OH or Na 2 Cr 2 O 7 /H 2 SO 4
CrO 3 (O)
O O
(Phenol) (p-Benzoquinone)
OH OH
- 3H (^2) 433 KNi
OH
- Zn
(Phenol) (Benzene)
2C 2 H 5 OH -H
2 O
H 2 SO 4 /413K C
2 H 5 -O-C 2 H 5
(ethanol) (Ethoxyethane)
C 2 H 5 OH -H
2 O
H 2 SO 4 /443K CH
2 = CH 2
(ethanol) (ethene)
vi. Oxidation of phenol : Phenol on oxidation
with chromic anhydride or sodium dichromate
in presence of H 2 SO 4 gives p-benzoquinone.
Do you know?
Sodium phenoxide is more reactive
than phenol towards electrophilic
substitution. Hence it is able to react
with a weak electrophile like CO 2 at high
temperature and pressure in Kolbe reaction.
Phenol oxidizes slowly giving a dark coloured
mixture in presence of air.
vii. Catalytic hydrogenation of phenol:
Phenol on catalytic hydrogenation gives
cyclohexanol. In this reaction a mixture of
vapours of phenol and hydrogen is passed over
nickel catalyst at 433 K.
viii. Reduction of phenol : Phenol is reduced
to benzene on heating with zinc dust.
11.5 Ethers
11.5.1 Preparation of ethers
a. Dehydration of alcohols : When alcohol is
heated with dehydrating agent like concentrated
H 2 SO 4 or H 3 PO 4 two products, either an ether
or an alkene, can form depending upon the
temperature. For example : dehydration of
ethanol by H 2 SO 4 gives ethoxyethane at 413
K, while ethene is formed at 443 K.
Symmetrical ethers can be obtained from
primary alcohols by this method. Use of higher
temperature or 2°/3° alcohols gives alkene as
the major product.
Do you know?
Dehydration of alcohols to form
ether is a SN^2 reaction. Protonated
alcohol species undergoes a backside
attack by second molecule of alcohol in a
slow step. Subsequent fast deprotonation
results in formation of ether.
C 2 H 5 - O-H H
⊕
C 2 H 5 - O
H
⊕
-H
C 2 H 5 - OH+CH 2 - O
H
⊕
-H (^) -H
H^2 O
3 C
C 2 H 5 - O
H
⊕
-CH 2 -CH 3
i. Protonation :
ii. SN^2 :
iii. Deprotonation :C 2 H 5 - O
H
⊕
-C 2 H 5
-H⊕
C 2 H 5 -O-C 2 H 5
b. Williamson Synthesis : Simple as well as
mixed ethers can be prepared in laboratory
by Williamson Synthesis. In this method
alkyl halide is treated with sodium alkoxide
or sodium phenoxide to give dialkyl ethers or
alkyl aryl ethers.
R-X + Na⊕ O-R R-O-R + NaX
R-X + Na⊕ O -Ar R-O-Ar + NaX
This reaction follows SN2 mechanism. Ether
is formed as a result of backside attack by
alkoxide/ phenoxide ion (a nucleophile) on
alkyl halide. The alkyl halide used in this
reaction must be primary. For example : t-butyl
methyl ether can be synthesised by reaction of
methyl bromide with sodium t-butoxide.
- ZnO
(Phenol) (Cyclohexanol)
(CH 3 ) 3 C-O Na⊕ + CH 3 -Br
(sodium t-butoxide) (methyl bromide)
(t-butyl methyl ether)
(CH 3 ) 3 C-O-CH 3 + NaBr
