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These are compounds containing C – O single bond. , The functional group present in alcohols and phenols is –OH (hydroxyl) group and that present in ethers is –O – group (oxy group)., Preparation of Alcohols, From alkenes: , By acid catalysed hydration: , Alkenes react with water in the presence of acid as catalyst to form alcohols. In the case of unsymmetrical alkenes, the addition reaction takes place in accordance with Markovnikov’s rule
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ii) By hydroboration–oxidation reaction: , Addition of diborane followed by oxidation using hydrogen peroxide in presence of alkali is called Hydroboration-oxidation reaction. , Alkenes add diborane to give trialkyl boranes which on oxidation by H2O2 in the presence of aqueous sodium hydroxide to form alcohols. , The net reaction is the addition of a water molecule to the alkene in a way opposite to the Markovnikov’s rule
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ii) By reduction of carboxylic acids and esters: , Carboxylic acids are reduced to primary alcohols by treating with LiAlH4. , R-COOH i)LiAlH4 R-CH2OH, ii)H2O , Commercially, acids are reduced to alcohols by converting them to the esters followed by their reduction using hydrogen in the presence of catalyst (catalytic hydrogenation).
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iii) From Grignard reagents: , Aldehydes and ketones add Grignard reagent followed by hydrolysis, we get alcohols., Formaldehyde (methanal) gives primary alcohols, aldehydes other than formaldehyde gives secondary alcohols and ketones give tertiary alcohols.
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Chemical Reactions of Alcohols , Reaction with hydrogen halides: , Alcohols react with hydrogen halides in presence of anhydrous zinc chloride (ZnCl2) to form alkyl halides., ROH + HX an. ZnCl2 R–X + H2O, The order of reactivity of alcohols with HCl is 30 alcohols > 20 alcohols > 10 alcohols. , This difference in reactivity of three classes of alcohols with HCl helps to distinguish them from one another (Lucas test)
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LUCAS TEST: , Lucas reagent is a mixture of conc. HCl and anhydrous ZnCl2. , Tertiary alcohols react with Lucas reagent and form turbidity immediately; , secondary alcohols form turbidity within 5 minutes , while primary alcohols do not produce turbidity at room temperature. , But they give turbidity on heating, 2. Dehydration: , On heating with Conc. H2SO4 or H3PO4, alcohols undergo dehydration (removal of a molecule of water) to form alkenes., Catalysts such as anhydrous zinc chloride or alumina can also be used for dehydaration.
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The relative ease of dehydration of alcohols follows the order: Tertiary > Secondary > Primary., For example ethanol undergoes dehydration by heating it with concentrated H2SO4 at 443 K, we get ethene., Secondary and tertiary alcohols are dehydrated under milder conditions.
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Chemical Reactions of Phenols , Nitration of Phenol: , Phenol reacts with Conc. Nitric acid to give an yellow precipitate of 2,4,6- trinitrophenol commonly called picric acid., For the preparation of ortho and para nitrophenols, phenol is treated with dil. HNO3 at low temperature
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The ortho and para isomers can be separated by steam distillation. o-Nitrophenol is steam volatile due to intramolecular hydrogen bonding while p-nitrophenol is less volatile due to intermolecular hydrogen bonding which causes the association of molecules.
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Commercially Important Alcohols – Ethanol , Ethanol is commonly known as spirit or grain alcohol. , It is obtained commercially by the fermentation of sugar. , The sugar in molasses, sugarcane or fruits like grapes is converted to glucose and fructose, in the presence of an enzyme, invertase. , Glucose and fructose undergo fermentation in the presence of another enzyme, zymase to give ethanol and carbondioxide. , Both the enzymes invertase and zymase are produced by yeast.
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Fermentation takes place in anaerobic conditions i.e. in absence of air. , If air gets into fermentation mixture, the oxygen of air oxidises ethanol to ethanoic acid (acetic acid), which destroys the taste of alcohol. , Ethanol is a colourless liquid. It is used as a solvent in paint industry and in the preparation of a large number of carbon compounds. , The commercial alcohol is made unfit for drinking by mixing it with some copper sulphate (to give it a colour) and pyridine (a foul smelling liquid). , It is known as denaturation of alcohol and the resulting alcohol is known as denatured spirit.
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Preparation of Ethers , Williamson synthesis: , Alkyl halide reacts with sodium alkoxide to form ether. This reaction is known as Williamson synthesis., Ethers containing secondary and tertiary alkyl groups can also be prepared by this method. For this, the alkyl halide used should be primary and the alkoxide should be secondary or tertiary. , For example for the preparation of tert-butyl methyl ether, methyl bromide should be treated with sodium tert-butoxide.
