Reactions of Alcohols
Dehydration and Oxidation Reactions
Dehydration of alcohols
Alcohols react with conc. H2SO4 to form different products under different conditions.
At 413 K, Ethers are formed : When alcohols are heated with conc. H2SO4 at 413 K, ethers (ROR') are formed.
At 433-443 K, Alkenes are formed : When alcohols are further heated at 433-443 K with conc. H2SO4, alkenes are formed.
Formation of alkene mechanism
The mechanism of formation of alkene by dehydration can be understood by using the example of ethanol (CH3CH2OH).
- Alcohols react with strong acids due to lone pairs on oxygen to form oxonium salts (in this case, protonated ethanol).
- Due to the presence of a positive charge on the oxygen atom of protonated ethanol, the C−O bond becomes weak resulting in the formation of ethyl
carbocation. This step being slow is the rate determining step of the reaction.
- The carbocation formed in step 2 being highly reactive, readily loses a proton to form ethene.
Try to write the mechanism of the formation of diethyl ether.
Reaction of alcohols with heated alumina
Different products are obtained when vapours of an alcohol are passed over heated alumina.
At 513-523 K : Ethers are formed
At 623 K : Alkenes are formed
Oxidation or Dehydrogenation
In oxidation, the oxygen atom of an alcohol and the carbon atom attached to the oxygen lose a hydrogen atom each to form carbon-oxygen double bond.
Alcohols can be oxidised using the following reagents :
- KMnO4 (neutral, acidic or alkaline)
- Acidified K2Cr2O7
Oxidation of primary alcohols
Primary alcohols are oxidised first to aldehydes and then to carboxylic acids
PCC (pyridinium chlorochromate) or Collin's reagents (CrO3.C5H5N, chromium trioxide pyridine) are used to obtain aldehydes.
Oxidation of secondary alcohols
Secondary alcohols are oxidised to carboxylic acid under drastic conditions containing fewer number of carbon atoms than the original alcohol.
Secondary alcohols can be converted to ketones easily using PCC or CrO3.
Oxidation of tertiary alcohols
Acidified K2Cr2O7 or acidified KMnO4 react with tertiary alcohols to form a mixture of a ketone and a carboxylic acid each containing fewer number of carbon atoms than the original alcohols.
Dehydrogenation of alcohols with Cu
The vapours of alcohols react with heated copper at 573 K to form different products depending on whether the alcohol is primary, secondary or tertiary.
Reaction of primary alcohols with Cu : Primary alcohols undergo dehydrogenation to form aldehydes.
Reaction of secondary alcohols with Cu : Secondary alcohols undergo dehydrogenation to form ketones.
Reacton of tertiary alcohols with Cu : Tertiary alcohols undergo dehydrogenation to form alkenes.
Question : Predict the major product when 1-methylcyclohexanol undergoes dehydration.
Answer : Dehydration of 1-methylcyclohexanol can give two products, viz, I and II.
According to Saytzeff rule, I is the major product because it is more highly substituted.
Question : Name the reagent used in the following reactions :
- Oxidation of a primary alcohol to carboxylic acid.
- Oxidation of a primary alcohol to aldehyde.
- Acidified potassium dichromate; or neutral, acidic or alkaline potassium permanganate can be used.
- PCC or Collin's reagents can be used.
Convert 1-propanol to 2-bromopropane
Convert ethanol to 2-propanol