Methods of Preparation of Alcohols Preparation and Properties
The important methods of preparation of alcohols are:
By hydrolysis of haloalkanes : When haloalkanes boiled with alkaline hydroxides like NaOH or KOH give alcohols.
→ If moist silver oxide (AgOH) is reacted with haloalkane then alcohols are formed.
→ Primary alkyl halide gives good yield of alcohols while tertiary alkyl halide give alkanes as major product.
This occurs due to dehydrohalogination of tertiary aleotiols.
→ Secondary alkyl halide gives mixture of alcohol and alkenes.
→ By Acid Catalysed Hydration : Alkenes react with water in the presence of dilute acid as catalyst (H2SO4, H3PO4) to form- alcohols.
→ In case of unsymmetrical alkenes, the addition reaction takes place in accordance with Markownikov’s rule. First member of alkene series contains two carbon atoms therefore methanol cannot be prepared from this method.
→ Mechanism of acid catalysed hydration reaction The mechanism of the hydration reaction involves the following steps:
→ Step 1 : Alkene gets protonated to form a carbocation by the electrophilic attack of H3O+.
→ Step 2 : Nucleophile (H2O) attacks the carbocation forming protonated alcohol .
→ Step 3 : Loss of H+ from oxygen (deprotonation) to form alcohol
By Hydroboratlon of alkene : By oxidation :
→ Diborane reacts with alkenes to give trialkyl boranes.
→ This is oxidised to alkanol by hydrogen peroxide in the presence of aqueous sodium hydroxide. In the complete reaction, addition of water to the alkene is according to Anti-markownikov’s rule.
→ The addition of borane to the double bond takes place in such a manner that the boron atom gets attached to the sp2 carbon carrying greatei aiiber of hydrogen atoms. In this reaction, alcohol is obtained in excellent yield.
→ By Oxy-Mercuratlon dmercuration : By reduction reaction of alkene with mercuric acetate (CH3COO)3Hg gives addition product, which on reduction in basic medium by NaBH4 yields alcohol.
From aldehyde and Ketones
→ From Grignard Reagents : Grignard reagent reacts with aldehydes and ketones to form addition products which decompose with dii. HCl or dii H2SO4 to give alcohols.
Preparation of Primary Alcohols
→ Reaction with formaldehyde : When formaldehyde reacts with Grignard reagent, formation of primary alcohol occurs.
→ Reaction with Ethylene Oxide or Oxirane : When Grignard reagent reacts with oxirane, an adduct is formed which on acidic hydrolysis forms primary alcohol.
Preparation of Secondary Alcohols
→ Reaction with Aldehyde Other than Formaldehyde Reaction of Grignard reagent with aldehyde other than formaldehyde, forms secondary alcohol.
→ Formation of secondary alcohol also takes place by the reaction of Grignard reagent with ester of formic acid. 1 mole of ester reacts with two moles of Grignard reagent in this reaction.
Preparation of Tertiary Alcohols
→ Ketones reacts with Grignard reagent to form tertiary alcohols.
→ Reaction with ester of an acid other than formic acid : Reaction of Grignard reagent with any ester of an acid other than formic acid, results in the formation of an adduct which on acidic hydrolysis yield tertiary alcohol. 2 moles of Grignard reagent are used up in the whole reaction.
→ By Reduction of Aldehydes and Ketones
→ Aldehydes give primary alcohols while ketones give secondary alcohols on reduction. Following reagents are used as reducing agent:
- Catalytic hydrogenation in the presence of Pt or Pd in inert solvents.
- Sodium amalgam (Na—Hg) and water.
- Lithium aluminium hydride (LiAlH4) or sodium horohydride (NaBH4).
- Bouveault-Blanc reduction by Na in the presence of ethanol.
→ By Reduction of Carboxylic Acids and Esters Carboxylic acids are reduced to primary alcohols by LiAlH4
→ Catalytic hydrogenation of ester also leads to the formation of primary alcohol.
→ From Haloalkaries: Haloalkanes, when reacts with aqueous alkali like NaOH/KOH or moist Ag2O, give alcohols or alkanols. Order of reactivity of haloakanes is 3° > 2° > 1° therefore tertiary alkyl halides are most reactive.
→ By Hydrolysis of Esters : By the hydrolysis of ester in the presence of dilute mineral acids formation of carboxylic acids and alcohol occurs.
→ From Aliphatic Primary Amines : Primary amines react with nitrous acid (HNO2) to form alkanols.
→ Methyl amine does not form methanol with HNO2. It forms methoxy methane. Formation of nitrous acid (HNO2) occurs by the reaction of NaNO2 and HCl during the reaction.
→ By Fermentation of Carbohydrates: Ethanol is manufactured by fermentation of starch or sugar using appropriate enzymes.
→ Fermentation done in absence of oxygen means it takes place under an aerobic conditions. During fermentation CO2 is evolved.