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11. Alcohols, Phenols and Ethers

Classification of alcohols and phenols: Mono, di, tri or polyhydric –  

On the basis of hybridisation –  

  3C sp

  – OH

  Primary, secondary and tertiary  Allylic alcohols

  Benzylic alcohols

  2C sp

  – OH

  Vinylic alcohol

  Phenol

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 Classification of ethers:

Simple or symmetrical –  C2H5OC2H5, C6H5OC6H5 

Mixed or unsymmetrical –  

C2H5OCH3, CH3OC6H5 

Common names of some phenols:

Alcohols and phenols:

Preparation of alcohols –  

  From alkenes  Acid catalysed hydration

[According to Markovnikov’s rule] 

  Hydroboration – oxidation

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  From carbonyl compounds  By reduction of aldehydes and ketones.

Aldehydes give 1 and ketones give 2 alcohols.Pd

2 2RCHO H RCH OH  

  By reduction of carboxylic acids and esters4

2

i LiAlH

2ii H O

catalyst

2 2catalystic hydrogenation

RCOOH RCH OH

RCOOR H RCH OH R OH 

  From Grignard reagents

Methanol gives 1 , other aldehydes give 2 , and ketones give 3 alcohols.

Preparation of phenols –    From haloarenes

  From benzenesulphonic acid

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  From diazonium salts

  From cumene

Physical properties of alcohols and phenols –    Boiling points:

  The boiling points of alcohols and phenols increase with increase in number

of carbon atoms as it involves increase in van der Waals forces.  The boiling points of alcohols decrease with increase of branching. This is

 because van der Waals forces decrease with decrease in surface area.  Solubility:

  Alcohols and phenols form H-bonds, so they are soluble in water.  Their solubility decreases with increase in size of alkyl/aryl groups.

Chemical reactions: Alcohols react both as nucleophiles and electrophiles

Reactions that involve cleavage of O – H bond  Acidity of alcohols and phenols

  Reaction with metals

22ROH 2Na 2R O Na H

Sodium

alkoxide

 

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  Acidity of alcoholsThe increasing order of acidity of alcohols is

  Acidity of phenolsPhenols are more acidic than alcohols and water.Electron-withdrawing groups increase the acidity of phenols whileelectron-releasing groups decrease the acidity of phenols.

  EsterificationPyridine

R/ArOH R COCl R/ArOCOR HCl    Acetylation of salicylic acid

Reactions that involve cleavage of C – O bond in alcohols  Reaction with hydrogen halide

2ROH HX RX H O  

  Lucas test – Used for distinguishing between 1°, 2° and 3° alcohols.(Lucas reagent – Concentrated HCl and ZnCl2)

  Reaction with phosphorus trihalide

3 3 33R OH PX 3R X H PO X Cl,Br    

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   Dehydration

Treated with concentrated H2SO4 or H3PO4 or Anhyd. ZnCl2, or Al2O3 

The order of increasing reactivity towards dehydration is

1 alcohol < 2 alcohol < 3 alcohol  Oxidation (also known as dehydrogenation)

It involves formation of a carbon – oxygen double bond.  Oxidising agent – Acidified KMnO4 

directlyAlcohol Carboxylic acid  

  Oxidising agent Anhyd. CrO3 

1 alcohol Aldehydealcohol Ketone

  Pyridinium chlorochromate (PCC)

1 alcohol Aldehyde

  Oxidising agent Heated Cu at 573 K

1 alcohol Aldehyde

alcohol Ketone

  3 alcohols do not undergo oxidation

Reactions of phenols:  Electrophilic aromatic substitution reaction

  Nitration

The o-isomer is steam volatile due to intramolecular H-bondingwhile the  p-isomer is less volatile due to intermolecular H-bonding.So, they can be separated by steam distillation.

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  Halogenation

  Kolbe’s reaction 

  Reimer – Tiemann reaction

  Reaction of phenol with zinc dust

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  Oxidation

Ethers:

Preparation of ethers –    By dehydration of alcohols

  Williamson synthesis

  But in case of 2 and 3 halides, instead of substitution, elimination takesplace, resulting in alkenes.

  Phenols are converted into ethers.

Physical properties of ethers –    Boiling points:

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 The boiling points of ethers are lower than those of alcohols of comparablemolecular masses due to the presence of H-bonding in alcohols.

Chemical reactions of ethers: 

Cleavage of C – O bond  Least reactive; under drastic conditions with excess of hydrogen halides, theC – O bond is cleaved

  The order of increasing reactivity of hydrogen halides isHCl < HBr < HI

  Electrophilic substitution  Halogenation

  Friedel Craft’s alkylation 

  Friedel Craft’s acylation

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  Nitration

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