overall reaction equation for the monobromination of benzene?
C6H6 + Br2 -> C6H5Br + HBr
What is the equation for the formation of the catalyst in the monobromination of benzene?
FeBr3 + Br2 -> Br+ + FeBr4-
AlBr3 + Br2 -> Br+ AlBr4-
What is the equation for the reformation of the catalyst in the monobromination of benzene?
H+ + FeBr4- -> HBr + FeBr3
What is the alkylation of benzene?
substitution of H atom in benzene ring with an alkyl group like a methyl, ethyl, propyl, ect.
What is acylation of benzene?
substitution of H atom in benzene ring with an acyl group like an ethanoyl, propnanoyl
What conditions does alkylation and acylation involve?
- electrophillic substitiution
- halogen carrier catalyst AlBr3
- Friedal-crafts reactions
- both increase the number of carbon atoms in the structure
What happens in the alkylation reaction?
React benzene with a haloalkane in the presence of AlCl3 catalyst
What is the reaction for the production of ethylbenzene?
benzene + CH3CH2CL -> ethyl benzene + HCl
What is the formation of the electrophile for the production of ethyl benzene?
CH3CH2Cl + AlCl3 -> CH3CH2+ + AlCl4 -
What is the reformation of the catalyst for the production of ethylbenzene?
H+ + AlCl4- -> AlCl3 + HCl
What happens in an acylation reaction?
react benzene with acyl chloride in presence of AlCl3 catalyst
What is the overall reaction equation for the production of phenylethanone?
benzene + CH3COCL -> phenyl ethanone + HCl
How is the electrophile formed in the mechanism for the production of phenylethanone?
CH3COCL + AlCl3 -> CH3CO+ + AlCl4-
What is the reaction for the reformation of the catalyst for the production of phenylethanone?
H+ + AlCl4- -> AlCl3 + HCl
Compare the reactivity of alkenes and arenes
Benzene
- subs.
- catalyst e.g AlCl3
- pi electrons delocalised into the ring
- lower electron dens.
- can’t polarise Br2
Alkene
- add.
- no catalyst
- pi electrons localised
- high electron density
- can polarise Br2
What is a phenol?
aromatic compounds with a hydroxyl bonded directly to a benzene ring
What type of acid is phenol?
weak
How does phenol react in water?
slightly soluble
partially dissociated
What is the reaction equation for phenols dissociation?
C6H5OH -> C6H5O- +H+
What type of organic compound is phenol less acidic than?
carboxylic acids
What is phenol more acidic than?
ethanol
What bases can phenol react with?
strong bases like NaOH
but not weak bases like Na2CO3
What is the reaction of phenol and NaOH?
neutralisation reaction
salt and water
What is the reaction equation for the reaction of phenol with NaOH?
Phenol + NaOH -> sodium phenoxide + H2O
C6H5OH + NaOH -> C6H5ONa + H2O
What happens in the reaction of phenol with bromine?
- Bromine is decolourisewd and a white precipitate of 2,4,6 - tribromophenol is formed
What happens when phenol reacts with nitric acid?
forms a mixture of 2-nitro phenol and 4-nitrophenol
What are the conditions for the reaction of phenol with nitric acid?
reacts at room temperature
nitric acid needs to be concentrated
conc sulfuric acid catalyst needed
temperature of 50-55 degrees
Why is their increased reactivity of phenol?
the presence of the lone pair of electrons in the oxygen p-orbital of the hydroxyl group
becomes delocalised in the delocalised ring of pi electrons of benzene ring
electron density of the ring is increased
ableto polarise / attarct electrophiles more easily
Compare how benzene and phenol react with bromine?
In benzene,
benzene needs a halogen carrier catalyst to generate a Br+ electrophile
In phenol,
reacts without bromine without the need for catalyst
Compare how benzene and phenol react with HNO3?
In benzene,
benzene needs a mixture of concentrated nitric acid and concentrated sulfuric acid and concentrated sulfuric acid at 50 degrees to generate a NO2+ electrophile
In phenol,
reacts at room temperature
Compare the electrons in benzene and phenol?
In benzene, pi electrons are delocalused
low electron densiuty
In phenol
lone pair of electrons from opxygen p orbital of the hydroxyl group become delocalised into the pi electron cloud of the benzene ring
increases electron density so more electrophiles attracted to phenol
What are the 2 types of aromatic directing groups?
- electron donating
- electron withdrawing
What are electron doanting groups?
- OH and NH2 substituent groups
-increase electron density at positions to 2,4,6 - electrophiles directed to these positions
What are electron withdrawing groups?
- NO2 group is electron withdrawing
- lowers electron density at positions 2,4,6
- electrophiles directed to 3 and 5
2 chemical tests are carried out on aqueous solution of an aromatic compound Y.
Results of the test show that Y decolourised Br2 and effervesed when reacted with sodium carbonate. What is the lowest nuber of caron atoms?
- Benzene cannot react with bromine without a halogen carrier but phenol can react without a halogen carrier : phenol
- the Na2CO3 reacts with acid to form salt and water and CO2, which is the effervescence seen so there is a COOH group at the bottom of the phenol.
Ans = 7
When bromine is reacted with nitrobenzene and phenylamine, what are the products>?
3- bromonitrobenzene
2-bromophenylamine
Are amide links ketones?
No as the carbon that it is bonded to the oxygen is not bonded to an alkyl group
Can phenols reacts with bases to form salts?
yes
phenols are weakly acidic so they can react with bases
Why can phenols react with bases?
L.P on oxygen partially delocalises into benzene ring
Is a student correct if they said FeCL3 is not needed for quinol as it is more reactive than benzene?
the lone pair of electrons on the oxygen atom
is partially delocalsed into the pi system
making quinol more susceptible to electrophilic attack
What is the NO2 group reduced to?
NH2
How would you obtain a pure sample of 3-nitrobenzoic acid from an impure solid?
1) Recyrstallisation
2) Dissolve solid in minimal amoiunt of hot solvent
3) cool solution and filter solid
4) Wash with cool solvent and pat dry
A student synthesises 3-bromophenylamine into nitrobenzene but reduction occurs before bromination, explain the result and the 2 structures of the 2 isomers found?>
NH2 has a 2,4 directing effect
so bromine will be at position 2 on one isomer
and position 4 on another isomer
What is phenyl?
C6H5
What is a CH2Cl attached to a benzene ring?
chloromethyl benzene
What is the overall reaction equation for the halogenation of benzene?
C6H6 + Cl2 -> C6H5Cl + HCl
What is the equation for reforming the catalyst in the halogenation of benzene?
H+ +AlBr4- -> AlBr3 + HBr
What is the equation for forming the catalyst in the halogenation of benzene?
H+ + AlCl4- -> AlCl3 + HCl
What are properties of phenol?
soluble in water
solid at RTP
How does phenol react with alkali?
phenol + alkali = salt and water
What is the equation for phenol and NaOH?
C6H5OH + NaOH = C6H5O- Na+ + H2O
How does phenol react with metal?
phenol + metal = salt and H2
What is the equation for phenol and K?
2C6H5OH + 2K = 2C6H5OK + H2
What is the observation of metal and phenol reacting?
solid / metal dissolves
effervesence / bubbles
What are observations for the reaction of phenol and bromine?
- white ppt
- bromine is decolourised
What is the reaction of phenol and bromine?
C6H5OH + 3Br2 = 2,4,6 tribromophenol + 3HBr
What is the mechanism for the reaction of phenol and bromine?
electrophilic substitution
Do you need a catalyst for the reaction of phenol and bromine?
no
Why is phenol more easily nitrated by benzene?
in phenol the electron density is hgiher than benzene
so phenol is more susceptible to electrophilic attack
phenol polarise electrophiles and accepts electrophiles more
How is phenol reactive?
the lone pair of electrons on the hydroxyl atom
is partially delocalised into the p system
making phenol more susceptible to electrophilic attack
What is the test for phenol?
Indicator/pH paper turns red / orange OR pH < 7
OR pH meter < 7
AND
No reaction with Na2CO3/CO3
2–/carbonate
