On the basis of structure of alcohols, their reactions may be divided into following types or groups for the purposes of study.
1. Reactions involving cleavage of oxygen-hydrogen bond.
2. Reactions involving cleavage of carbon-oxygen bond.
3. Reactions involving both the alkyl and hydroxyl groups.
1. Reactions involving cleavage of oxygen-hydrogen bond.
i) reaction with active metals
ii) reaction with metal hydrides
iii) reaction with carboxylic acids (esterification)
iv) reaction with Grignard reagents
v) Reaction with acyl chloride or acid anhydride (acylation)
2. Reactions involving cleavage of carbon-oxygen bond.
i) reaction with hydrogen haldies
ii) reaction with phosphorus halides
iii) reaction with thionyl chloride
3. Reactions involving both the alkyl and hydroxyl groups.
i) dehydration
ii) oxidation,
iii) dehydrogenation
reaction with ZnCl2/conc.-HCl, (Lucas test)
conversion of alcohols into aldehydes and ketones;
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1. Reactions involving cleavage of oxygen-hydrogen bond.
2. Reactions involving cleavage of carbon-oxygen bond.
3. Reactions involving both the alkyl and hydroxyl groups.
1. Reactions involving cleavage of oxygen-hydrogen bond.
i) reaction with active metals
ii) reaction with metal hydrides
iii) reaction with carboxylic acids (esterification)
iv) reaction with Grignard reagents
v) Reaction with acyl chloride or acid anhydride (acylation)
2. Reactions involving cleavage of carbon-oxygen bond.
i) reaction with hydrogen haldies
ii) reaction with phosphorus halides
iii) reaction with thionyl chloride
3. Reactions involving both the alkyl and hydroxyl groups.
i) dehydration
ii) oxidation,
iii) dehydrogenation
reaction with ZnCl2/conc.-HCl, (Lucas test)
conversion of alcohols into aldehydes and ketones;
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Alcohols -Reaction with active metals - acidic character
Alcohols are weakly acidic in nature.
They react with active metals such as sodium, potassium, magnesium, aluminium, etc. to liberate hydrogen gas and form metal alkoxide.
Liberation of hydrogen shows that alcohols are acidic in nature.
The acidic nature of alcohols is due to the presence of polar O-H bond.
As oxygen withdraws shared electron pair between O and H atoms towards itself, it can lose the proton (H+).
However, alcohols are weak acids
They react with active metals such as sodium, potassium, magnesium, aluminium, etc. to liberate hydrogen gas and form metal alkoxide.
Liberation of hydrogen shows that alcohols are acidic in nature.
The acidic nature of alcohols is due to the presence of polar O-H bond.
As oxygen withdraws shared electron pair between O and H atoms towards itself, it can lose the proton (H+).
However, alcohols are weak acids
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Esterification
Alcohols react with monocarboxylic acids, in the presence of concentrated sulphuric acid or dry HCL gas as catalyst, to from esters. This reaction is known as esterification.
Alcohols react with monocarboxylic acids, in the presence of concentrated sulphuric acid or dry HCL gas as catalyst, to from esters. This reaction is known as esterification.
The function of concentrated sulphuric acid is to act as protonating agent as well as a dehydrating agent.
RCOOH + HOR' ↔ RCOOR' + H2O with H2SO4 as catalyst
CH3COOH + HOC2H5 ↔ CH3COOC2H5 + H2O with H2SO4 as catalyst
The reaction is reversible is nature. Double headed arrow is used to indicate it. The equilibrium can be shifted toward the forward direction by removing water as soon as it is formed.
If dry HCL gas is used as a catalyst, the reaction is called Fisher=Speier esterification.
It is is difficult to prepare esters of tertiary alcohols becasue bulky groups in the alcohol decrease rate of reaction or esterification. This is termed as stearic hindrance of bulky groups.
As noted above in the reactions, esterification involved the cleavage of the O-H bonds of the alcohol. This was proved by using alcohol with isotopic O18 which can be tracked using isotopic tracer technique. It was found that this oxygen is present in the resulting ester which means that the oxygen in the alcohol is going into the ester and hydrogen is going into the water molecules.
RCOOH + HOR' ↔ RCOOR' + H2O with H2SO4 as catalyst
CH3COOH + HOC2H5 ↔ CH3COOC2H5 + H2O with H2SO4 as catalyst
The reaction is reversible is nature. Double headed arrow is used to indicate it. The equilibrium can be shifted toward the forward direction by removing water as soon as it is formed.
If dry HCL gas is used as a catalyst, the reaction is called Fisher=Speier esterification.
It is is difficult to prepare esters of tertiary alcohols becasue bulky groups in the alcohol decrease rate of reaction or esterification. This is termed as stearic hindrance of bulky groups.
As noted above in the reactions, esterification involved the cleavage of the O-H bonds of the alcohol. This was proved by using alcohol with isotopic O18 which can be tracked using isotopic tracer technique. It was found that this oxygen is present in the resulting ester which means that the oxygen in the alcohol is going into the ester and hydrogen is going into the water molecules.
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Dehydration
When alcohols are heated with conc. or H3PO4, at 443 K, they get dehydrated to form alkenes.
The ease of dehydration of alcohol follows the order 3>2>1 which is also the order of stability of carbocation.
When alcohols are heated with conc. or H3PO4, at 443 K, they get dehydrated to form alkenes.
The ease of dehydration of alcohol follows the order 3>2>1 which is also the order of stability of carbocation.
Dehydration of alcohols to ethers or alkenes can also be brought about by passing the vapour of the alcohols over heated alumina catalyst under different conditions
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Oxidation
The oxidation of alcohols can be carried out by a number of reagents such as acqueous, alkalineor acidified KMnO4, acidified Na2Cr2O7, nitric acid, chromic acid, etc.
Different classes of alcohols differ from each other in their ease of oxidation and also give different products.
(i) Primary alcohols: Primary alcohols are easily oxidized. First an aldehyde is formed and then from it carboxylic acid is formed. Both the aldehyde and the resulting acid contain the same number of carbon atoms as the starting alcohol.
(ii) Secondary alcohols: Ease of oxidation is still there. But they are oxidized to ketone and under strong conditions they are further oxidized to form a mixture of acids. While the ketone contains the same number of carbon atoms as the starting alcohol, the acids formed contain lesser number of carbon atoms.
(iii) It is difficult to oxidize tertiarly alcohols.
When treated with acidic oxidizing agents under very strong conditions they form first ketones and then acids.
Both the ketones and acids contain lesser number of carbon atoms than the starting alcohols.
The oxidation of alcohols can be carried out by a number of reagents such as acqueous, alkalineor acidified KMnO4, acidified Na2Cr2O7, nitric acid, chromic acid, etc.
Different classes of alcohols differ from each other in their ease of oxidation and also give different products.
(i) Primary alcohols: Primary alcohols are easily oxidized. First an aldehyde is formed and then from it carboxylic acid is formed. Both the aldehyde and the resulting acid contain the same number of carbon atoms as the starting alcohol.
(ii) Secondary alcohols: Ease of oxidation is still there. But they are oxidized to ketone and under strong conditions they are further oxidized to form a mixture of acids. While the ketone contains the same number of carbon atoms as the starting alcohol, the acids formed contain lesser number of carbon atoms.
(iii) It is difficult to oxidize tertiarly alcohols.
When treated with acidic oxidizing agents under very strong conditions they form first ketones and then acids.
Both the ketones and acids contain lesser number of carbon atoms than the starting alcohols.
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Reaction with sodium
The cleavage in this reaction will be in the OH bond. Alcohols react with active metals to liberate hydrogen gas an form metal alkoxide.
Ethanol or Ethyl alcohol reacts with sodium to gibve Sodium ethoxide and hydrogen
This reaction shows that alcohols are acidic in nature.
The acidic nature is due to the presence of polar O-H bond.
Alcohols are weak acids even weaker than water.
Reaction with phosphorus halides
Phosphorus halides such as PCl5, Pcl3, PBr3 and PI3 react with alcohols to form corresponding haloalkanes.
Haloalkanes : Chloroethane, Bromoethane, Iodoethane
Reaction with ZnCl2/conc.-HCl
This is a reaction or test to distinguish various categories of alcohols and is termed Lucas test.
In this test, an alcohol is treated with an equimolar mixture of concentrated hydrochloric acid and anhydrous ZnCl2 (called Lucas reagent).
Alcohols get converted into alkylhalides. As alkyl halides are insoluble in water, their presence is indicated by the appearance of turbidity in the reaction mixture.
The time required for the formation of alkyl halides and appearance of turbidity is very less in the tertiary alcohols.
in the case of secondary alcohols, it takes five minutes.
A primary alcohol produces turbidity only after heating.
Thus alcohols can be distinguished using Lucas test.
Conversion of alcohols into aldehydes and ketones
This topic was already covered in the topic of oxidation.
Oxidation of primary alcohol gives aldehydes.
Oxidation of secondary alcohols gives ketones.
It is difficult to oxidize tertiary alcohols.
The cleavage in this reaction will be in the OH bond. Alcohols react with active metals to liberate hydrogen gas an form metal alkoxide.
Ethanol or Ethyl alcohol reacts with sodium to gibve Sodium ethoxide and hydrogen
This reaction shows that alcohols are acidic in nature.
The acidic nature is due to the presence of polar O-H bond.
Alcohols are weak acids even weaker than water.
Reaction with phosphorus halides
Phosphorus halides such as PCl5, Pcl3, PBr3 and PI3 react with alcohols to form corresponding haloalkanes.
Haloalkanes : Chloroethane, Bromoethane, Iodoethane
Reaction with ZnCl2/conc.-HCl
This is a reaction or test to distinguish various categories of alcohols and is termed Lucas test.
In this test, an alcohol is treated with an equimolar mixture of concentrated hydrochloric acid and anhydrous ZnCl2 (called Lucas reagent).
Alcohols get converted into alkylhalides. As alkyl halides are insoluble in water, their presence is indicated by the appearance of turbidity in the reaction mixture.
The time required for the formation of alkyl halides and appearance of turbidity is very less in the tertiary alcohols.
in the case of secondary alcohols, it takes five minutes.
A primary alcohol produces turbidity only after heating.
Thus alcohols can be distinguished using Lucas test.
Conversion of alcohols into aldehydes and ketones
This topic was already covered in the topic of oxidation.
Oxidation of primary alcohol gives aldehydes.
Oxidation of secondary alcohols gives ketones.
It is difficult to oxidize tertiary alcohols.
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