Chemistry. Organic Compounds Containing Oxygen - III Session

Chemistry

Organic Compounds Containing Oxygen - III

Session

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 :

6 5 6 6C H OH C H + ZnO

Nitration

OH

Conc. HNO3

Conc. H2SO4

OH

NO2O2N

NO2

With dilute HNO3, it gives ortho and para-isomers which can be separated easily by distillation.

O H

Dil. HNO3

O HN O2

O H

N O2

+

With concentrated HNO3 phenol is converted to 2,4,6-trinitrophenol.

Bromination of phenol

OHaqueous medium

Br23

CHCl3/CCl4

OH

BrBr

Br

2, 4, 6 tribromophenol

Br2

OH

Br

p-Bromophenol

OH

Br

o-Bromophenol(minor product)

(major product)

+

+ 3HBr

Kolbe’s reaction

ONa

CO2

OH

COONa H+

OH

COOH

+

Salicylic acid

400 K

4 –7 atm

Reimer-Tiemann Reaction Mechanism OH

CHCl3

NaOH/H2Oheat

C

O

H

OH

salicylaldehyde

CHCl3 + OH- CCl3- + H2O

CCl3- CCl2 + Cl-

a carbene

+ OH-

OOH O

H

O

H CCl2+

O

H

CCl2 CHCl2

O-

CHCl2

O-

H2O

heat

OHC

O

H+ HCl2

Overall:

Reimer Tiemann Reaction

O

CHO

OH OH

CHOCHCl3

O

CHCl2

H+

Salicylaldehyde(main product)

aq. NaOH, 70°C

The mechanism involves dichlorocarbene as an intermediate

3OH CHCl 2 3 2H O : CCl Cl : CCl

On treating phenol with chloroform in presence of sodium hydroxide, a —CHO group is introduced at ortho position of benzene ring.

Fries rearrangement

Esters of phenols yield phenolic ketones on treatment with anhydrous aluminium chloride.

O H

3

O HO

+(CH CO ) O3 2

C O C H3

AlC l

O H

O C O C H 3

O C O C H 3

Coupling Reaction

N N Cl OH+

N N OH

–OH

p-hydroxy azo benzene

Phenol Reactions: A Summary OH

NaOH

or

Na

O- Na+ 1. NaOH

2.RX (primary)

OR

RCOCl

or

(RCO)2O

OC

O

R

AlCl3

OH

C

O

RArN2

+

OH

N

NAr

CO24-7 atmheat

OH

C

O

O-Na+

CHCl3

O-

C

O

H

NaOH

Na+

Ethers

• Formula R-O-R where R is alkyl or aryl.• Symmetrical or unsymmetrical• Examples:

O CH3CH3 O CH3

O

Introduction

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.

• Grignard reagents

O B

H

H

H

+ _

BH3 THF

Ether complexes

• Crown ethers

• Electrophiles

Nomenclature

CH3CH2 O CH2CH3

diethyl ether orethyl ether

CH3 O C

CH3

CH3

CH3

t-butyl methyl ether ormethyl t-butyl ether =>

• Alkyl alkyl ether

• Current rule: alphabetical order

• Old rule: order of increasing complexity

• Symmetrical: use dialkyl, or just alkyl.

• Examples:

Common name

IUPAC Names

CH3 O C

CH3

CH3

CH3

2-methyl-2-methoxypropane

O CH3

Methoxycyclohexane

• Alkoxy alkane

• Examples:

Preparation

By dehyration of alcohols

CH3CH2OH

C2H5OC2H5

CH2=CH2

H2SO4

410 K

H2SO4

443 K

Williamson’s Process

ROH + Na – +RO Na 21

H2

–R – O – R` X– –RO R`—X 2

NS

0

0 0

Must be 1

2 and 3 Alkene will be the major

product

But R`X .

R`X

Important laboratory method for the preparation of symmetrical and unsymmetrical ethers.

Williamsons ProcessCH3

CH3

– +3H C O Na

|

|– C – + 2 5C H Br Path 1

CH3

CH3

3 2 5H C OC H|

|– C –

CH3

CH3

3H C|

|– C – Br–

2 5C H O Na + Path 2

H2CCH3

CCH3

(Major)

CH3

CH3

3 2 5H C OC H|

|– C –

(Minor)

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.

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

CH3 O CH3 H Br CH3 O CH3

H_

Br_

++

Br_ +

CH3 O CH3

H

Br CH3 + H O CH3

Mechanism

Alcohol is protonated, halide attacks, and another molecule of alkyl bromide is formed.

Phenyl Ether Cleavage

O CH2CH3HBr

OH

+ CH3CH2 Br

• Phenol cannot react further to become halide.

• Example:

Alkyl aryl ethers are cleaved at the alkyl oxygen bond due to the low reactivity of aryl-oxygen bond.

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.

O R

O R

–

–

–+ O R

+ O R

+

Helogenation

Anisole undergoes bromination with bromine in ethanoic acid even in absence of iron(III) bromide catalyst.

O C H3

A n iso le

Br2

O C H 3

B r

Friedel Craft reaction

O C H3

A n iso le

O C H3

+ C H C l3

3AlC l

O C H3

C H 3

C H 3+

Alkylation

Acylation O C H

3

+ CH CO Cl3

AlC l

O C O C H 3

O C O C H 3

3 O C H3O C H3

Nitration

O C H

HNO

N O2

N O2

+4

3

3 O C H3O C H3

H SO2

Obtained mixture of ortho and para isomers.

Illustrative Example

Give the major products that are formed by heating each of the following ethers with HI.

C H — C H — C H — C H — O — C H C H 323 22

C H3

C H C H C H O C C H C H

C H 3

3 2 2 2 3

(i)

(ii)

Solution

C H — C H — C H — C H O H 323 22

C H 3

C H C H I+(i)

C H C H — C — I

C H 3

3 22 23

C H3

C H C H C H O H +(ii)

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

OO

O O

Na+

In this reaction crown ether is host and metal ion is guest.Crown ethers allow inorganic salts to dissolve in non-polar solvents.

O

OO

O O

Na+

Inclusion compound

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

CHO

OCH3

Vanillin

Thank you