25 - Aromatic Compounds Flashcards
What is an aromatic hydrocarbon/ arene
Hydrocarbon within alternating double and single bonds between C atoms, forming rings
What is kekules structure of benzene
Consists of a hexagonal planar ring of 6 carbons
Each C atom is joined to 2 other C’s and 1 hydrogen
With alternating double and single bonds
Adapted to say constantly flipping between 2 isomers
What evidence is there to disprove kekules model
3
1- lack of reactivity of benzene
(Doesn’t decoularise benzene)
2- the lengths of carbon bonds
(Using x Ray diffraction)
3- unexpected hydrogenation enthalpies
What are some features of benzene
It is colourless Sweet smelling Naturally found in crude oil Found in cigarette smoke and petrol Classified as a carcinogen
What are the 2 possible structures of benzene
Kekulé model
Delocalised model
What does the lack of reactivity of benzene suggest
Unlike alkenes
Benzene doesn’t undergo electrophilic addition reactions
Bromine can not be added across the double bonds
Doesn’t decolourise bromine water
Supports evidence to delocalised model
Why does a difference in length of carbon carbon bonds disprove kekule model
X Ray diffraction
If it was right you would expect 3 bonds with length of a carbon single bond and 3 the length of carbon double bond
However
All bond lengths are the same in benzene
Length of 140pm
How does benzenes hydrogenation enthalpies disprove kekule
Would be expected to have enthalpy of hydrogenation 3 x that of cyclohexene (3x-120=360)
But it is actually -208kjmol-1
This is less exothermic than expected
Meaning more energy was inputted to break bonds of benezene than would be expected to break kekules bonds
What does benzene breaking bonds being far less exothermic than expected suggest
It’s resistance to reaction suggests it’s more stable
Stability brought by delocalised ring of electron density
What is the delocalised model of benzene
- Benzene is a hexagonal planar made up of 6 carbons.
- Each carbon uses 3 of its 4 valence electrons in bonding to 2 other carbons and 1 hydrogen
- Each C has 1 electron in a P orbital at RA to benzene planar
- Adjacent P orbitals overlap sideways in both directions, above and below the planar
- Creates pi bonds that spread over all 6 carbons = pi bonds are delocalised
Substituent définition
Atom/group of atoms which replaces 1 or more H atoms n the parent chain of a hydocarbon
Parent chain = benzene
1 = monosubstituted 2= disubstituted
Prefixes to benzene (parent chain)
Alkyl
Halogens = bromo iodo chloro
Nitro = NO2
When is the prefix phenyl used instead of benzene
When a benzene ring is attached to
An alkyl chain with 7 or more Cs
Alkyl chain with functional group
Exceptions in naming compounds
Benzoic acid = carboxylic acid functional group on benzene
Phenylamine = NH2 on benzene
Benzaldehyde = CHO on benzene
Naming disubstituted compounds
Numbers
Add to use lowest numbers
What makes benzene stable
The delocalised model has pi bond electron density spread out over the 6 carbons
Compounds containing delocalised electrons are more stable than those that don’t
What happens in benzenes electrophilic substitutions
A hydrogen atom on benzene is replaced by another atom/group of
What is an electrophile
An electron pair acceptor
that is attracted to areas of rich electron density, where it accepts a pair of electrons to form new covalent bonds
What conditions are needed in the nitration of benzene
H is replaced by NO2 group
Reacts slowly with HNO3= nitric acid
In presence of H2SO4 = sulfuric acid
Heated to 50 in water bath
What happens it temperature rises over 50* in nitration of benzene
Further substitution reactions will happen
At 70* dinitrobenzene is formed
What is nitrobenzene useful for
Dyes
Pharmaceuticals
Pesticides
Paracetamol
What is the electrophile in the nitration of benzene
NO2+
How is the electrophile produced in the nitration of benzene
First step
Reaction of conc. nitric acid and conc. sulfuric acid
HNO3 + H2SO4 —> HSO4- + H2O + NO2+
What happens in step 2 of the nitration of benzene
- NO2+ accepts a pair of electrons from the benzene ring to form a dative covalent bond
- the organic intermediate formed is unstable, and breaks down to form the organic product
What is the organic product made at the end of step 2 in the nitration of benzene
Nitrobenzene + H+ ion
What happens in step 3 of the reaction mechanism of nitration of benzene
The catalyst is regenerated at the end
H+ reacts with HSO4- to reform H2SO4
Why do halogens need a halogen carrier to react with benzene
An electrophile is not polarised enough by the benzene ring, because there is insufficient electron density around any 2 carbons due to the delocalised ring
The halogen carrier makes the electrophile strong enough to attack the stable benzene ring
How does a halogen carrier help to make good electrophiles
They accept lone pair of electrons from a halogen atom on the electrophile
As the lone pair is pulled away, the polarisation increases, and can form a carbocation
This makes the electrophile strong enough to react with the stable ring
What is a halogen carrier
And examples
A halogen compound used during the halogénation of benzene
AlCl3
FeBr3
Fe
What conditions are needed for the bromination of benzene
Room temp and pressure
Presence of halogen carrier
What is the electrophile in bromination of benzene
Br+
What is step 1 of the bromination of benzene
Br+ is generated by halogen carrier catalyst reacting with Bromine
Because benzene is too stable to react with non polar Bromine
Br2 + FeBr3 —> FeBr4- + Br+
What is step 2 in the bromination of benzene
Bromonium ion accepts a pair of electrons from the benzene ring to form a dative covalent bond
This forms an unstable intermediate, which breaks down to produce the organic product and H+
What is the organic product made in step 2 of bromination of benzene
Bromobenzene + H+
What is step 3 of the bromination of benzene
The halogen carrier reforms by reacting with H+
FeBr4- + H+ —> FeBr3 + HBr
What is an alkylation reaction
Friedel - crafts
The substitution of a hydrogen atom in the benzene ring by an alkyl group
It increases the number of carbon atoms in compound, by forming c-c bonds
Conditions needed for the alkylation of benzene
Reaction of a halo alkane in the presence of AlCl3
Under reflux
What is an acylation reaction
Hydrogen is replaced with an acyl chloride in presence of catalyst AlCl3
What does an acylation reaction form
An aromatic ketone
Named phenyl
Electrophilic addition in alkenes
- Pi bond in alkene is localised above and below plane = high electron density
- localised electrons induce dipole in non polar bromine, making one slightly positive
- slightly positive bromine enables the bromine molecule to act like an electrophile
What can’t benzene undergo electrophilic addition
- Benzene has delocalised electrons spread above and below plane of carbon atoms in ring structure
- The electron density is less than that in double bond of alkene
- there is insufficient electron density around any 2 Cs to polarisé non polar bromine
- prevents react taking place
What are phenols
Type of organic chemical contains a hydroxy functional group
That is directly bonded to an aromatic ring
When is a compound named an alcohol rather than phenol
Aromatic Alcohol =
OH group attached to a alkyl chain
2-phenylethanol
What is phenol used in
Paints Detergents Plastics Aspirin Disinfectants
How was phenol original manufactured
Benzene + H2SO4 + NaOH —> C6H5OH + Na2SO3 + 2H2O
How is phenol made nowadays
C6H6 + C3H6 +O2 —> CH3COCH3 + C6H5OH
Why is phenol a weak acid
Is less soluble in water than alcohols
Because or non polar benzene ring
Only partially dissociates into a phenoxide ion and a proton when dissolved in water
What is phenol more acidic than
Ethanol
Because does not react with strong base = NaOH
Or weak base = Na2CO3
What is phenol less acidic than
Carboxylic acids
Strong enough to react with weak base = sodium carbonate Na2CO3
This reaction can be used to distinguish a phenol and carboxylic acid = carboxylic produces CO2
Phenol in neutralisation reactions
Room temp
Phenol reacts with NaOH solution
Forming sodium phenoxide (salt) and water
Bromination of phenol
Phenol reacts with bromine water - 3Br2 - aqueous solution of bromine)
Forms a white precipitate = decolourises bromine water
Of 246-Tribromophenol
And 3HBr
Conditions needed for bromination of phenol
No halogen carrier catalyst
Room temp
What happens in Nitration of phenol
Phenol readily reacts with DILUTE nitric acid
No sulfuric acid needed
Produces 2 isomers and water
2 nitrophenol
4 nitrophenol
What are the differences in reactivity of phenol and benzene
Bromine and nitric acid react more readily
Phenol does not require a halogen carrier catalyst - or need CONCENTRATED nitric acid with a sulphuric acid catalyst
Why does phenol have and increased reactivity
Compared to benzene
Lone pair of electrons in p orbital of oxygen on hydroxy overlaps with delocalised ring of electron in benzene ring
The lone pair is partially delocalised into the pisystem (donated)
Increases the electron density in benzene ring
Attracts more electrophiles
Why does phenol not need a halogen carrier in bromination
The electron density in the pisystem has increased
Making it more susceptible to attack from electrophiles than in benzene
The electron density is sufficient enough to polarise bromine
So no need for halogen carrier
What is the difference between unsubstituted benzene ring and substituted benzene ring
Unsubstituted
- electrophiles can react with any of the C atoms
Substituted
- the functional group can change the electron density at certain C atoms, making them more or less likely to react
Conditions for bromination with phenylamine NH2
+ Br2
Tribromophenylamine + 3HBr
Reacts readily
Bromination with nitrobenzene
Requires halogen catalyst
High temperature
Electron donating groups
activates the ring
Activates the ring as the aromatic ring reacts more readily with electrophiles
Have electrons in orbitals that overlap with delocalised ring
Increase the density
At 2,4,6
Example of electron donating groups
2,4,6 directing
NH2
OH
Electron withdrawing groups
Deactivate
Deactivates the aroma tingling, as the aromatic ring reacts less readily with electrophiles
Are electronegative
So withdraw electron density at carbons 246
So becomes unlikely to react in these positisions
Examples of electron withdrawing groups
3,5 directing
NO2
