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Module 6 - Chapter 25 - Aromatic compounds II - Coggle Diagram
Module 6 - Chapter 25 - Aromatic compounds II
Alkylation
Substitution of a hydrogen atom in the benzene ring by an alkyl group
Benzene reacts with a haloalkane in the present of AlCl3 (generates the electrophile)
Acylation
Acyl chloride reacts with benzene in the present of AlCl3, aromatic ketone is formed
Electrophilic substitution
Comparing alkene/ arene reactivity
Alkenes decolourise bromine by electrophilic addition
Reaction
Pi bond in alkene has localised electrons above and below the plane of the two carbon atoms - area of high electron density
Localised electrons in the pi bond induce a dipole in the non-polar bromine molecule
Slightly positive bromine atoms enables the bromine molecule to act as an electrophile
Benzene doesn't react with bromine unless a halogen carrier catalyst is present - this is because of the delocalised pi electrons above and below the plane
Electron density around any two carbon atoms is less than in a C=C double bond in an alkene
When the non polar bromine approaches benzene, the insufficient pi electron density means no polarisation happens
Phenols
Organic compound containing hydroxyl function group bonded to an aromatic ring
Compound containing a -OH group bonded to a carbon side chain rather than aromatic ring are classified as aromatic alcohols
Phenol reactions
Weak acid
Phenol partially dissociates forming the phenoxide ions and proton
Ethanol doesn't react with sodium hydroxide or sodium carbonate (strong/weak bases)
Phenols and carboxylic acids react with strong bases
Only carboxylic acids are strong enough to react with weak bases
The dissociation constant of a phenol is greater than ethanol by less than ethanoic acid
Non polar benzene ring makes phenol less soluble in water than alcohols
Sodium hydroxide
Phenol reacts with sodium hydroxide to from sodium phenoxide and water
Electrophilic substitution
Bromination
Phenol reacts with bromine water to form white precipitate of 2,4,6, tribromophenol
Bromine water is decolourised
No catalyst is required and RTP is used
Nitration
Phenol reacts with dilute nitric acid at RTP
Mixture of 2-nitrophenol and 4-nitrophenol is formed
Reactivity of phenol and benzene
Phenol is nitrated with dilute nitric acid while benzene required concentrated nitric and sulfuric acid
Increase reactivity is due to lone pair of electron from oxygen p orbital from -OH group donated into the pi system
Electron density of phenol is increased - electrophiles are attracted more strongly
Electron density in phenol ring is sufficient to polarise bromine molecules so no halogen carrier catalyst is needed
Directing groups
Activating groups
Electron donating groups likes OH and NH2 direct electrophiles to substitute at the 2, 4 positions
All 2- and 4- directing groups are activating
Activating groups - increase electron density of the benzene ring at carbons 2,4,6
Deactivating groups
Electron withdrawing groupslikes NO2 are 3-directing in electrophilic substitution of aromatic compounds
All 3-directing groups are deactivating groups. They withdraw election density from beznene ring, making it less nucleophilic
Otho- and para- position become more electron deficient than meta position
Meta- position in the only viable site for electrophilic attack, although its a very slow rate of reaction
Ortho - position 2
Meta - position 3
Para - position 3
