Coupon Accepted Successfully!


Preparation of alcohols

Monohydric alcohols can be prepared by the following methods:
  1. Oxymercurationdemercuration
    Description: 48624.png
    For example,
    Description: 48631.png
  2. Hydroborationoxidation
    Description: 49841.png
  3. Grignard synthesis: The carbon–magnesium bond of the Grignard reagent is a highly polar bond—carbon being negative relative to electropositive magnesium. When Grignard reagent is added to carbonyl compounds, the organic group attaches to carbon and the magnesium attaches to oxygen.
    Description: 49852.png
    The product is a magnesium salt of weakly acidic alcohol and is easily converted into the alcohol by the addition of a stronger acid, water. The Mg(OH)X thus formed is a gelatinous material, which forms coating over carbonyl compound. Therefore, dilute mineral acid (HCl or H2SO4) is commonly used instead of water, so that water-soluble magnesium salts are formed.
    Description: 49862.png
    Description: 49874.png
    Description: 49883.png
  4. Hydrolysis of alkyl halides: Alkyl halides on hydrolysis gives alcohols either by SN1 or SN2 route depending on the structure of alkyl halide and the reaction conditions employed.
    R – X + OH (or H2O) R – OH + X (or HX)
  5. Reduction of carbonyl compounds: Aldehydes can be reduced to primary alcohols, and ketones can be reduced to secondary alcohols, either by catalytic hydrogenation or by the use of reducing agents such as lithium aluminium hydride (LiAlH4), H2/Ni, and B2H6/THF.
  6. Reduction of esters: In chemical reduction of esters, the acid portion of ester is converted into primary alcohol.
    CH3(CH2)8COOCH3 Description: 47036.png CH3(CH2)8CH2 – OH + CH3 – OH
    CH3(CH2)10COOC2H5 Description: 47044.png CH3(CH2)10CH2 – OH + C2H5 – OH

Chemical properties of the alcohols

  1. Reaction with alkali metals: Active metals (Na, K, Mg, Al, etc.) when treated with alcohols give hydrogen gas. In this reaction, the order of reactivity of alcohols is CH3OH > 1° > 2° > 3°. This reaction exhibits acidic character of alcohols.
    RO – H + Na RONa+ + ½ H2
  2. Reaction with H – X
    Description: 47056.png
    Description: 47069.png
  3. Reaction with PX3 and PX5
    Description: 47077.png
    R – OH + PCl5 R – Cl + POCl3 + HCl
  4. Reaction with SOCl2
    R – OH + SOCl2 R – Cl + SO2 + HCl
  5. Acid catalyzed dehydration of alcohols
    Description: 49893.png
  6. Oxidation of 1°, 2°, 3° alcohols with CrO3 or K2Cr2O7 in acid: Alcohols with at least one hydrogen atom on 1° and 2° carbinol are oxidized to carbonyl compounds (aldehydes and ketones). PCC (a mixture of pyridine, HCl, and CrO3) oxidizes 1° alcohol to aldehydes, but K2Cr2O7 or KMnO4 in acid converts 1° alcohol directly into carboxylic acids. Under mild conditions, 3° alcohols are not oxidized.
  7. Cleavage of 1,2-glycols: Lead tetraacetate or periodic acid are commonly used for the cleavage of 1,2-glycols. The former reagent is used in anhydrous solvent, whereas the latter is in organic solvent. Periodic acid is more selective and readily cleaves 1,2-glycols at room temperature.
    Description: 49903.png
  8. Haloform reaction:
    Description: 49912.png

Methods for distinguishing primary, secondary, and tertiary alcohols

  1. Lucas test: Alcohols react with concentrated hydrochloric acid in the presence of anhydrous zinc chloride to form alkyl chlorides. The alkyl chlorides appear as a cloudy dispersion because of its insolubility in Lucas reagent.
      Description: 47091.png
    The three types of alcohols undergo this reaction at different rates. The rates of reaction with Lucas reagent [conc. HCl + ZnCl2 (anhydrous)] follow the given order:
      Tertiary alcohol > Secondary alcohol > Primary alcohol
    An unknown alcohol (monohydric) is mixed with conc. HCl and anhydrous ZnCl2 at room temperature. The alkyl chloride formed is insoluble in the medium, thus the solution becomes cloudy before it separates out as a distinct layer. The following observations are made:
    • If cloudiness (white turbidity) appears immediately, the alcohol is tertiary.
    • If cloudiness appears within 5 min, the alcohol is secondary.
    • If the solution remains clear, i.e., no cloudiness is formed, the alcohol is primary.
  2. Victor Meyer’s test:
    Description: 49952.png
    Description: 49561.png
    Description: 49568.png

Test Your Skills Now!
Take a Quiz now
Reviewer Name