Alcohols, Phenols and Ethers, Class 12, Chemistry Detailed Chapter Notes PDF Download

Introduction

Physical Properties

(1) Boiling point :

(a) Alcohols show increase in boiling point with increase in molecular weight amongst homologues.

(b) Alcohols have higher boiling point than hydrocarbons of the same molecular weight. The reason for higher boiling point is the intermolecular H-bonding present in alcohols.

Intermolecular H bonds in

(2) Solubility in water :

As molecular weight increases solubility in water decreases. The lower alcohols are miscible with water. This is due to intermolecular hydrogen bonding between alcohol and water molecules.

Intermolecular H bond between water & alcohol molecules

Preparation of alcohols

(1) From alkenes

(a) By acid catalyzed hydration of alkenes : Formation of carbocation intermediate (Markovnikov addition, rearrangement possible)

General reacion

e.g.

(b) By Oxymercuration - demercuration process :

(1) Oxymercuration involves an electrophilic attack on the double bond by the positively charged mercury species. The product is a mercurinium ion, an organometallic cation containing a three-membered ring.

(2) In the second step, water from the solvent attacks the mercurinium ion to give (after deprotonation) an organomercurial alcohol.

(3) The third step is demercuration to remove the Hg. Sodium borohydride (NaBH₄, a reducing agent) re-places the mercuric acetate fragment with hydrogen.

General reaction

OH^Q NaBH₄

e.g.

(c) By Hydroboration - oxidation process : (Forms anti-markovnikov alcohol, no rearrangement)

General reaction R-Ch=CH₂ R-CH₂-CH₂-OH

e.g.

Ex. Give the major product of the following reaction

Ans. Major product is

(a) because 3^o carbocation is more stable.

(b)

(c)

(2) From alkyl halides : By nucleophilic substitution reactions

(a) By SN² mechanism (second-order substitution) : It is given by primary (and some
secondary) halides

General reaction : R-CH₂-Br R-CH₂-OH

e.g. (CH₃)₂CHCH₂CH ₂-Br (CH₃)₂CHCH₂CH ₂- OH

e.g.

(b) By SN¹ mechanism : It is given by tertiary and some secondary halides

General reaction :

e.g.

(3) From Grignard reagents

(a) From air

A Grignard reagent may be used to synthesize an alcohol by treating it with dry oxygen and decomposing the product with acid :

General reaction RMgX RO₂MgX 2ROMgX 2ROH

e.g. C₂H₅MgBrC₂H₅O₂ MgX2C₂H₅OMgX 2C₂H₅OH MgBr(OH)

(b) From ethylene oxide

Addition of Grignard reagent to ethylene oxide gives a primary alcohol (with two carbon atoms added)

General reaction RCH₂CH₂OMgX RCH₂CH₂OH MgX(OH)

e.g. C₂H₅MgBr C₂H₅CH₂CH₂ OMgX MgBr(OH)

(c) From carbonyl compounds : Nucleophilic addition to the carbonyl groups by Grignard reagent

General reaction Mg(OH)X

(i) Addition of formaldehyde gives a primary alcohol

General reaction = O RMgX RCH₂-OH

(ii) Addition to an aldehyde (other than formaldehyde) gives a secondary alcohol

General reaction

(sec-alcohol)

e.g.

(iii) Addition to a ketone gives a tertiary alcohol

General reaction

e.g. CH₃CH₂MgCl

(iv) Addition to an acid halide or an ester gives a tertiary alcohol

Esters on treatment with Grignard ragent first form ketones which then react with second
molecule of Grignard reagent and form tertiary alcohol.

General reaction

(4) By reduction of carbonyl compounds

(a) Catalytic hydrogenation of aldehydes and ketones

General reaction

e.g. CH₃CHO 2H CH₃CH₂OH

CH₃CH₂CH₂OH

4H

(b) Lithium aluminium hydride reduction of aldehydes and ketones

General reaction (i)

(ii)

(iii) RCOOH 4H RCH₂OH H₂O

(iv) RCH₂OH HCl

(v) R-CH₂OH R'OH

e.g. CH₃CH₂OH

C₂H₅CH₂OH HCl

(c) By NaBH₄ (sodium borohydride) : It is insoluble in ether and is used in aqueous ethanolic solution to reduce carbonyl compounds. It does not reduce esters and acids.

R-CH₂-OH

e.g.

CH=CHCHO 4H CH₃CH=CHCH₂OH

(ii) Reduction of a ketone gives a secondary alcohol

(d) Bouveault-Blanc reduction : The reduction of aldehydes, ketones or esters by means of excess of sodium and ethanol or n-butanol as the reducing agent.

General reaction

(i) Aldehyde RCHO RCH₂OH

(ii) Esters R'CO₂R'' R'CH₂OH R''OH

(iii) Ketones R₂CO R₂CHOH

The Bouveault-Blanc reduction is believed to occur in steps involving transfer of one electron at a time.

Mechanism

RCH₂OH O

e.g. CH₃CHO 2H CH₃CH₂OH

CH₃COOC₂H₅ 4H2CH₃CH₂OH

Ex. Identify (X) in the following reaction

X

Ans.

Ex. What are the product A, B, C, D and E in the following reactions?

A

D E

Ans. A : (EtOH)

B : Ester part is not affected by NaBH₄

C : Ester part and keto parts are affected by LiAlH₄

D :

E :

(5) By reaction of nitrous acid on aliphatic primary amines

General reaction R-NH₂ HONO R-OH N₂ H₂O

Mech.

R-NH₂ (R) ROH N₂

e.g. (i) C₂H₅NH₂ HNO₂ C₂H₅OH N₂ H₂O

(ii) CH₃-CH₂- HONO CH₃-CH₂- N₂ H₂O

Mech.

CH₃-CH₂- CH₃-CH₂- CH₃-CH₂-

(6) Hydroxylation : Forms vicinal diols (glycols)

Converting an alkene to a glycol requires adding a hydroxy group to each end of the double bond. This addition is called hydroxylation of the double bond.

(a) Syn hydroxylation, using KMnO₄ / NaOH or using OsO₄/H₂O₂

General reaction :

e.g.

(b) Anti hydroxylation, using per acids

Chemical reactions of alcohols

1. Reaction with hydrogen halides

General reaction :

R - OH HX R - X H₂O (R may rearrange)

Reactivity of HX : Hl > HBr > HCl

Reactivity of ROH: allyl, benzyl > 3^o > 2^o > 1^o

Mechanism R - OH R- R-X

e.g.

e.g.

e.g.

2. Reaction with Phosphorus trihalides

(1) Several phosphorus halides are useful for converting alcohols to alkyl halides. PBr₃, PCl₃, & PCl₅ work well and are commercially available.

(2) Phosphorus halides produce good yields of most primary and secondary alkyl halides, but none works well with ter. alcohols. The two phosphorus halides used most often are PBr₃ and the

P₄ / I₂ combination.
General reaction :

3R - OH PX₃ 3R - X H₃PO₃

Mechanism

The mechanism for the reaction involves attach of the alcohol group on the phosphorus atom, displacing a bromide ion and forming a protonated alkyl dibromophosphite (see following reaction).

In second step a bromide ion acts as nucleophile to displace HOPBr₂, a good leaving group due to the electronegative atoms bonded to the phosphorus.

RCH₂X HOPX₂

e.g.

3. Reaction with thionyl chloride

R-Cl SO₂ HCl

4. Dehydration of alcohols

H₂O (Rearrangement may occur)

Mechanism

Step 1 :

Step 2 :

Step 3 :

Reactivity of ROH : 3º > 2º > 1º

5. Reaction with metals

e.g.

6. Ester formation

General reaction

H₂O

e.g. CH₃CH₂O - H H₂O

7. Oxidation reactions

(a) oxidation of primary alcohols

Oxidation of a primary alcohol initially forms an aldehyde. obtaining the aldehyde is often difficult, since most oxidizing agents are strong enough to oxidize the adehydes formed. CrO₃ acid generally oxidizes a primary alcohol all the way upto the carboxylic acid

(b) oxidation of secondary aldohols

Sec. alcohols are easily oxidized to give excellent yields of ketones. The chromic acid reagent is often best for laboratory oxidations of secondary alcohols. The active species in the mixture is probably chromic acid,

H₂CrO₄, or the acid chromate ion,

(c) Resistance of tertiary alcohols to oxidation

Oxidation of ter-alcohol is not an important reaction is organic chemistry. Ter-alcohols have hydrogen atoms on the carbinol carbon atom, so oxidation must take place by breaking C-C bonds. These oxidations require severe conditions and result in mixtures of products.

Primary

Secondary

Tertiary no reaction

e.g.

Ex. A

Identify A

Ans.

ETHER

Structure of ether

Classificaion of Acyclic ethers

IUPAC Nomenclature of ether "Alkoxy Alkane"

Method of Preparation of Ether

(1) Williamson synthesis

General reaction

RX R - OR'

e.g. (i) n-PrOH

(ii) MeOH Me

(iii) t-BuOH t-Bu t-Butyl ethyl ether

(This reaction produces a poor yield of ether because of the bulkiness of t-BuO^-)

2. Williamson's Continuous Etherification process or by Dehydration of Alcohols

(i) ROH RO

ROR

(ii) ....

e.g.

Mechanism

Step-1:

This is an acid-base reaction in which the alcohol accepts a proton from the sulfuric acid

Step-2 :

Another molecule of the alcohol acts as a nucleophile and attacks the protonated alcohol in an reaction.

Step-3 :

Another acid-base reaction converts the protonated ether to an ether by transferring a proton to a molecule of water (or to another molecule of the alcohol).

Only one combination of alkythalide and alkoxide is appropriate for the preparation of each of the following ethers by Willianson ether synthesis. What is the correct combination in each case ?

3. Form alkenes

(a) By addition of alcohols in alkenes

When alcohol is added to alkenes in presence of acid, we get ethers.

General reaction

(I) carbocation

(II) Carbocation alcohol

e.g. (I) = CH₂ (H₂SO₄) Me₃

(II) Me₃ EtOH Me₃COEt

(b) Alkoxymercuration - demercuration

e.g.

(i) RCH=CH₂ R'OH RCH(OR')CH₃

(ii) CH₂=CHCH₃ CH₃CH(OH)CH ₃

(iii) CH₃CH=CHCH₃ CH₃CH(OH)CH ₂CH₃

Reactions of ethers

1. With HX

General reaction

X-R X-R'

e.g. (i) CH₃CH₂CH₂CH₃ 2CH₃-CH₂Br

2. Reaction with sulphuric acid

Ethers dissolve in concentrated solutions of strong inorganic acids to from oxonium salts, i.e. ether behave as bronsted Lowry bases.

R₂O H₂SO₄ R-OH

When heated with dilute H₂SO₄

R₂O H₂SO₄ 2ROH

e.g. C₂H₅OC₂H₅ H₂SO₄ C₂H₅OH C₂H₅OHSO₄ C₂H₅OH H₂SO₄

3. Autoxidation of ethers :

When ethers are stored in the presence of atmospheric oxygen, they slowly oxidize to produce hydroperoxides and dialkyl peroxides, both of which are explosive. Such a spontaneous oxidation by atmospheric oxygen is called an autoxidation.

General reaction

Ex. (i)

(ii)

4. Reaction with acid chlorides and anhydrides

Reagent : ZnCl₂, AlCl₃ etc.

General reaction (i) R-O-R R-CO-Cl R-Cl RCOOR

Mech.

RCOCl AlCl₃ RCO

RCO RCOOR' R"Cl AlCl₃

e.g. C₂H₅OC₂H₅ CH₃COCl C₂H₅Cl CH₃COOC₂H₅

(ii) R₂O 2CH₃COOR

e.g. C₂H₅OC₂H₅ (CH₃CO)₂O 2CH₃COOC₂H₅

5. Reaction with carbon monoxide :

Ether react with CO at 125-180^oC and at a pressure of 500 atm, in the presence of BF₃ plus a little water.

R₂O CO RCOOR

6. Reaction with halogens :

When treated with chlorine or Br, ether undergo substitution, the extent of which depends on the conditions.

CH₃CH₂OCH₂CH₃ CH₃CHClOCH₂CH₃ CH₃CHClOCHClCH₃

in presence of light

(C₂H₅)₂O

Mech.

The reaction proceeds by a free-radical mechanism, and a-substitution occurs readily because of resonance stabilization of the intermediate radical

, etc.