Class 12 Exam  >  Class 12 Notes  >  Detailed Chapter Notes - Alcohols, Phenols and Ethers, Class 12, Chemistry

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.

chemistry

Intermolecular H bonds in chemistry

(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.

chemistry

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 chemistry chemistry chemistry

e.g. chemistry

(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

chemistry

chemistry OHQ NaBH4 chemistry chemistry

e.g. chemistrychemistry chemistrychemistry chemistry

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

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

e.g. chemistrychemistry chemistry

 

Ex. Give the major product of the following reaction

chemistry chemistry

Ans. Major product is

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

(b) chemistry

(c) chemistry

 

(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 chemistry R-CH2-OH

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

e.g. chemistry chemistry chemistry

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

General reaction : chemistry chemistry chemistry

e.g. chemistrychemistry chemistry

 

(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 chemistry RO2MgX chemistry 2ROMgX chemistry 2ROH

e.g. C2H5MgBrchemistryC2H5O2 MgXchemistry2C2H5OMgX chemistry2C2H5OH MgBr(OH)

(b) From ethylene oxide

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

General reaction chemistry chemistry RCH2CH2OMgX chemistry RCH2CH2OH MgX(OH)

e.g. chemistry C2H5MgBr chemistry C2H5CH2CH2 OMgX chemistry chemistry MgBr(OH)

 

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

 

General reaction chemistrychemistrychemistrychemistrychemistry Mg(OH)X

 

 

(i) Addition of formaldehyde gives a primary alcohol

General reaction chemistry= O RMgX chemistrychemistry chemistryRCH2-OH

 

 

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

General reaction chemistrychemistrychemistrychemistrychemistry

(sec-alcohol)

e.g. chemistry chemistrychemistry chemistry

 

 

(iii) Addition to a ketone gives a tertiary alcohol

General reaction chemistrychemistrychemistry

e.g. CH3CH2MgCl chemistrychemistry chemistry

 

(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

chemistrychemistrychemistrychemistrychemistry


 

chemistrychemistrychemistry

 

chemistrychemistrychemistrychemistrychemistry

 

(4) By reduction of carbonyl compounds

(a) Catalytic hydrogenation of aldehydes and ketones

General reaction chemistry chemistry chemistry

chemistry chemistry chemistry

e.g. CH3CHO 2H chemistry CH3CH2OH

chemistry chemistry CH3CH2CH2OH

chemistry 4H chemistry chemistry

 

(b) Lithium aluminium hydride reduction of aldehydes and ketones

General reaction (i) chemistry chemistry chemistry

(ii) chemistry chemistry chemistry

(iii) RCOOH 4H chemistry RCH2OH H2O

(iv) chemistry chemistry RCH2OH HCl

(v) chemistry chemistry R-CH2OH R'OH

e.g. chemistry chemistry CH3CH2OH

chemistry chemistry 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.

 

chemistry chemistry R-CH2-OH

e.g. chemistry chemistry chemistry

chemistryCH=CHCHO 4H chemistry CH3CH=CHCH2OH

(ii) Reduction of a ketone gives a secondary alcohol

chemistry chemistry chemistry

 

(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 chemistryRCH2OH

(ii) Esters R'CO2R'' chemistryR'CH2OH R''OH

(iii) Ketones R2CO chemistryR2CHOH

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

Mechanism

chemistrychemistrychemistrychemistrychemistry

RCH2OH Ochemistry

e.g. CH3CHO 2H chemistryCH3CH2OH

CH3COOC2H5 4Hchemistry2CH3CH2OH

chemistrychemistrychemistry

Ex. Identify (X) in the following reaction

chemistrychemistryX

Ans. chemistry

 

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

chemistrychemistryA chemistry

chemistrychemistryD chemistrychemistryE

Ans. A : chemistry (EtOH)

B : chemistry Ester part is not affected by NaBH4

C : chemistry Ester part and keto parts are affected by LiAlH4

D : chemistry

E : chemistry

 

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

General reaction R-NH2 HONO chemistry R-OH N2 H2O

Mech.

R-NH2 chemistry (Rchemistry) chemistry ROH N2 chemistry

e.g. (i) C2H5NH2 HNO2 chemistry C2H5OH N2 H2O

 

(ii) CH3-CH2-chemistry HONO chemistry CH3-CH2-chemistry N2 H2O

Mech.

CH3-CH2-chemistry chemistry CH3-CH2-chemistry chemistryCH3-CH2-chemistry

 

(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 :

chemistry chemistry chemistry

e.g. chemistry chemistry chemistry

chemistrychemistry chemistry

 

(b) Anti hydroxylation, using per acids

chemistrychemistrychemistry

 

Chemical reactions of alcohols

 

1. Reaction with hydrogen halides

General reaction :

R - OH HX chemistry R - X H2O (R may rearrange)

Reactivity of HX : Hl > HBr > HCl

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

Mechanism R - OH chemistry R-chemistry chemistry chemistry chemistry R-X

e.g. chemistry chemistry chemistry

e.g. chemistry chemistry chemistry

e.g. chemistry chemistry chemistry

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 chemistry 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).

chemistry chemistry chemistry

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.

chemistrychemistry RCH2X HOPX2

e.g. chemistry chemistry chemistry

 

chemistry chemistry chemistry

 

3. Reaction with thionyl chloride

chemistry chemistry R-Cl SO2 HCl

 

4. Dehydration of alcohols

chemistrychemistry chemistry H2O (Rearrangement may occur)

Mechanism

Step 1 : chemistry chemistry chemistry

Step 2 : chemistry chemistry chemistry

Step 3 : chemistry chemistry chemistry

Reactivity of ROH : 3º > 2º > 1º

 

5. Reaction with metals

chemistry chemistry

e.g. chemistry chemistry chemistry

chemistry chemistry chemistry chemistry

chemistry chemistry chemistry

 

6. Ester formation

General reaction

chemistry chemistry chemistry chemistry H2O

e.g. CH3CH2O - H chemistry chemistry chemistry H2O

chemistry chemistrychemistrychemistry

 

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, chemistry

(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 chemistry

Secondary chemistrychemistrychemistry

Tertiary chemistry chemistryno reaction

e.g. chemistrychemistrychemistry

chemistrychemistrychemistry

chemistry chemistry chemistry

Ex. chemistrychemistryA

Identify A

Ans. chemistrychemistrychemistry


 

ETHER

Structure of ether

chemistry chemistry

Classificaion of Acyclic ethers

 

IUPAC Nomenclature of ether "Alkoxy Alkane"

chemistry

 

Method of Preparation of Ether

(1) Williamson synthesis

General reaction

 

RX chemistry chemistry R - OR'

 

e.g. (i) n-PrOH chemistry chemistry

(ii) MeOH chemistry Mechemistry chemistry chemistry

(iii) t-BuOH chemistryt-Buchemistry chemistry 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 chemistry chemistry ROchemistry

chemistry chemistrychemistry chemistry ROR chemistry

(ii) chemistry chemistry chemistrychemistry chemistry ....chemistry

chemistry

e.g. chemistry

Mechanism

Step-1: chemistry

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

Step-2 : chemistry

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

Step-3 : chemistry

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) chemistry chemistry carbocation

(II) Carbocation alcohol chemistry chemistry

e.g. (I) chemistry = CH2 chemistry(H2SO4) chemistry Me3chemistry

(II) Me3chemistry EtOH chemistry chemistry chemistry Me3COEt

(b) Alkoxymercuration - demercuration

chemistry chemistry chemistry chemistry chemistry

e.g.

(i) RCH=CH2 R'OH chemistry RCH(OR')CH3

(ii) CH2=CHCH3 CH3CH(OH)CH 3 chemistry chemistry

(iii) CH3CH=CHCH3 CH3CH(OH)CH 2CH3 chemistry chemistry

Reactions of ethers

1. With HX

General reaction

chemistry chemistry chemistry chemistrychemistry chemistry chemistry X-R X-R'

e.g. (i) CH3CH2CH2CH3 chemistry 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 chemistry chemistry chemistry R-OH chemistry

When heated with dilute H2SO4

R2O H2SO4 chemistry 2ROH

e.g. C2H5OC2H5 H2SO4 chemistry C2H5OH C2H5OHSO4 chemistry 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

chemistry chemistry chemistry chemistry

Ex. (i) chemistry chemistry chemistry chemistry

(ii) chemistry chemistry chemistry

 

4. Reaction with acid chlorides and anhydrides

Reagent : ZnCl2, AlCl3 etc.

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

Mech.

RCOCl AlCl3 chemistry RCO chemistry

chemistry RCO chemistry chemistry chemistry RCOOR' chemistry chemistry R"Cl AlCl3

e.g. C2H5OC2H5 CH3COCl chemistry C2H5Cl CH3COOC2H5

(ii) R2O chemistrychemistry 2CH3COOR

e.g. C2H5OC2H5 (CH3CO)2O chemistry 2CH3COOC2H5

 

5. Reaction with carbon monoxide :

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

R2O CO chemistry RCOOR

 

6. Reaction with halogens :

 

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

CH3CH2OCH2CH3 chemistry CH3CHClOCH2CH3 chemistry CH3CHClOCHClCH3

in presence of light

(C2H5)2O chemistry chemistry

Mech.

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

chemistry chemistry chemistry chemistry chemistry chemistry chemistry, etc.

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FAQs on Alcohols, Phenols and Ethers, Class 12, Chemistry Detailed Chapter Notes

1. What are alcohols?
Ans. Alcohols are organic compounds that contain one or more hydroxyl (-OH) group(s) attached to a carbon atom. They are classified based on the number of hydroxyl groups attached to the carbon atom.
2. What are phenols?
Ans. Phenols are a type of organic compounds that contain a hydroxyl (-OH) group attached to a benzene ring. They are stronger acids than alcohols and can react with bases to form salts.
3. What are ethers?
Ans. Ethers are organic compounds that contain an oxygen atom bonded to two carbon atoms. They are classified as either symmetrical or unsymmetrical, depending on whether the two carbon atoms are identical or not.
4. What is the general formula for alcohols?
Ans. The general formula for alcohols is CnH2n+1OH, where n is the number of carbon atoms in the molecule.
5. What are the uses of alcohols, phenols, and ethers?
Ans. Alcohols are used as solvents, fuels, antiseptics, and in the production of chemicals such as ethylene oxide and ethylene glycol. Phenols are used in the production of plastics, resins, and pharmaceuticals. Ethers are used as solvents, anesthetics, and as starting materials for the synthesis of other organic compounds.
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