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Alcohols, phenols, ethers, aldehydes, ketones and carboxylic acids are compounds with
functional groups containing oxygen.
An alcohol contains one or more hydroxyl (OH) groups(s) attached to aliphatic carbon atom(s).
For e.g., ethyl alcohol formula CH3, CH2, OH is an alcohol.
A phenol contains -OH group(s) directly attached to an aryl carbon atom(s). The simplest phenol
is hydroxybenzene also called phenol with formula C6H5OH.
In an ether, an oxygen atom is attached to two carbon atoms of two alkyl or an alkyl and an aryl
or two aryl groups. The simplest ether is CH3-O-CH3.
The compounds under these 3 classes have wide application in our daily life as well as in
industry. For e.g., ethanol is widely used, as an antiseptic in the form of rectified spirit. It is an
important component important of all alcoholic beverages and is widely used as a solvent for
lacquers and varnishes.
Simple phenol is an antiseptic. A phenolic compound hexachlorophene is a constituent of
several mouthwashes, deodorant soaps and medicinal skin cleansers.
The simple ether ethoxyethane has been used as an anaesthetic for a long time and is widely
used as a solvent too.
Some alcohols, phenols and ethers occur in nature and are used in the manufacture of
perfumes and flavors due to their pleasant odors.
From alcohols various other classes of organic compounds can be synthesized. Phenols are
used in the manufacture of dyes are resins like bakelite.
[It must be noted that the aromatic compounds in which -OH group is not directly attached to
benzene ring are not phenols but are called aromatic alcohols. These may be regarded as aryl
derivatives of aliphatic alcohols.]
Alcohols end phenols are classified as mono-, di- and trihydric alcohols according to the number
of -OH grounds contained in their molecules.
Some examples are shown below:
Monohydric alcohols are classified as primary, secondary or tertiary alcohols depending upon
whether the hydroxyl group is attached to primary, secondary or tertiary carbon atom. For e.g.,
In 1o alcohol, only one carbon atom is attached to the carbon carrying the -OH group.
In 2o alcohols two carbon are attached to the carbon carrying the -OH group.
In 3o alcohols, the carbon atom carrying the -OH group is a tertiary carbon, i.e., it is attached to
three other carbon atoms.
Ethers are known as simple or symmetrical, if the two alkyl or aryl groups attached to the
oxygen atom are same.
Example: C2H5 - O - C2H5 is called diethyl ether and is a simple ether.
Ethers are known as mixed or unsymmetrical if the two groups attached to the oxygen atom are
different.
Example: C2H5 - O - CH3 called ethyl methyl ether is a mixed ether.
In the common system, alcohols are named as alkyl alcohols. The word alcohol is added after
the name of the alkyl group to which the hydroxyl group is attached. For e.g., CH3OH is methyl
alcohol.
In the IUPAC system, the names of saturated alcohols are derived from corresponding alkenes
by replacing 'e' of alkenes by 'ol'
Some examples are shown below.
The numbering is done such that the carbon atom attached to the
-OH group gets the lowest number.
For naming polyhydric alcohols, the name of the alkane is retained and the ending -e is not
dropped. Thus dihydric alcohols are named as alkane diols and trihydric alcohols are named as
alkene triols.
The position of carbon atoms carrying -OH groups are indicated by locants written after the
name of alkene. The number of hydroxyl groups is indicated by adding the multiplicative prefix
di, tri, tetra etc., before the suffix-ol.
Phenols are named as derivatives of the simplest compound of this class i.e., phenol.
Examples:
Methyl phenols are commonly called as cresols.
Dihydroxy derivatives
Trihydroxy derivatives
Common names of ethers follow after the names of alkyl / aryl groups written as separate words
in alphabetical order. The word ether is added at the end.
In case of simple ethers, the prefix di is attached before the name of the alkyl group.
Examples:
C2H5 - O - C2H5 is Diethyl ether
C6H5 - O - C6H5 is Diphenyl ether
C2H5 - O - C6H5 is Dthyl phenyl ether.
According to the IUPAC nomenclature ethers are regarded as hydrocarbon derivatives in which
a hydrogen atom is replaced by an alkoxy group - OR, the larger group (R) being chosen as the
parent hydrocarbon. Ethers are named as alkoxyalkenes. The larger alkyl group forms the part
of parent chain while lower alkyl group constitutes the alkoxy radical.
Examples:
The numbering of the parent chain is done so that the carbon atom linked to the -O-atom gets
the lowest number.
Structure of Functional Groups
n alcohol, the oxygen of the -OH group is attached to sp3 hybridized carbon by a sigma (s) bond
formed by the overlap of sp3 hybrid orbital of carbon with an sp3 hybrid orbital of oxygen.
Since the oxygen atom of the hydroxyl group has two bond pairs and two lone pairs of
electrons, there is repulsion between the unshared electron pairs of oxygen. Thus the C-O-H
bonds in alcohols as well as phenols is not linear. The bond angle
in alcohols is slightly less than the tetrahedral angle (109o 28')
In phenols the -OH group is attached to sp2 hybrid carbon of an aromatic ring.
The bond angle
in phenol is 109o. The C - O bond length (136 pm) in phenol is slightly less than in methanol.
This is due to partial double bond character on account of the conjugation of unshared electron
pair of oxygen with the aromatic ring
further stabilized by resonance.
As oxygen is more electronegative then carbon and hydrogen, therefore C-O and O-H bonds in
alcohols and phenols are polar bonds and hence both alcohols and phenols possess a net
dipole moment.
Methanol has a dipole moment of 1.71 D whereas phenol has a dipole moment of 1.54 D. The
smaller dipole moment of phenol is due to the electron withdrawing effect of phenyl group in
contrast to the electron releasing effect of alkyl group in alcohols.
In ethers, the four electron pairs i.e., the two bond pairs and two lone pairs of electrons around
the sp3 hybridized oxygen are arranged approximately in a tetrahedral arrangement. This bond
angle is slightly greater than the tetrahedral angle due to the repulsive interaction
between the two bulky (-R) groups.
The C - O bond length (141 pm) in ethers is almost as same as in alcohols.
Thus ethers have bent structure and their dipole moment is greater than zero. Hence their
molecules are polar in nature.
Isomerism in Alcohols and Ethers
Alcohols exhibit following types of isomerism:
1. Chain isomerism
Alcohols with four or more carbon atoms exhibit this type of isomerism in which the carbon
skeleton is different.
2. Position isomerism
Alcohols with three or more carbon atoms can exhibit position isomerism. In this type of
isomerism the position of the functional group i.e., the -OH group varies. In other words the
carbon atoms to which the -OH group is attached is different.
3. Functional isomerism
Alcohols with two or more carbon atoms can exhibit functional isomerism with ethers. Thus
ethers and alcohols have the same molecular formula but have different functional groups,
hence they are called functional isomers.
4. Optical isomerism
Alcohols containing chiral centrescen exhibit enantiomerismor optical isomerism. The optical
isomers can rotate the plane of plane polarized angles in different directions.
* represents an asymmetric carbon atom.
Isomerism in Aliphatic Ethers
Aliphatic Ethers can give two different types of isomers.
1. Chain isomerism
Ethers with the same formula and having different carbon chain skeletons are called chain
isomers.
Examples:
2. Functional isomers
Ethers are isomeric with alcohols.
Example:
is isomeric with ethyl alcohol C2H5OH.
3. Metamerism
Isomers with the same molecular formula but different alkyl groups (around the functional group)
are called metamers. An ether with formula C4H10O has 3 metamers.
Preparation of Alcohols
1. From aldehyde and ketones
Aldehydes and ketones are reduced to the corresponding alcohols by
a) Addition of hydrogen in the presence of catalysts like finely divided platinum, palladium,
nickel and ruthenium.
This method is called catalytic hydrogenation.
b) Treatment with chemical reagents such as sodium borohydride (NaBH4) or Lithium aluminium
hydride (LiAlH4).
Aldehydes yield primary alcohols while ketones give secondary alcohols.
2. From carboxylic acids and esters
Carboxylic acids are reduced to primary alcohols in the presence of strong reducing agent like
lithium aluminium hydride.
The yield of alcohol here is high but LiAlH4 being an expensive reagent, this method is not
commonly used.
Commercially acids are reduced to alcohols by converting them to the esters followed by their
reduction using either:
a) Hydrogen in the presence of a catalyst (Catalytic hydrogenation) or
b) Sodium and alcohol.
From alkenes
1) Hydration
Alkenes undergo hydration (addition of water across C=C bond) in the presence of dilute H2SO4
to produce alcohols. The alkyl hydrogen sulphate is formed which on hydrolysis with hot water
gives alcohol.
The addition of water to the double bond is in accordance with Markownikov's rule. The alkene
is obtained by cracking of hydrocarbons. The alkene is then absorbed by passing it into
sulphuric acid at 353 K and 30 atmosphere pressure. The acid is diluted and treated with stream
to release the alcohol.
The preparation of ethyl alcohol is done starting with ethane.
Similarly isopropyl alcohol is prepared from Propene.
b) Oxymercuration - demercuration
Alkenes react with mercuric acetate in presence of water to yield hydroxy mercurial compounds.
These are reduced to alcohols by sodium borohydride.
This reaction gives a good yield of alcohol.
The alcohol obtained corresponds to Markownikov's addition of water to an alkene.
c) Hydroboration
Alkenes react with diborane (B2H6), which is an electron deficient molecule to yield alkylboranes
(R3B). These are oxidised to alcohols on reaction with hydrogen peroxide in presence of alkali.
In each addition step, the boron atom is attached to the sp2 carbon atom that is bonded to
greater number of hydrogen atoms. The hydrogen atom of the boron atom attaches to the other
carbon of the double bond. Thus this is anti-Markovnikov's addition.
During the oxidation of trialkyl borane, boron is replaced by -OH group.
The yield of alcohol in this method is good and the product is easy to isolate.
d) From Grignard's reagent
Grignard reagents (R MgX) are alkyl or aryl magnesium halides.
The C
Mg bondin Grignard reagent is a highly polar bond as carbon is electronegative
relative to electropositive magnesium. Due to this polar nature of C-Mg bond, Grignard reagents
are very versatile magnets in organic synthesis.
Grignard reagents regents react with aldehydes and ketones to form products, which
decompose with dil HCl or dil H2SO4 to give primary secondary and tertiary alcohols.
The overall result is to bind the alkyl group of Grignard reagent to carbon of the carbonyl group
and hydrogen to oxygen. Formaldehyde gives primary alcohol where as all other aldehydes give
secondary alcohols and ketones furnish tertiary alcohols.
Preparation of Phenols
In the early nineteenth century, phenol was selected from coal tar by destructive distillation.
Now phenol is commercially produced synthetically. The laboratory methods of preparation of
phenols are:
i) From aryl sulphonic acids
An aryl sulphonic acid yields the corresponding phenol on heating it with molten sodium
hydroxide at 570 - 620 K. The sodium salt is obtained which is hydrolysed with acid to obtain
free phenol.
ii) From haloarenes
Chlorobenzene (an haloarene) is hydrolysed by treating it with 10% NaOH at 623 K and 320
atmospheric pressure in presence of Cu catalyst. Phenol is obtained by acidification of sodium
phenoxide.
iii) Hydrolysis of diazonium salts
A diazonium salt is formed by treating an aromatic primary amine with nitrous acid (obtained
from a mixture of NaNO2 and HCl) at low temperature of 273 K to 278 K.
Diazonium salts are hydrolysed to phenols treating with dilute acids.
Physical Properties of Alcohols and Phenols
Alcohols and Phenols consist of two parts, an alkyl/aryl group and a hydroxyl group. The
properties of alcohols and phenols are due to the -OH group.
The alkyl and aryl groups modify these properties.
1. The lower members of alcohols are colourless, volatile liquids with a characteristic alcoholic
smell and burning taste whereas higher alcohols are odourless and tasteless.
Higher alcohols having 12 or more carbon atoms are colourless waxy solids. Phenols are
colourless, crystalline solids or liquids. (They may become coloured due to slow oxidation with
air).
2. Solubility of alcohols
The first three members are completely miscible with water. The solubility rapidly decreases
with increase in molecular mass. The higher members are almost insoluble in water but are
soluble in organic solvents like benzene, ether etc.
The solubility of lower alcohols is due to the existence of hydrogen bonds between water and
polar -OH group of alcohol molecules. Phenols too are sparingly soluble in water.
The -OH group in alcohols and phenols contain a hydrogen bonded to an electronegative
oxygen atom. Thus they form hydrogen bonds with water molecules.
The solubility of alcohols in water decreases with increase in molecular mass because the
increase in molecular mass, the non polar alkyl group becomes predominant and masks the
effect of polar -OH group.
In addition, among the isomeric alcohols, the solubility increases with branching of chain. As the
surface area of the non-polar part in the molecule decreases, the solubility increases.
Phenols are sparingly soluble in water but readily soluble in organic solvents such as alcohol
and ether.
Boiling Point of Alcohols and Phenols
Boiling point of alcohols are much higher than those of alkenes, halo alkenes or ethers of
comparable molecular masses. This is because in alcohols intermolecular hydrogen bonding
exists due to which a large amount of energy is required to break these bonds.
Phenols boil at higher temperatures them the arenes of comparable molecular masses. The
higher boiling point of phenols is due to the pressure of intermolecular hydrogen bonding in
them.
Among isomeric alcohols, the boiling point decreases with increase in branching in the alkyl
group.
For isomeric alcohols the boiling points decreases with increase in branching in the alkyl group.
For isomeric alcohols, the boiling points follow the order.
Primary alcohol > secondary alcohol > tertiary alcohol
Intoxicating effect
Alcohols are known to have intoxicating effect. Methanol is poisonous and is not good for
drinking purposes. It causes blindness. Ethanol can be used for drinking purposes.
Chemical Properties of Alcohols and Phenols
In alcohols and phenols, -OH group is the functional group. Thus the chemical properties of
alcohols generally involve the reactions of -OH group. They can undergo substitution as well as
elimination reaction.
Reactions of alcohols are classified into three types.
1) Reactions involving cleavage of -OH bond.
Reaction with metals
Alcohols and phenols react with metals such as sodium, potassium and aluminium to yield
corresponding alkoxides and hydrogen.
In addition to this, phenols react with aqueous sodium hydroxide to form sodium phenoxides.
The above reactions show that alcohols and phenols are acidic in nature. In fact, alcohols and
phenols are Bronsted acids i.e., they can donate a proton to a strong base (B:).
On treating in alkoxide with water, the starting alcohol is obtained.
This reaction shows that water is a better proton donar than alcohol i.e., alcohols are very weak
acids even feeble than water. They do not turn blue litmus to red but treated with active metals
liberate hydrogen.
The acidic character of alcohols is due to the presence of polar O-H group. Due to greater
electro negativity of oxygen atom, the shared pair between O and H is drawn towards the
oxygen atom helping in release of H+ ion.
An electron - releasing alkyl group (-CH3, -C2H5) increases electron density over the oxygen
atom tending to decrease the polarity of O-H bond. This decreases the acid strength. In
addition, the alkoxide ion formed is also destabilised due to concentration of negative charge on
oxygen atom by electron releasing inductive effect of alkyl group. Thus the release of H+ ion is
difficult.
The acid strength of alcohols decreases in the following order:
The tertiary alcohols are least acidic while primary alcohols (with only one alkyl group) are most
acidic.
Consider the reaction
This reaction shows that alkoxide ion is a better proton acceptor then hydroxide ion, which show
that alkoxides are stronger bases (sodium ethoxide is a stronger base than sodium hydroxide)
The basic strength of the alkoxides follow the order
Alcohols act as Bronsted bases as well. It is due to the presence of unshared electron pairs
over oxygen, which makes alcohols proton acceptors.
Acidity of Phenols
Phenols turn blue litmus red and react with metals liberating hydrogen. However they do not
react with carbonates or bicarbonates.
Phenols behave as acids because of the presence of polar O-H group in them. They ionise in
aqueous solutions to give H+ ions.
Phenols as well as phenoxide ion both are resonance stabilised. The various contributing
structures of phenol and phenoxide ion are given below:
Phenoxide
Phenoxide is more stabilised by resonance than phenol. In phenol, three contributing structures
involve charge separation whereas in case of phenoxide ion there is no charge separation.
Since phenoxide ion is more stabilised than phenol, therefore the equilibrium move to the
dissociated form and hence phenols furnish a high concentration of H+ ions and behave as fairly
strong acids.
On the other hand, in the case of alcohols neither alcohol nor alkoxide ion is stabilised by
resonance and hence they behave as weaker acids than phenols.
In substituted phenols, the presence of electron withdrawing groups at ortho and para positions
such as nitro group, stabilises the phenoxide ion by dispersal of negative charge resulting in an
increase in acid strength. Thus ortho and para nitrophenols are more acidic than phenol.
On the other hand, the electron releasing groups such as alkyl groups destabilise the phenoxide
ion by concentrating the negative charge. This causes a decrease in the acid strength. Thus
cresols are less acidic than phenol.
Esterification of Alcohols and Phenols
Alcohols and Phenols react with carboxylic acids, acid chlorides and acid anhydrides to form
esters. The reactions between alcohols and acids are carried out in the presence of a small
amount of concentrated sulphuric acid. H2SO4 acts as a protonating agent as well as a
dehydrating agent. This reaction is called esterification.
The reaction is reversible and therefore water is removed as soon it is formed.
The reaction with acid chloride is carried out in presence of a base (pyridine) so in presence of a
base (pyridine) so as to remove HCl. It also shifts the equilibrium to the right hand side.
The introduction of acetyl (CH3CO-) group in alcohols or phenols in known as acetylation.
Acetylation of salicylic acid produces aspirin, which possesses analgesic, anti-inflammatory and
antipyretic properties.
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