Alcohols, Phenols And Ethers

- A quick revision of all the important concepts
1
Classification of Alcohols, Phenols and Ethers
Classification of alcohols can be made on the number of -OH groups attached.
post image
expand
Monohydric alcohols can be further classified on the basis of Hybridization of the Carbon to which the -OH group is attached
post image
expand
Allylic alcohols: In these alcohols, the -OH group is attached to a hybridised carbon adjacent to the carbon-carbon double bond
post image
expand
Benzylic alcohols: In these alcohols, the -OH group is attached to a hybridised carbon atom next to an aromatic ring.
post image
expand
post image
expand
Phenols can be classified on the basis of number of OH groups attached directly to the aromatic system as
post image
expand
Ethers are classified on the basis of the symmetry of the molecules around the C-O-C bond
  • Simple or symmetrical
post image
expand
Mixed or unsymmetrical
post image
expand
2
Nomenclature
Alcohols
  • The name of an alcohol is derived from the name of the alkane from which the alcohol is derived, by substituting 'e' of alkane with the suffix 'ol'.
  • The position of substituents are indicated by numerals.
  • The longest carbon chain is numbered starting at the end nearest to the OH group.
  • The positions of the -OH group and other substituents are indicated by numbering the carbon atoms to which these are attached.
  • For naming polyhydric alcohols, the number of -OH groups is indicated by adding prefixs, di, tri, etc., before 'ol'.
post image
expand
Phenols
  • In IUPAC nomenclature, the parent molecule is called phenol

  • The substituents are always numbered such that the -OH group gets the lowest number.

post image
expand
  • Benzene rings attached to more than one hydroxyl group are labelled with the Greek numerical prefixes such as di, tri, tetra to denote the number of similar hydroxyl groups attached to the benzene ring.
post image
expand
Ethers
  • The alkoxy groups with shorter carbon chain is a substituent
  • The longest carbon chain is the parent chain
  • The substituents are placed alphabetically
post image
expand
3
Alcohols
Preparation
From Alkenes
Acid catalysed hydration:- Addition of water molecule across the double bond in the presence of Acid.
post image
expand
  • In case of unsymmetrical alkenes, the addition reaction takes place in accordance with Markovnikov's rule
post image
expand
Hydroboration-oxidation:-Diborane reacts with alkenes to give trialkyl boranes as addition product.
  • This is oxidised to alcohol by hydrogen peroxide in the presence of aqueous sodium hydroxide.
  • Alcohol is formed by the anti Markovnikov rule
post image
expand
From Carbonyl compounds
Reduction of aldehydes and ketones:- Aldehydes and ketones are reduced to the corresponding alcohols by addition of hydrogen in the presence of catalysts such as Platinum
post image
expand
Aldehydes and ketones can also be reduced by with Sodium borohydride () or Lithium aluminum hydride ().
post image
expand
Reduction of carboxylic acids and esters :- Carboxylic acids are reduced to primary alcohols by lithium aluminium hydride
post image
expand
From Grignard reagent:-
post image
expand
Physical Properties
  • Boiling points of alcohols are higher in comparison to other hydrocarbons.
  • This is due to the presence of intermolecular hydrogen bonding
post image
expand
  • Solubility of alcohols in water is due to their ability to form hydrogen bonds with water molecules.
  • The solubility decreases with increase in size of alkyl groups
post image
expand
Chemical Properties
  • Reactions involving cleavage of O-H bond

  • The acidic character of alcohols is due to the polar nature of O-H bond.

  • Electron-releasing groups ( , ) increase electron density on oxygen tending to decrease in the polarity of O-H bond - reducing the acidic nature
post image
expand
Reaction with metals: Alcohols and phenols react with metals such as sodium, potassium and aluminium to yield corresponding alkoxides/phenoxides and hydrogen
post image
expand
Esterification Reaction:- Alcohols and phenols react with carboxylic acids, acid chlorides and acid anhydrides to form esters.
post image
expand
  • Reactions involving cleavage of carbon-oxygen (C-O) bond in alcohols
  • Alcohols react as electrophiles when the bond between C-O is broken
post image
expand
Reaction with hydrogen halides:- Alcohols react with hydrogen halides to form alkyl halides
post image
expand
post image
expand
  • Oxidation

Oxidation of alcohols involves the formation of a carbon-oxygen double bond with cleavage of an O-H and C-H bonds
post image
expand
  • Primary alcohols are oxidised to aldehydes which in turn get oxidised to carboxylic acids depending on the oxidising agent used
post image
expand
  • Secondary alcohols are oxidised to ketones by chromic anhydride
post image
expand
  • Tertiary alcohols do not undergo oxidation reactions easily.
  • Strong oxidising agents () and elevated temperatures lead to cleavage of various C-C bonds
  • Mixture of carboxylic acids containing lesser number of carbon atoms is formed
post image
expand
  • When primary or a secondary alcohol vapours are passed over heated copper at 573 K, dehydrogenation takes place and an aldehyde or a ketone is formed respectively
  • While tertiary alcohols undergo dehydration.
post image
expand
4
Phenols
Preparation
From Haloarenes
  • Chlorobenzene is fused with NaOH at 623K and 320 atmospheric pressure.
  • Phenol is obtained by acidification of sodium phenoxide so produced
post image
expand
From Benzenesulphonic acid
  • Benzene is sulphonated with oleum
  • Benzenesulphonic acid so formed is converted to sodium phenoxide on heating with molten sodium hydroxide.
  • Acidification of the sodium salt gives phenol.
post image
expand
From Diazonium salts
  • A diazonium salt is formed by treating an aromatic primary amine with nitrous acid () at 273-278 K.
  • Diazonium salts are hydrolysed to phenols by warming with water or by treating with dilute acids
post image
expand
From Cumene
  • Cumene (isopropyl benzene) is oxidised in the presence of air to cumene hydroperoxide.
  • It is converted to phenol and acetone by treating it with dilute acid
post image
expand
Physical Properties
Boiling points of phenols are higher in comparison to other hydrocarbons due to the presence of intermolecular hydrogen bonding.
post image
expand
Solubility of phenols in water is due to their ability to form hydrogen bonds with water molecules
post image
expand
Chemical Properties
Acidic Character of Phenols
  • Phenol is more acidic than alcohols due to stabilisation of phenoxide ion through resonance
In alkoxide ion, the negative charge is localised on oxygen
post image
expand
In phenoxide ion, the charge is delocalised.
post image
expand
  • In substituted phenols, the presence of electron withdrawing groups enhances the acidic strength of phenol.
  • This effect is more pronounced when such a group is present at ortho and para positions.
post image
expand
Electron releasing groups do not favour the formation of phenoxide ion resulting in decrease in acid strength
post image
expand
Electrophilic aromatic substitution
  • The -OH group attached to the benzene ring activates it towards electrophilic substitution.
  • It directs the incoming groups to ortho and para positions in the ring as these positions become electron rich due to the resonance effect caused by -OH group
post image
expand
Nitration
With dilute nitric acid at low temperature, phenol yields a mixture of ortho and para nitrophenols.
post image
expand
With concentrated nitric acid, phenol is converted to 2,4,6-trinitrophenol or picric acid
post image
expand
Halogenation
When the reaction is carried out in solvents of low polarity such as or and at low temperature, mono-bromophenols are formed.
post image
expand
When phenol is treated with bromine water, 2,4,6-tribromophenol is formed as white precipitate
post image
expand
Reimer-Tiemann reaction
On treating phenol with chloroform in the presence of sodium hydroxide, a -CHO group is introduced at the ortho position of benzene ring.
post image
expand
Reaction with zinc dust
Phenol is converted to benzene on heating with zinc dust.
post image
expand
Oxidation
Phenol gets oxidised by chromic acid to give a diketone known as benzoquinone.
post image
expand
Kolbe's Reaction
post image
expand
5
Ethers
Preparation
Dehydration of alcohols
  • Reaction conditions influence the end products
post image
expand
  • Method is suitable for the preparation of ethers having primary alkyl groups only.
Williamson synthesis
  • Alkyl halide is allowed to react with sodium alkoxide
post image
expand
Ethers containing substituted alkyl groups (secondary or tertiary) can also be prepared by this method
Physical Properties
  • The C-O bonds in ethers are polar
  • They have a net dipole moment
post image
expand
Boiling point is comparable to those of the alkanes of similar molecular masses
post image
expand
Solubility in water is similar to that of alcohols because of their ability to form hydrogen bond with water.
post image
expand
Chemical Properties
Cleavage of C-O bond
  • Least reactive functional group
  • The cleavage of C-O bond in ethers takes place under drastic conditions
  • The reaction of dialkyl ether gives two alkyl halide molecules.
post image
expand
  • Alkyl aryl ethers are cleaved at the alkyl-oxygen bond due to the more stable aryl-oxygen bond.
  • The reaction yields phenol and alkyl halide.
post image
expand
Ethers with two different alkyl groups are also cleaved in the same manner.
post image
expand
  • Order of reactivity of hydrogen halides is HI > HBr > HCl

Electrophilic substitution


  • The alkoxy group (-OR) is ortho, para directing and activates the aromatic ring towards electrophilic substitution
post image
expand
Halogenation
  • Phenyl alkyl ethers undergo usual halogenation in the benzene ring
post image
expand
Alkylation and Acylation
  • Alkyl and acyl groups are introduced at ortho and para positions by reaction with alkyl halide and acyl halide in the presence of anhydrous aluminium chloride as catalyst
post image
expand
Nitration
Anisole reacts with a mixture of concentrated sulphuric and nitric acids to yield a mixture of ortho and para nitro anisole.
post image
expand