25-Aromatic Compounds Flashcards
Benzene properties
• Colourless with a sweet smell
• Found naturally in crude oil
• Carcinogenic
Kekule model
Evidence to disprove kekule
- lack of reactivity
- length of c-c bonds
- hydrogenation enthalpies
• Lack of reactivity
o Double bonds should decolourise bromine water
o Benzene does not discolourise bromine water
o Therefore doesn’t undergo electrophilic addition reactions
o Suggests benzene cant have any double bonds
• Length of C-C bonds
o Bond lengths can be measured using x-ray diffraction
o The bond length of a single bond = 0.153 nm
o Bond length of double bond = 0.134 nm
o In benzene, all the bonds were the same length (0.139 nm)
• Hydrogenation enthalpies
o The hydrogenation enthalpy of benzene is less exothermic than expected.
o The structure is more stable than the Kekulé model of benzene suggests.
Shape of benzene
trigonal planar
How is benzene formed
• The p-orbitals are above and below the plane of the benzene ring
• Sideways overlap of p-orbitals creates a delocalised ring of electron density above and below the plane of the benzene ring
Features of new benzene model
- Planar, cyclic, hexagonal hydrocarbon with 6 Cs and 6 Hs.
Main difference between kekules and new model
• In kekule or any double bond // electrons in pi bond are localized + in benzene they are delocalized
When is benzene a parent chain
Prefixes to benzene:
o Alkyl groups (any hydrocarbon chain with less than 7 C’s) – E.g. CH3, C2H5
o Halogens - F, Cl, Br
o Nitro groups - NO2
Name
propylbenzene
Name
bromobenzene
When is benzene considered a substituent chain
• An alkyl chain with a functional group
• An alkyl chain with seven or more carbon atoms
What to do when Benzene is a substituent chain
• In these cases, the prefix phenyl- is used
Name
2-phenyldecane
Name
3-phenylpropanoic acid
What to do with Compounds with more then 1 substituent on con benzene ring
• Need to number the carbon ring
• Numbering begins with one of the substituent groups
• Working out where the number starts – look to see if it is based on methylbenzene, chlorobenzene or nitrobenzene etc.
• The substituent groups are listed in alphabetical order using the smallest numbers possible
Practise
Explain the difference in reactivity between arenes and alkenes
Electrophilic substitution
• When benzene reacts with an electrophile, a hydrogen atom is replaced by another atom or group of atoms.
• The electrophile is attracted to the electron rich benzene ring.
What is the electrophile attracted to
electron rich benzene ring.
Electrophilic substitution of benzene and chloride ion
Electrophilic substitution with benzene - four types
- Nitration of Benzene
- Halogenation of Benzene (bromination and chlorination)
- Alkylation Reactions
- Acylation Reactions
Nitration of benzene
• reacts slowly with nitric acid to form nitrobenzene.
Conditions of Nitration of benzene
sulfuric acid catalyst and heat to 50°C (water bath)
Overall reaction Nitration of benzene
• One hydrogen atom is replaced by a nitro (-NO2) group
What happens at higher temps - Nitration of benzene
benzene will react with the excess nitric acid to form 1,3-dinitrobenzene.
Mechanism of nitration of benzene
Step 1 - Mechanism of nitration of benzene
formation of the electrophile = nitronium ion (NO2+)
• Formed by reacting concentrated nitric acid with concentrated sulfuric acid
Step 2 - Nitration of benzene
Electrophile (NO2+) reacts with benzene
• Electrophile accepts a pair of electrons from the benzene ring to form a dative covalent bond.
• Forms an unstable intermediate, which breaks down to form nitrobenzene and H+ ion
Horseshoe in intermediate rules
- mist take up more then half of the ring
- must be open at the position where substitution occurs
Don’t forget….. (nitration of benzene)
Ion in last drawing in step 2 (H+)
Step 3 - nitration of benzene
• H+ ion reacts with HSO4- generated in step 1 to reform the sulfuric acid catalyst.
• Halogens will ONLY react with benzene if…
there is a halogen carrier catalyst
Examples of halogen carrier catalysts
AlCl3, FeCl3, AlBr3, FeBr3
How are halogen carrier catalysts generated
in situ from the metal and the halogen
Halogenation of benzene - conditions
halogen carrier catalyst appropriate one), room temp + pressure.
Bromination - overall reaction
Why is the bromonium ion = Br+ made
• Benzene too stable to react with bromine on own
Bromination of benzene mechanism
Chlorination of benzene mechanism
Alkylation of benzene
• H atom replaced with an alkyl group - increases the number of carbon-carbon atoms in a compound
Alkylation of benzene conditions
• React benzene with a haloalkane in the presence of AlCl3 which acts as a halogen carrier catalyst.
Reaction of benzene and chloroethane:
Acylation of benzene + conditions
• React benzene with an acyl chloride in the presence of halogen carrier catalyst
What does acylation of benzene form
aromatic ketone
Draw acyl chloride
Acylation of benzene - mechanism
Name
What is a phenol
• -OH bonded directly to an aromatic ring (in this case benzene)
What happens if -OH on side chain
Alcohol
Formula for phenol
• C6H50H
Name
Why are alcohols and phenols different
due to the closeness of the delocalised ring affecting the –OH group in phenol
What is more soluble in water - alcohol or phenol
Alcohol o
Why is phenol less soluble
• Due to the presence of the non-polar (insoluble) benzene ring
• Phenol partially dissociates in water, releasing protons and phenoxide ions.
o Phenol is a weak acid
What does this show
Phenol is a weak acid
The higher the Ka,
The higher the acidity
Acidity order
Alcohols < Phenols < Carboxylic Acids
How to distinguish between Alcohols Phenols Carboxylic Acids
react with a weak base – sodium carbonate – and a strong base – sodium hydroxide
Weak base
Sodium carbonate
Strong base
Sodium hydroxide
Results for distinguishing between Alcohols Phenols Carboxylic Acids
Reaction of phenol with NaOH
• Neutralisation reaction / electrophilic substitution
Reaction of phenol with NaOH - what does it form
• Forms salt – sodium phenoxide
Overall reaction - phenol with NaOH
What’s weird
No line between O- and Na +
Why is there No line between O- and Na +
not a covalent bond
Bromination of phenol - conditions
• Phenol reacts with bromine water: room temperature, no halogen carrier catalyst
Observations in Bromination of phenol
• Bromine water will decolourise + white precipitate will form (2,4,6-tribromophenol)
What forms when Bromination of phenol happens
2,4,6-tribromophenol
Overall reaction Bromination of phenol
Nitration of phenol + conditions
• reacts with dilute nitric acid at room temperature, no catalyst required
Nitration of phenol - products
mixture of products: 2-nitrophenol, 4-nitrophenol
Overall reaction - Nitration of phenol
What reacts more readily phenol or benzene
Phenol
Example of phenol reacting more readily then benzene
What happens in phenol that makes it more readily react
Why is no catalyst required for Bromination of phenol
Electron density of phenol ring is sufficient enough to polarise the bromine molecules
Directing groups depend on…
functional group ALREADY present – not one being added on
what does electron withdrawing functional groups mean
functional groups that withdraw electrons from the delocalised ring in benzene
electron withdrawing groups also known as
deactivating groups
what positions do electron withdrawing direct substitution at
3- position
(5- position too but thats technically the 3 position when flipped)
example of electron withdrawing functional groups
NO2
what does electron donating functional groups mean
functional groups that donate electrons from the delocalised ring in benzene
electron donating groups also known as
activating groups
what positions do electron donating direct substitution at
2, 4, 6
examples of electron donating groups
OH
NH2
is phenol electron donating / withdrawing + what position
OH
donating
2,4,6
is NO2 electron donating / withdrawing + what position
withdrawing
3 position
is NH2 electron donating / withdrawing + what position
donating
2,4,6
