Alcohols, Phenols And Ethers

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Alcohols, Phenols And Ethers

- A quick revision of all the important concepts

Classification of Alcohols, Phenols and Ethers
Classification of alcohols can be made on the number of -OH groups attached.

Monohydric alcohols can be further classified on the basis of Hybridization of the Carbon to which the -OH group is attached

Allylic alcohols: In these alcohols, the -OH group is attached to a

s p^{3}

hybridised carbon adjacent to the carbon-carbon double bond

Benzylic alcohols: In these alcohols, the -OH group is attached to a

s p^{3}

hybridised carbon atom next to an aromatic ring.

Phenols can be classified on the basis of number of OH groups attached directly to the aromatic system as

Ethers are classified on the basis of the symmetry of the molecules around the C-O-C bond

Simple or symmetrical

Mixed or unsymmetrical

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'.

Phenols

In IUPAC nomenclature, the parent molecule is called phenol

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

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.

Ethers

The alkoxy groups with shorter carbon chain is a substituent
The longest carbon chain is the parent chain
The substituents are placed alphabetically

Alcohols
Preparation
From Alkenes
Acid catalysed hydration:- Addition of water molecule across the double bond in the presence of Acid.

In case of unsymmetrical alkenes, the addition reaction takes place in accordance with Markovnikov's rule

Hydroboration-oxidation:-Diborane

(B H_{3})_{2}

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

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

Aldehydes and ketones can also be reduced by with Sodium borohydride (

N a B H_{4}

) or Lithium aluminum hydride (

L i A l H_{4}

Reduction of carboxylic acids and esters :- Carboxylic acids are reduced to primary alcohols by lithium aluminium hydride

From Grignard reagent:-

Physical Properties

Boiling points of alcohols are higher in comparison to other hydrocarbons.
This is due to the presence of intermolecular hydrogen bonding

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

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 ( $- C H_{3}$ , $- C_{2} H_{5}$ ) increase electron density on oxygen tending to decrease in the polarity of O-H bond - reducing the acidic nature

Reaction with metals: Alcohols and phenols react with metals such as sodium, potassium and aluminium to yield corresponding alkoxides/phenoxides and hydrogen

Esterification Reaction:- Alcohols and phenols react with carboxylic acids, acid chlorides and acid anhydrides to form esters.

Reactions involving cleavage of carbon-oxygen (C-O) bond in alcohols

Alcohols react as electrophiles when the bond between C-O is broken

Reaction with hydrogen halides:- Alcohols react with hydrogen halides to form alkyl halides

Oxidation

Oxidation of alcohols involves the formation of a carbon-oxygen double bond with cleavage of an O-H and C-H bonds

Primary alcohols are oxidised to aldehydes which in turn get oxidised to carboxylic acids depending on the oxidising agent used

Secondary alcohols are oxidised to ketones by chromic anhydride

Tertiary alcohols do not undergo oxidation reactions easily.
Strong oxidising agents ( $K M n O_{4}$ ) and elevated temperatures lead to cleavage of various C-C bonds
Mixture of carboxylic acids containing lesser number of carbon atoms is formed

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.

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

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.

From Diazonium salts

A diazonium salt is formed by treating an aromatic primary amine with nitrous acid ( $N a N O_{2} + H C l$ ) at 273-278 K.
Diazonium salts are hydrolysed to phenols by warming with water or by treating with dilute acids

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

Physical Properties
Boiling points of phenols are higher in comparison to other hydrocarbons due to the presence of intermolecular hydrogen bonding.

Solubility of phenols in water is due to their ability to form hydrogen bonds with water molecules

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

In phenoxide ion, the charge is delocalised.

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.

Electron releasing groups do not favour the formation of phenoxide ion resulting in decrease in acid strength

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

Nitration
With dilute nitric acid at low temperature, phenol yields a mixture of ortho and para nitrophenols.

With concentrated nitric acid, phenol is converted to 2,4,6-trinitrophenol or picric acid

Halogenation
When the reaction is carried out in solvents of low polarity such as

C H C l_{3}

C S_{2}

and at low temperature, mono-bromophenols are formed.

When phenol is treated with bromine water, 2,4,6-tribromophenol is formed as white precipitate

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.

Reaction with zinc dust
Phenol is converted to benzene on heating with zinc dust.

Oxidation
Phenol gets oxidised by chromic acid to give a diketone known as benzoquinone.

Kolbe's Reaction

Ethers
Preparation
Dehydration of alcohols

Reaction conditions influence the end products

Method is suitable for the preparation of ethers having primary alkyl groups only.

Williamson synthesis

Alkyl halide is allowed to react with sodium alkoxide

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

Boiling point is comparable to those of the alkanes of similar molecular masses

Solubility in water is similar to that of alcohols because of their ability to form hydrogen bond with water.

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.

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.

Ethers with two different alkyl groups are also cleaved in the same manner.

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

Halogenation

Phenyl alkyl ethers undergo usual halogenation in the benzene ring

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

Nitration
Anisole reacts with a mixture of concentrated sulphuric and nitric acids to yield a mixture of ortho and para nitro anisole.