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Reactions of Alcohols
Dehydration and Oxidation Reactions

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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.

Alcohols are heated at 413 K with H2SO4 to produce ethers

At 433-443 K, Alkenes are formed : When alcohols are further heated at 433-443 K with conc. H2SO4, alkenes are formed.

Alcohols are heated at 433-443 K with H2SO4 to produce alkenes

Formation of alkene mechanism

The mechanism of formation of alkene by dehydration can be understood by using the example of ethanol (CH3CH2OH).

  1. Alcohols react with strong acids due to lone pairs on oxygen to form oxonium salts (in this case, protonated ethanol).
    mechanism of formation of alkenes from alcohols by dehydration step 1 : H+ ion is attracted towards alcohols
  2. 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.
    mechanism of formation of alkenes from alcohols by dehydration step 2 : formation of carbocation
  3. The carbocation formed in step 2 being highly reactive, readily loses a proton to form ethene.
    mechanism of formation of alkenes from alcohols by dehydration step 3 : alkene is formed

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

Alcohols react with Al2O3 at 513 K to form ether

At 623 K : Alkenes are formed

Alcohols react with Al2O3 at 623 K to form alkenes

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.

Oxidation or dehydrogenation concept : carbon oxygen are linked by a double bond

Alcohols can be oxidised using the following reagents :

  1. KMnO4 (neutral, acidic or alkaline)
  2. Acidified K2Cr2O7

Oxidation of primary alcohols

Primary alcohols are oxidised first to aldehydes and then to carboxylic acids

Oxidation of primary alcohols with K2Cr2O7/KMnO4 yields carboxylic acids

Note : The oxygen coming from oxidising agent can be represented as [O].

PCC (pyridinium chlorochromate) or Collin's reagents (CrO3.C5H5N, chromium trioxide pyridine) are used to obtain aldehydes.

Oxidation of primary alcohols with PCC/Collin's reagents yields aldehyde

Oxidation of secondary alcohols

Secondary alcohols are oxidised to carboxylic acid under drastic conditions containing fewer number of carbon atoms than the original alcohol.

Oxidation of secondary alcohols with K2Cr2O7/KMnO4 yields carboxylic acids

Secondary alcohols can be converted to ketones easily using PCC or CrO3.

Oxidation of secondary alcohols with PCC/Collin's reagents yields Ketones

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.

Tertiary alcohols upon oxidation yield a mixture of ketones and carboxylic acids

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.

Dehydrogenation of primary alcohols with Cu yields aldehydes

Reaction of secondary alcohols with Cu : Secondary alcohols undergo dehydrogenation to form ketones.

Dehydrogenation of secondary alcohols with Cu yields ketones

Reacton of tertiary alcohols with Cu : Tertiary alcohols undergo dehydrogenation to form alkenes.

Dehydrogenation of tertiary alcohols with Cu yields alkenes

Questions

Question : Predict the major product when 1-methylcyclohexanol undergoes dehydration.

Answer : Dehydration of 1-methylcyclohexanol can give two products, viz, I and II.

Dehydration of 1-methylcyclohexanol

According to Saytzeff rule, I is the major product because it is more highly substituted.

Question : Name the reagent used in the following reactions :

  1. Oxidation of a primary alcohol to carboxylic acid.
  2. Oxidation of a primary alcohol to aldehyde.

Answer :

  1. Acidified potassium dichromate; or neutral, acidic or alkaline potassium permanganate can be used.
  2. PCC or Collin's reagents can be used.

Conversions

Convert 1-propanol to 2-bromopropane

Convert ethanol to 2-propanol

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