Oxidation of alcohol

Test to distinguish primary, secondary and tertiary alcohol

Oxidation of alcohols involves the formation of a carbon oxygen double bond with the cleavage of an O-H and C-H bonds.
The products of oxidizing reaction depends on the nature of alcohols (primary, secondary or tertiary nature of the carbon) and also the oxidizing agent used. Hence this reaction can be utilized for distinguishing the primary, secondary and tertiary alcohols from one another
A primary alcohol when treated with potassium to an aldehyde and then to an acid. Both the aldehyde and acid contain the same number of carbon atoms as the parent alcohol.

A secondary alcohol on oxidation with acidified K2Cr2O7 or chromium trioxide (CrO3) forms a ketone with the same number of carbon atoms as the parent alcohol. The ketone is further oxidized under strong conditions to form a mixture of acids, each containing lesser number of carbon atoms than the parent alcohol.

Tertiary alcohols do not undergo oxidation reaction under normal conditions since they have no hydrogen on carbon bearing the –OH group. But, when treated with strong oxidizing agents like HNO3 at elevated temperature they are oxidized to ketones first and then to acids, both containing fewer number of carbon atoms than the parent alcohol due to cleavage of various C-C bonds.

Related post Chemical properties of Alcohols and Phenols

Dehydration of alcohol

Alcohols undergo dehydration (removal of a molecule of water) on treating them with protonic acids(conc. H₂SO₄ or H₃PO₄) or catalysts such as anhydrous zinc chloride or alumina to form alkenes.

Ethanol undergoes dehydration to form ethylene by heating it with concentrated H₂SO₄ at 443 K.

C₂H5OH (ethanol)----(conc.H₂SO₄, 443 K)-----> CH₂ = CH₂(ethylene) + H₂O

Secondary and tertiary alcohols are dehydrated under milder conditions.

For example






The relative ease of dehydration of alcohols follows the order:
Tertiary alcohol > Secondary alcohol > Primary alcohol

In the above reaction, if alcohol is in excess acidic dehydration takes place in a different way to yield an ether.



The ether is formed by the elimination of one water between molecule from two hydroxyl groups.

 Related post Chemical properties of Alcohols and Phenols

Reaction of alcohol with phosphorus halides and SOCl2

Reaction with phosphorus halides and SOCl₂
Alcohols react with phosphorus tri or penta halides or thionyl chloride in pyridine to form the corresponding alkyl halides.

3R - OH + PX₃ --------------> 3R - X + H₃PO₃
R - OH + PX₅ --------------> R - X + POX₃ + HX
R - OH + SOCl₂ -------(pyridine)---------> R - Cl + SO₂ + HCl

Alcohols are converted to alkyl bromides by reaction with phosphorus tribromide

3ROH + PBr₃ ---------------> 2RBr + H₃PO₃

Alkyl iodides are obtained by reacting and alcohol with PI₃

3ROH + PI₃ -------(red p + I₂)-------> 3RI + H₃PO₃

3C₂H₅OH + PI₃ -------(red p + I₂)-------> 3C₂H₅OH + H₃PO₃

PBr and PI₃ are obtained in the reaction mixture by the action of red phosphorous and bromine or iodine.

Alkyl chlorides are prepared by reacting an alcohol with phosphorus trichloride, phosphorus penta chloride or thionyl chloride.

3R - OH + PCl₃ ----------------> 3R - Cl + H₃PO₃

3C₂H₅OH + PCl₃ ----------------> 3C₂H₅Cl + H₃PO₃

R - OH + PCl₅ ----------------> R - Cl + POCl₃ + HCl

C₂H₅OH + PCl₅ ----------------> C₂H₅OH + POCl₃ + HCl

C₂H₅OH + SOCl₂ --------(pyridine)--------> C₂H₅Cl + SO₂ + HCl

The reaction with thionyl chloride is preferred as the by products (SO₂ and HCl) formed are gases and are easily removed from the reaction mixture.

 Related post Chemical properties of Alcohols and Phenols

Reaction of alcohol with hydrogen halide

Alcohols react with hydrogen halides to give alkylhalides.
ROH + HX --------> R-X + H2O
The alcohol may be primary, secondary or tertiary and hydrogen halide may be HCl, HBr or HI.
For example,
CH₃CH₂CH₂OH + HI ------------> CH₃CH₂CH₂I + H₂O






To increase the yield of haloalkane a dehydrating agent like anhydrous ZnCl2 is used to remove the water formed.
In this reaction, the reactivity of alcohols decreases in the order tertiary > secondary > primary. Among hydrogen halides reactivity decreases in the order HI > HBr > HCl

 Related post Chemical properties of Alcohols and Phenols