Class 12 Chemistry
Alcohols, Phenols and Ethers
Chemical properties of phenols
- Acidic nature of phenols –
- It is sp2 hybridized that is it has 33% s character and 67% p character that shows it is more electronegative . As a result electron density on oxygen decreases and Polarity of O – H bond increase therefore Hydrogen is not easily removed.
- Although phenol is acidic but it is less acidic than carboxylic acid therefore It doesn’t react with Na2CO3 and NaHCO3 but reacts with sodium metal as shown below.
- As compared with alcohol, phenol is more acidic than alcohol. Because in alcohol it is sp2 hybridised in phenol and Sp3 hybridised C atom in alcohol.
- The phenoxide ion is resonantly stabilized due to resonance as shown :
In this only one negative charge is getting dispersed.
On the other hand:
C2H5OH --> C2H5 O- + H+
ethanol ethoxide ion
No such resonance is possible therefore stability of alkoxide is less than phenoxide as a result phenol has more tendency to form phenoxide ion due to which phenol is more acidic than alcohols.
If electron withdrawing group is present at ortho or para position like NO2 , CN , Cl etc then they disperse this negative charge and will enhance acidic strength, whereas if electron donating group’s like CH3, alkyl group, NH2 (amine) or OR etc are present they decrease the acidic strength.
1. Reacting with zinc dust: Benzene is formed .
(phenol) (Zinc) (benzene)
- Reaction with NH3: aniline is formed
- Reaction with Acid chloride: Phenyl ethanoate is formed .The reaction is called as acetylation. If we react this phenyl ethanoate with ethyl anhydride then we get aspirin :
- Fries rearrangement
Phenyl ethanoate O – (hydroxy acetophenone)
- Shorten baumem reaction
(Phenol) ( benzoyl chloride) (phenyl benzoate)
In all these reaction cleavage takes place between O and H.
Reactions of benzene ring
In this Electrophile substitution reaction takes place .As we know OH- group is Ortho and Para directing so according to resonating structures the negative charge is on the Ortho and Para position. Therefore Electrophile attacks at O and P position.
Reactions involved:
- Bromination: Substitution by bromine
If it occurs in presence of CS2 then mixture of ortho and para isomers are formed:
- Nitration: Substitution by nitro group by adding nitric acid. If we use Concentrated HNO3 we get tri-substituted
- Friedel craft alkylation reaction
- Sulphonation: Substitution by SO3H group.
There are certain reactions that phenol undergoes –
- Kolbe’s reaction
By this Aspirin and salol can be formed a shown :
- Aspirin :
- Salol – phenyl salicylate
- Reimer – tieman reaction –
- Reaction with carbon tetra chloride
- Coupling reaction – in this we get dyes of different colours.
- Hydrogenation reaction –
- With Zn dust
- Oxidation reaction
Class 12 Chemistry Alcohols, Phenols and Ethers NCERT Chapter 7 Free Notes for Best Revision
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