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).

1. Alcohols react with strong acids due to lone pairs on oxygen to form oxonium salts (in this case, protonated ethanol).
   
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.
   
3. The carbocation formed in step 2 being highly reactive, readily loses a proton to form ethene.
   

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 :

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

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.

Questions

Conversions