Download 61. Organic Compounds with Functional Groups Containing Oxygen

Chemistry
Session
Organic Compounds
Containing Oxygen - III
Session Objectives
1. Properties of phenols
2. Reaction of phenols
3. Preparation of ethers
4. Properties and reactions of ethers
5. Some useful ethers
6. Crown ethers
Acidity of phenol
Phenol is more acidic than aliphatic alcohols because
conjugate base is stabilized by resonance.
O
O



O

 O–
–
–

–
Reactions of phenol
Electrophilic aromatic substitution
—OH group is ortho, para- directing group and
activates the benzene rings.
Chemical reaction of phenol
Fries rearrangement
Distillation with Zn dust :
C6H5OH
C6H6 + ZnO
Nitration
With dilute HNO3, it gives ortho and para-isomers which
can be separated easily by distillation.
OH
OH
OH
Dil. HNO3
NO2
+
NO2
With concentrated HNO3 phenol is converted to 2,4,6-trinitrophenol.
OH
OH
NO2
O2N
Conc. HNO3
Conc. H2SO4
NO2
Bromination of phenol
OH
3 Br2
Br
Br
+ 3HBr
aqueous medium
OH
Br
2, 4, 6 tribromophenol
OH
OH
Br2
Br
+
CHCl3/CCl4
o-Bromophenol
(minor product)
Br
p-Bromophenol
(major product)
Kolbe’s reaction
ONa
OH
OH
+
CO2
400 K
COONa
+
H
COOH
4 –7 atm
Salicylic acid
Reimer-Tiemann Reaction Mechanism
O
C
CHCl 3
Overall:
OH
NaOH/H
heat
CHCl 3 + OH
H
salicylaldehyde
2O
OH
-
CCl3
CCl3 -
-
+ H 2O
CCl2 + Cl a carbene
O
OH
O
+ OH -
H
O-
O
O
CCl2
H +
CHCl 2
H
CCl2
O-
OH
CHCl 2
H 2O
heat
O
C
H
+
2 HCl
Reimer Tiemann Reaction
On treating phenol with chloroform in presence of
sodium hydroxide, a —CHO group is introduced at
ortho position of benzene ring.
O
O
OH
CHCl3
CHCl2
OH
CHO
+
H
CHO
aq. NaOH, 70°C
Salicylaldehyde
(main product)
The mechanism involves dichlorocarbene as an
intermediate
OH  CHCl3
H2O  : CCl3  Cl  : CCl2
Fries rearrangement
Esters of phenols yield phenolic ketones on treatment
with anhydrous aluminium chloride.
OCOCH3
OH
(CH3CO)2O
AlCl3
OH
OH
OCOCH3
+
OCOCH3
Coupling Reaction
N
N
Cl +
OH–
OH 

N
N
OH
p-hydroxy azo benzene
Phenol Reactions: A Summary
OR
-
O Na
1. NaOH
+
O
2. RX (primary)
NaOH
CO2
4-7 atm
heat
OH
or
(RCO) 2 O
Na
AlCl
OH
-
C O Na
O
+
ArN
CHCl 3
2
R
OH
O- Na +
C
O
H
N
N
Ar
3
OH
+
NaOH
C
O
RCOCl
or
R
C
O
Ethers
Introduction
• Formula R-O-R where R is alkyl or aryl.
• Symmetrical or unsymmetrical
• Examples:
CH3
O CH3
O
O CH3
Structure and Polarity
•
Bent molecular geometry
•
Oxygen is sp3 hybridized
•
Tetrahedral angle
Hydrogen Bond Acceptor
Ethers cannot H-bond to each other.
In the presence of -OH or -NH (donor), the
lone pair of electrons from ether forms a
hydrogen bond with the -OH or -NH.
Solvent properties
•
Nonpolar solutes dissolve better in ether than in alcohol.
•
Ether has large dipole moment, so polar solutes also dissolve.
•
Ethers solvate cations.
•
Ethers do not react with strong bases.
Ether complexes
• Grignard reagents
•
Electrophiles
H
_
+
O B H
H
BH3 THF
•
Crown ethers
Nomenclature
Common name
•
Alkyl alkyl ether
•
Current rule: alphabetical order
•
Old rule: order of increasing complexity
•
Symmetrical: use dialkyl, or just alkyl.
•
Examples:
CH3
CH3CH2
O CH2CH3
diethyl ether or
ethyl ether
CH3
O C CH3
CH3
t-butyl methyl ether or
methyl t-butyl ether =>
IUPAC Names
•
Alkoxy alkane
•
Examples:
CH3
CH3
O CH3
O C CH3
CH3
2-methyl-2-methoxypropane
Methoxycyclohexane
Preparation
By dehyration of alcohols
H2SO4
443 K
CH2=CH2
CH3CH2OH
H2SO4
410 K
C2H5OC2H5
Williamson’s Process
Important laboratory method for the preparation
of symmetrical and unsymmetrical ethers.
RO–Na+ 
ROH + Na
RO –  R`—X –

2
SN
1
H2
2
R – O – R` X –
But R`X  Must be 10.
20 and 30 R`X  Alkene will be the major
product
Williamsons Process
CH3
CH3
|
|
–
+
Path
1
H3C – C – O Na + C2H5Br  H3C – C – OC2H5
|
|
CH3
CH3
Best results are obtained if the alkyl halide is primary. If
tertiary alkyl halide is used, an alkene is the only reaction
product and no ether is formed.
CH3
|
H3C – C – OC2H5
CH3
|
C2H5O– Na + H3 C – C – Br
|
CH3
Path 2

|
CH3
(Minor)
CH3
H2C
C
CH3
(Major)
Cleavage of Ethers
•
Ethers are unreactive toward base, but protonated ethers
can undergo substitution reactions with strong acids.
•
Alcohol leaving group is replaced by a halide.
•
Reactivity: HI > HBr >> HCl
Mechanism
CH3
O CH3
H Br
CH3
H
+
O CH3
_
Br
CH3
H
+
O CH3
_
+ Br
Br CH3 + H O CH3
Alcohol is protonated, halide attacks, and another
molecule of alkyl bromide is formed.
_
Phenyl Ether Cleavage
Alkyl aryl ethers are cleaved at the alkyl oxygen bond
due to the low reactivity of aryl-oxygen bond.
•
Phenol cannot react further to become halide.
•
Example:
OH
O CH2CH3
HBr
+ CH3CH2
Br
Electrophilic substitution in
alkyl aryl ethers
The alkoxy group(-OR) is ortho, para directing and
activate the aromatic ring towards electrophilic
substitution in the same way as phenol.
+
OR
+
OR




 OR

+
OR
–
–

–
Helogenation
Anisole undergoes bromination with bromine in ethanoic
acid even in absence of iron(III) bromide catalyst.
OCH3
OCH3
Br2
Anisole
Br
Friedel Craft reaction
Alkylation
OCH3
OCH3
+ CH3Cl
AlCl3
+
Anisole
Acylation
CH3
OCH3
OCH3
OCH3
CH3COCl
OCH3
CH3
OCOCH3
+
AlCl3
OCOCH3
Nitration
Obtained mixture of ortho and para isomers.
OCH3
OCH3
OCH3
H2SO4
+
HNO3
NO2
NO2
Illustrative Example
Give the major products that are formed by heating each of
the following ethers with HI.
CH3
(i)
CH3—CH2—CH—CH2—O—CH2CH3
C
H
3
(ii)
C
H
C
H
C
H
O
C
C
H
C
H
2
3
2
2
3
Solution
CH3
(i)
CH3—CH2—CH—CH2OH + CH3CH2I
CH3
(ii)
CH3CH2CH2OH + CH3CH2—C—I
CH3
Crown ethers
Cyclic polyethers containing four or more ether linkages in a
ring of twelve or more atoms.
Crown ethers bind certain metal ions depending on size of the cavity
O
Na+
Na+
O
O
O
O
O
O
O
O
O
Inclusion compound
In this reaction crown ether is host and metal ion is guest.
Crown ethers allow inorganic salts to dissolve in non-polar solvents.
Uses of ethers
As solvent and inhalation anaesthetic.
A number of naturally occurring phenol and ethers are used as
flavourings and in perfumes of their pleasant odour.
OH
OCH3
CHO
Vanillin
Thank you