catalyst

 

➡️Preparation of Alcohols

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• Preparation of Alcohols
  1. From alkenes
     • By acid catalysed hydration:
       1. Alkenes react with water in the presence of acid as catalyst to form alcohols.
       2. In case of unsymmetrical alkenes, the addition reaction takes place in accordance with Markovnikov’s rule (Unit 13, Class XI).
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     • By hydroboration–oxidation:
       1. Diborane (BH3)2 reacts with alkenes to give trialkyl boranes as addition product. This is oxidised to alcohol by hydrogen peroxide in the presence of aqueous sodium hydroxide. The
       2. .
          
          
          
       3. The addition of borane to the double bond takes place in such a manner that the boron atom gets attached to the sp2 carbon carrying greater number of hydrogen atoms. The alcohol so formed looks as if it has been formed by the addition of water to the alkene in a way opposite to the Markovnikov’s rule. In this reaction, alcohol is obtained in excellent yield.
  2. From carbonyl compounds
     • (i) By reduction of aldehydes and ketones:
       1. Aldehydes and ketones are reduced to the corresponding alcohols by addition of hydrogen in the presence of catalysts (catalytic hydrogenation).
       2. The usual catalyst is a finely divided metal such as platinum, palladium or nickel. It is also prepared by treating aldehydes and ketones with sodium borohydride (NaBH4) or lithium aluminium hydride (LiAlH4)
       3. Aldehydes yield primary alcohols whereas ketones give secondary alcohols.
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     • By reduction of carboxylic acids and esters:
       1. Carboxylic acids are reduced to primary alcohols in excellent yields by lithium aluminium hydride, a strong reducing agent.
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       3. However, LiAlH4 is an expensive reagent, and therefore, used for preparing special chemicals only.
       4. Commercially, acids are reduced to alcohols by converting them to the esters (Section 11.4.4), followed by their reduction using hydrogen in the presence of catalyst (catalytic hydrogenation).
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  3. From Grignard reagents
     1. Alcohols are produced by the reaction of Grignard reagents (Unit 10, Class XII) with aldehydes and ketones.
     2. The first step of the reaction is the nucleophilic addition of Grignard reagent to the carbonyl group to form an adduct. Hydrolysis of the adduct yields an alcohol.
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     4. You will notice that the reaction produces a primary alcohol with methanal, a secondary alcohol with other aldehydes and tertiary alcohol with ketones.

➡️Some Commercially Important Alcohols

Methanol and ethanol are among the two commercially important

alcohols.

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• Some Commercially Important Alcohols
  • Methanol
    1. Methanol, CH3OH, also known as ‘wood spirit’, was produced by destructive distillation of wood.
    2. Today, most of the methanol is produced by catalytic hydrogenation of carbon monoxide at high pressure and temperature and in the presence of ZnO – Cr2O3 catalyst.
    3. .
    4. Methanol is a colourless liquid and boils at 337 K.
    5. It is highly poisonous in nature. Ingestion of even small quantities of methanol can cause blindness and large quantities causes even death.
    6. Methanol is used as a solvent in paints, varnishes and chiefly for making formaldehyde.
• Ethanol
  1. Ethanol, C2H5OH, is obtained commercially by fermentation, the oldest method is from sugars. The sugar in molasses, sugarcane or fruits such as grapes is converted to glucose and fructose, (both of which have the formula C6H12O6), in the presence of an enzyme, invertase. Glucose and fructose undergo fermentation in the presence of another enzyme, zymase, which is found in yeast.
  2. .
  3. In wine making, grapes are the source of sugars and yeast. As grapes ripen, the quantity of sugar increases and yeast grows on the outer skin. When grapes are crushed, sugar and the enzyme come in contact and fermentation starts. Fermentation takes place in anaerobic conditions i.e. in absence of air. Carbon dioxide is released during fermentation
  4. The action of zymase is inhibited once the percentage of alcohol formed exceeds 14 percent. If air gets into fermentation mixture, the oxygen of air oxidises ethanol to ethanoic acid which in turn destroys the taste of alcoholic drinks.
  5. Ethanol is a colourless liquid with boiling point 351 K. It is used as a solvent in paint industry and in the preparation of a number of carbon compounds. The commercial alcohol is made unfit for drinking by mixing in it some copper sulphate (to give it a colour) and pyridine (a foul smelling liquid). It is known as denaturation of alcohol.
  6. Nowadays, large quantities of ethanol are obtained by hydration of ethene (Section 11.4).