After studying this Unit, you will be

able to

? name alcohols, phenols and

ethers according to the IUPAC

system of nomenclature;

? discuss the reactions involved in

the preparation of alcohols from

(i) alkenes (ii) aldehydes, ketones

and carboxylic acids;

? discuss the reactions involved in

the preparation of phenols from

(i) haloarenes (ii) benzene

sulphonic acids (iii) diazonium

salts and (iv) cumene;

? discuss the reactions for

preparation of ethers from

(i) alcohols and (ii) alkyl halides

and sodium alkoxides/aryloxides;

? correlate physical properties of

alcohols, phenols and ethers with

their structures;

? discuss chemical reactions of the

three classes of compounds on

the basis of their functional

groups.

Objectives

Alcohols, Phenols

and Ethers

You have learnt that substitution of one or more

hydrogen atom(s) from a hydrocarbon by another atom

or a group of atoms result in the formation of an entirely

new compound having altogether different properties

when a hydrogen atom in a hydrocarbon, aliphatic and

aromatic respectively, is replaced by ?OH group. These

classes of compounds find wide applications in industry

as well as in day-to-day life. For instance, have you

ever noticed that ordinary spirit used for polishing

wooden furniture is chiefly a compound containing

hydroxyl group, ethanol. The sugar we eat, the cotton

used for fabrics, the paper we use for writing, are all

made up of compounds containing ?OH groups. Just

think of life without paper; no note-books, books, newspapers,

currency notes, cheques, certificates, etc. The

magazines carrying beautiful photographs and

interesting stories would disappear from our life. It

would have been really a different world.

An alcohol contains one or more hydroxyl (OH)

group(s) directly attached to carbon atom(s), of an

group(s) directly attached to carbon atom(s) of an

The subsitution of a hydrogen atom in a

hydrocarbon by an alkoxy or aryloxy group

(R?O/Ar?O) yields another class of compounds known

may also visualise ethers as compounds formed by

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The classification of compounds makes their study systematic and

hence simpler. Therefore, let us first learn how are alcohols, phenols

and ethers classified?

Alcohols and phenols may be classified as mono?, di?, tri- or

polyhydric compounds depending on whether they contain one, two,

three or many hydroxyl groups respectively in their structures as

given below:

substituting the hydrogen atom of hydroxyl group of an alcohol or

phenol by an alkyl or aryl group.

In this unit, we shall discuss the chemistry of three classes of

compounds, namely ? alcohols, phenols and ethers.

11.1 Classification

Monohydric alcohols may be further classified according to the

hybridisation of the carbon atom to which the hydroxyl group is

attached.

alkyl group. They are further classified as follows:

alcohols, the ?OH group is attached to primary, secondary and

tertiary carbon atom, respectively as depicted below:

that is to an allylic carbon. For example

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Allylic and benzylic alcohols may be primary, secondary or tertiary.

?OH group bonded to a carbon-carbon double bond i.e., to a

vinylic carbon or to an aryl carbon. These alcohols are also known

as vinylic alcohols.

are unsymmetrical ethers.

CH

OH

(iv)

OH

(v)

CH CH C OH

(vi)

Intext Questions

common name of the alkyl group and adding the word alcohol to it.

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According to IUPAC system (Unit 12, Class XI), the name of an alcohol

is derived from the name of the alkane from which the alcohol is derived,

substituents are indicated by numerals. For this, the longest carbon

chain (parent chain) is numbered starting at the end nearest to the

hydroxyl group. The positions of the ?OH group and other substituents

are indicated by using the numbers of carbon atoms to which these are

attached. For naming polyhydric alcohols, the ?e? of alkane is retained

adding the multiplicative prefix, di, tri, etc., before ?ol?. The positions of

is named as ethane?1, 2-diol. Table 11.1 gives common and IUPAC

names of a few alcohols as examples.

Isopropyl alcohol Propan-2-ol

Isobutyl alcohol 2-Methylpropan-1-ol

Glycerol Propane -1, 2, 3-triol

Cyclic alcohols are named using the prefix cyclo and considering

the ?OH group attached to C?1.

OH

OH

Cyclohexanol 2-Methylcyclopentanol

It is its common name and also an accepted IUPAC name. As structure

of phenol involves a benzene ring, in its substituted compounds the

(1,4-disubstituted) are often used in the common names.

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Dihydroxy derivatives of benzene are known as 1, 2-, 1, 3- and

1, 4-benzenediol.

OH

OH

OH

OH

OH

OH

OH

OH

OH

Benzene-1,2-diol

Resorcinol

Benzene-1,3-diol

Hydroquinone or quinol

aryl groups written as separate words in alphabetical order and adding the

(Anisole) (Anisole)

(Phenetole)

Methyl isopropyl ether 2-Methoxypropane

Phenylisopentyl ether 3- Methylbutoxybenzene

? 2-Ethoxy-

-1,1-dimethylcyclohexane

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If both the alkyl groups are the same, the prefix ?di? is added before the alkyl

According to IUPAC system of nomenclature, ethers are regarded as

hydrocarbon derivatives in which a hydrogen atom is replaced by an

respectively. The larger (R) group is chosen as the parent hydrocarbon.

The names of a few ethers are given as examples in Table 11.2.

(i) 4-Chloro-2,3-dimethylpentan-1-ol (ii) 2-Ethoxypropane

(iii) 2,6-Dimethylphenol (iv) 1-Ethoxy-2-nitrocyclohexane

Example 11.1

Solution

OH

(i)

(iii)

Cl

CH CH

(iv)

Intext Question

(i) (ii)

(iii) (iv) (v)

structural aspects of methanol, phenol and methoxymethane.

11.3 Structures of

Functional

Groups

Give IUPAC names of the following compounds:

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The bond angle in alcohols is slightly less than the tetrahedral

hybridised carbon of an aromatic ring. The carbon? oxygen bond

length (136 pm) in phenol is slightly less than that in methanol. This

is due to (i) partial double bond character on account of the conjugation

of unshared electron pair of oxygen with the aromatic ring (Section

attached.

In ethers, the four electron pairs, i.e., the two bond pairs and two

lone pairs of electrons on oxygen are arranged approximately in a

tetrahedral arrangement. The bond angle is slightly greater than the

tetrahedral angle due to the repulsive interaction between the two

as in alcohols.

Alcohols are prepared by the following methods:

presence of acid as catalyst to form alcohols. In case of

unsymmetrical alkenes, the addition reaction takes place in

accordance with Markovnikov?s rule (Unit 13, Class XI).

The mechanism of the reaction involves the following three steps:

Step 1: Protonation of alkene to form carbocation by electrophilic

Step 2: Nucleophilic attack of water on carbocation.

Step 3: Deprotonation to form an alcohol.

11.4 Alcohols and

Phenols

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to give trialkyl boranes as addition product. This is oxidised to

alcohol by hydrogen peroxide in the presence of aqueous sodium

hydroxide.

The addition of borane to the double bond takes place in such

carrying greater number of hydrogen atoms. The alcohol so formed

looks as if it has been formed by the addition of water to the

alkene in a way opposite to the Markovnikov?s rule. In this reaction,

alcohol is obtained in excellent yield.

are reduced to the corresponding alcohols by addition of

hydrogen in the presence of catalysts (catalytic hydrogenation).

The usual catalyst is a finely divided metal such as platinum,

palladium or nickel. It is also prepared by treating aldehydes

whereas ketones give secondary alcohols.

are reduced to primary alcohols in excellent yields by lithium

aluminium hydride, a strong reducing agent.

for preparing special chemicals only. Commercially, acids are

reduced to alcohols by converting them to the esters (Section

11.4.4), followed by their reduction using hydrogen in the

presence of catalyst (catalytic hydrogenation).

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Alcohols are produced by the reaction of Grignard reagents (Unit 10,

Class XII) with aldehydes and ketones.

The first step of the reaction is the nucleophilic addition of Grignard

reagent to the carbonyl group to form an adduct. Hydrolysis of the

adduct yields an alcohol.

... (i)

...(ii)

The overall reactions using different aldehydes and ketones are as

follows:

You will notice that the reaction produces a primary alcohol with

methanal, a secondary alcohol with other aldehydes and tertiary alcohol

with ketones.

Butan-1-ol

Give the structures and IUPAC names of the products expected from

the following reactions:

(a) Catalytic reduction of butanal.

(b) Hydration of propene in the presence of dilute sulphuric acid.

(c) Reaction of propanone with methylmagnesium bromide followed

by hydrolysis.

Example 11.2

Solution

2-Methylpropan-2-ol

C OH

OH

Propan-2-ol

(a) (b) (c)

Phenol, also known as carbolic acid, was first isolated in the early

nineteenth century from coal tar. Nowadays, phenol is commercially

produced synthetically. In the laboratory, phenols are prepared from

benzene derivatives by any of the following methods:

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Chlorobenzene is fused with NaOH at 623K and 320 atmospheric

pressure. Phenol is obtained by acidification of sodium phenoxide so

produced (Unit 10, Class XII).

Benzene is sulphonated with oleum and benzene sulphonic acid so

formed is converted to sodium phenoxide on heating with molten

sodium hydroxide. Acidification of the sodium salt gives phenol.

A diazonium salt is formed by treating an aromatic primary amine

hydrolysed to phenols by warming with water or by treating with

dilute acids (Unit 13, Class XII).

H O

+HCl

Aniline

Benzene diazonium

chloride

Warm

+ ?

Phenol is manufactured from the hydrocarbon, cumene. Cumene

(isopropylbenzene) is oxidised in the presence of air to cumene

hydroperoxide. It is converted to phenol and acetone by treating it

with dilute acid. Acetone, a by-product of this reaction, is also

obtained in large quantities by this method.

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Alcohols and phenols consist of two parts, an alkyl/aryl group and a

hydroxyl group. The properties of alcohols and phenols are chiefly due

to the hydroxyl group. The nature of alkyl and aryl groups simply

modify these properties.

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

number of carbon atoms (increase in van der Waals forces). In alcohols,

the boiling points decrease with increase of branching in carbon chain

(because of decrease in van der Waals forces with decrease in surface

area).

The ?OH group in alcohols and phenols is involved in intermolecular

hydrogen bonding as shown below:

It is interesting to note that boiling points of alcohols and phenols

are higher in comparison to other classes of compounds, namely

hydrocarbons, ethers, haloalkanes and haloarenes of comparable

molecular masses. For example, ethanol and propane have comparable

molecular masses but their boiling points differ widely. The boiling

point of methoxymethane is intermediate of the two boiling points.

Grignard reagent on methanal ?

Intext Questions

(ii)

(iii)

(i)

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The high boiling points of alcohols are mainly due to the presence

of intermolecular hydrogen bonding in them which is lacking in ethers

and hydrocarbons.

Solubility of alcohols and phenols in

water is due to their ability to form

hydrogen bonds with water molecules

as shown. The solubility decreases with

increase in size of alkyl/aryl (hydrophobic)

groups. Several of the lower

molecular mass alcohols are miscible

with water in all proportions.

Arrange the following sets of compounds in order of their increasing

boiling points:

(a) Pentan-1-ol, butan-1-ol, butan-2-ol, ethanol, propan-1-ol, methanol.

(b) Pentan-1-ol, n-butane, pentanal, ethoxyethane.

(a) Methanol, ethanol, propan-1-ol, butan-2-ol, butan-1-ol, pentan-1-ol.

(b) n-Butane, ethoxyethane, pentanal and pentan-1-ol.

Example 11.3

Solution

Alcohols are versatile compounds. They react both as nucleophiles and

electrophiles. The bond between O?H is broken when alcohols react as

nucleophiles.

(ii) The bond between C?O is broken when they react as

electrophiles. Protonated alcohols react in this manner.

Based on the cleavage of O?H and C?O bonds, the reactions

of alcohols and phenols may be divided into two groups:

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metals such as sodium, potassium and aluminium to yield

corresponding alkoxides/phenoxides and hydrogen.

In addition to this, phenols react with aqueous sodium

hydroxide to form sodium phenoxides.

Sodium phenoxide

+ NaOH

The above reactions show that alcohols and phenols are

acidic in nature. In fact, alcohols and phenols are Brönsted

acids i.e., they can donate a proton to a stronger base (B:).

the polar nature of O?H bond. An electron-releasing group

decrease the polarity of O-H bond. This decreases the acid

strength. For this reason, the acid strength of alcohols decreases

in the following order:

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Alcohols are, however, weaker acids than water. This can be

illustrated by the reaction of water with an alkoxide.

This reaction shows that water is a better proton donor (i.e.,

stronger acid) than alcohol. Also, in the above reaction, we note

that an alkoxide ion is a better proton acceptor than hydroxide

ion, which suggests that alkoxides are stronger bases (sodium

ethoxide is a stronger base than sodium hydroxide).

Alcohols act as Bronsted bases as well. It is due to the

presence of unshared electron pairs on oxygen, which makes

them proton acceptors.

sodium, aluminium) and sodium hydroxide indicate its acidic

nature. The hydroxyl group, in phenol is directly attached to

electron withdrawing group. Due to this, the charge distribution

in phenol molecule, as depicted in its resonance structures,

causes the oxygen of ?OH group to be positive.

The reaction of phenol with aqueous sodium hydroxide

indicates that phenols are stronger acids than alcohols and water.

Let us examine how a compound in which hydroxyl group

attached to an aromatic ring is more acidic than the one in

which hydroxyl group is attached to an alkyl group.

The ionisation of an alcohol and a phenol takes place as follows:

of phenol to which ?OH is attached, electron density decreases

on oxygen. This increases the polarity of O?H bond and results

in an increase in ionisation of phenols than that of alcohols.

Now let us examine the stabilities of alkoxide and phenoxide

ions. In alkoxide ion, the negative charge is localised on oxygen

while in phenoxide ion, the charge is delocalised.

The delocalisation of negative charge (structures I-V) makes

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phenoxide ion more stable and favours the ionisation of phenol.

Although there is also charge delocalisation in phenol, its

resonance structures have charge separation due to which the

phenol molecule is less stable than phenoxide ion.

From the above data, you will note that phenol is million times

more acidic than ethanol.

Arrange the following compounds in increasing order of their acid strength:

Propan-1-ol, 2,4,6-trinitrophenol, 3-nitrophenol, 3,5-dinitrophenol,

phenol, 4-methylphenol.

Propan-1-ol, 4-methylphenol, phenol, 3-nitrophenol, 3,5-dinitrophenol,

2,4, 6-trinitrophenol.

Example 11.4

Solution

Alcohols and phenols react with carboxylic acids, acid chlorides and

acid anhydrides to form esters.

In substituted phenols, the presence of electron withdrawing

groups such as nitro group, enhances the acidic strength of

phenol. This effect is more pronounced when such a group is

delocalisation of negative charge in phenoxide ion. On the other

hand, electron releasing groups, such as alkyl groups, in

general, do not favour the formation of phenoxide ion resulting

in decrease in acid strength. Cresols, for example, are less acidic

than phenol.

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Pyridine

The reaction with carboxylic acid and acid anhydride is carried

out in the presence of a small amount of concentrated sulphuric

acid. The reaction is reversible, and therefore, water is removed as

soon as it is formed. The reaction with acid chloride is carried out in

the presence of a base (pyridine) so as to neutralise HCl which is

formed during the reaction. It shifts the equilibrium to the right

phenols is known as acetylation. Acetylation of salicylic acid

produces aspirin.

The reactions involving cleavage of C?O bond take place only in

alcohols. Phenols show this type of reaction only with zinc.

halides to form alkyl halides (Refer Unit 10, Class XII).

The difference in reactivity of three classes of alcohols with HCl

and produce turbidity in solution. In case of tertiary alcohols, turbidity

is produced immediately as they form the halides easily. Primary

alcohols do not produce turbidity at room temperature.

alkyl bromides by reaction with phosphorus tribromide (Refer Unit

10, Class XII).

of water) to form alkenes on treating with a protic acid e.g.,

chloride or alumina (Unit 13, Class XI).

Ethanol undergoes dehydration by heating it with concentrated

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Secondary and tertiary alcohols are dehydrated under milder

conditions. For example

Thus, the relative ease of dehydration of alcohols follows the following

order:

Tertiary Secondary Primar> > y

The mechanism of dehydration of ethanol involves the following steps:

rate determining step of the reaction.

The acid used in step 1 is released in step 3. To drive the equilibrium

to the right, ethene is removed as it is formed.

double bond with cleavage of an O-H and C-H bonds.

Such a cleavage and formation of bonds occur in oxidation

these involve loss of dihydrogen from an alcohol molecule. Depending

on the oxidising agent used, a primary alcohol is oxidised to an

aldehyde which in turn is oxidised to a carboxylic acid.

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Strong oxidising agents such as acidified potassium permanganate

anhydrous medium is used as the oxidising agent for the isolation

of aldehydes.

3

2

A better reagent for oxidation of primary alcohols to aldehydes in

good yield is pyridinium chlorochromate (PCC), a complex of

chromium trioxide with pyridine and HCl.

3 2 3

Secondary alcohols are oxidised to ketones by chromic anhyride

Tertiary alcohols do not undergo oxidation reaction. Under strong

elevated temperatures, cleavage of various C-C bonds takes place

and a mixture of carboxylic

acids containing lesser number

of carbon atoms is formed.

When the vapours of a

primary or a secondary alcohol

are passed over heated copper

at 573 K, dehydrogenation

takes place and an aldehyde or

a ketone is formed while tertiary

alcohols undergo dehydration.

Biological oxidation of methanol and ethanol in the body produces the corresponding

aldehyde followed by the acid. At times the alcoholics, by mistake, drink ethanol,

mixed with methanol also called denatured alcohol. In the body, methanol is oxidised

first to methanal and then to methanoic acid, which may cause blindness and

death. A methanol poisoned patient is treated by giving intravenous infusions of

diluted ethanol. The enzyme responsible for oxidation of aldehyde (HCHO) to acid

is swamped allowing time for kidneys to excrete methanol.

Following reactions are shown by phenols only.

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In phenols, the reactions that take place on the aromatic ring are

electrophilic substitution reactions (Unit 13, Class XI). The ?OH group

attached to the benzene ring activates it towards electrophilic

positions in the ring as these positions become electron rich due to

the resonance effect caused by ?OH group. The resonance structures

are shown under acidity of phenols.

Common electrophilic aromatic substitution reactions taking place

in phenol are as follows:

intermolecular hydrogen bonding which causes the association

of molecules.

With concentrated nitric acid, phenol is converted to

2,4,6-trinitrophenol. The product is commonly known as picric

acid. The yield of the reaction product is poor.

Nowadays picric acid is prepared by treating phenol first

with concentrated sulphuric acid which converts it to

phenol-2,4-disulphonic acid, and then with concentrated nitric

acid to get 2,4,6-trinitrophenol. Can you write the equations of

the reactions involved?

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products are formed under different experimental conditions.

(a) When the reaction is carried out in solvents of low polarity

monobromophenols are formed.

The usual halogenation of benzene takes place in the

which polarises the halogen molecule. In case of phenol, the

polarisation of bromine molecule takes place even in the

absence of Lewis acid. It is due to the highly activating

effect of ?OH group attached to the benzene ring.

(b) When phenol is treated with bromine water,

2,4,6-tribromophenol is formed as white precipitate.

2,4,6-Tribromophenol

OH

Br

OH

Br

Br

2

Write the structures of the major products expected from the following

reactions:

(a) Mononitration of 3-methylphenol

(b) Dinitration of 3-methylphenol

(c) Mononitration of phenyl methanoate.

position of the incoming group.

Example 11.5

Solution

Phenoxide ion generated by treating phenol with sodium hydroxide

is even more reactive than phenol towards electrophilic aromatic

substitution. Hence, it undergoes electrophilic substitution with

formed as the main reaction product.

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On treating phenol with chloroform in the presence of sodium

The intermediate substituted benzal chloride is hydrolysed in the

presence of alkali to produce salicylaldehyde.

Phenol is converted to benzene on heating with zinc dust.

Oxidation of phenol with chromic

acid produces a conjugated diketone

known as benzoquinone. In the

presence of air, phenols are slowly

oxidised to dark coloured mixtures

containing quinones.

(i) Butan-1-ol (ii) 2-Methylbutan-2-ol

(i) 1-methylcyclohex