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MHT-CET : Chemistry Entrance Exam

MHT - CET : Chemistry - Organic Hydroxy Compounds Page 1

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Organic Hydroxy Compounds


The organic compounds which contain hydroxy groups (-OH groups) in their molecules are known as organic hydroxy compounds.







The hydroxy derivatives of alkanes in which one or more hydrogen atoms are replaced by hydroxyl groups are called alcohols.




Classification of Alcohols




Monohydric Alcohols (R-OH), G.F. CnH2n + 1OH


The alcohols containing only one hydroxyl group in the molecule are called monohydric alcohols. e.g, CH3OH, C2H5OH.
They show chain, optical and position isomerism. They are functional isomers of ether.



Classification of Monohydric Alcohols




Preparation of Alcohols


(a)From Alkyl Halides (by Hydrolysis of Alkyl Halides)
It is a nucleophilic substitution reaction.


(b)From Alkenes (by Hydration of Alkenes, Using an Acid)
It is an electrophilic addition reaction.

(c) From Aldehydes and Ketones (by Reduction or Catalytic Hydrogenation of Carbonyl Compounds)

Catalytic reduction using nickel or reduction by sodium amalgam and water.
Aldehydes give primary alcohols, while ketones give secondary alcohols.





Physical Properties of Alcohols


  1. Lower members are colourless liquids with characteristic pleasant odour.
  2. Higher members are colourless and odourless solids.
  3. Lower members are water soluble and the solubility decreases as the molecular weight increases.
  4. Alcohols, except ethanol, are poisonous.
  5. They are neutral in nature but act as Lewis bases due to the presence of a lone pair of electrons on oxygen atom.



Chemical Properties of Alcohols


a) Reaction with Sodium Metal

-OH + 2Na 2R-ONa + H2
Example: 2CH3
- CH2 -OH + 2Na 2CH3 - CH2 - ONa + H2

b) Reaction with Hydrogen Halide

Anhydrous ZnCl2 (a dehydrating agent) is used only for primary and secondary alcohols. Tertiary alcohols react readily even in the absence of ZnCl2.


c) Reaction with Phosphorus Halides (PCl3 and PCl5)









d) Dehydration of Alcohols
     It is an elimination reaction. The ease of dehydration of alcohols is 3 > 2 > 1.

     Alcohols are dehydrated using conc. H2SO4, Al2O3, P2O5 or H3PO4.


When excess ethanol is heated with conc. H2SO4 at about 413K, diethyl ether is formed by dehydration of ethyl alcohol.

e) Oxidation of Alcohols [Distinguishing Test of Alcohols]
     The order of ease of oxidation of alcohols is 1 > 2 > 3.

     K2Cr2O7 + 4H2SO4
K2SO4 + Cr2(SO4)3 + 4H2O + 3[O]

     (i) Oxidation of Primary Alcohols:


     (ii) Oxidation of Secondary Alcohols:

     (iii) Oxidation of Tertiary Alcohols:



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