📝 Alcohols

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Alcohols are saturated molecules containing an – OH group. Ethanol can be manufactured industrially in two ways – fermentation of sugars and hydration of ethene. At 35–55^oC, glucose can be fermented by yeast and turned into ethanol and carbon dioxide. This process has a number of advantages:

it is a low-technology process, which means it can be used anywhere.
it does not use much energy.
it uses sugar cane as a raw material, which is a renewable resource.

At 300^oC and 60 atmospheres with a concentrated H₃PO₄ catalyst, H₂O can be added to ethene to make ethanol. There are also a number of disadvantages associated with this process:

it requires fairly high technology
it uses a lot of energy
the ethene comes from crude oil, which is a non-renewable resource.

Ethanol is a useful biofuel. Biofuels are carbon neutral. Although they release carbon dioxide when they are burned, they come from plant sources which absorb carbon dioxide from the atmosphere during photosynthesis while they are growing. Thus, there are no net emissions of carbon dioxide during the process from growing to combustion.
Primary alcohols are those in which the carbon attached to the OH is attached to 0 or 1 other carbon atom. Secondary alcohols are those in which the carbon attached to the OH is attached to 2 other carbon atoms. Tertiary alcohols are those in which the carbon atom attached to the OH is attached to 3 other carbon atoms.
The complete combustion of alcohol forms carbon dioxide and water. A nucleophilic substitution reaction takes place between alcohols and hydrogen halides to form halogenoalkanes. Alcohols react with sodium metal to give sodium alkoxides and hydrogen gas. Alcohol will react with a carboxylic acid, in the presence of a strong acid catalyst, to form an ester and water. Esters are used as solvents, perfumes, and flavourings. They can be hydrolysed by an acid or by a base. Acid hydrolysis is a reversible reaction, but base hydrolysis is not reversible. Elimination of water from an alcohol produces an alkene; the reaction is dehydration. Dehydration may be carried out by passing alcohol vapour over heated pumice, porous pot or aluminium oxide catalysts. A primary alcohol can be oxidised to an aldehyde by heating the alcohol gently with acidified potassium dichromate (VI). A primary alcohol can be further oxidised to a carboxylic acid by refluxing the alcohol with excess acidified potassium dichromate (VI). A secondary alcohol can be oxidised to a ketone by heating the alcohol with acidified potassium dichromate (VI). Acidified potassium dichromate (VI) changes colour from orange to green when a primary or secondary alcohol is oxidised by it. Tertiary alcohols cannot be oxidised by refluxing with acidified potassium dichromate (VI).
Ammoniacal silver nitrate, or Tollen’s reagent, is an oxidising agent and will react with aldehydes on boiling. In the presence of aldehydes, the colourless Ag⁺ ions are reduced to metallic silver, which forms on the surface of the test tube. The presence of a “silver mirror” indicates that an aldehyde is present. Aldehydes and ketones can also be distinguished by their reaction with Fehling’s solution. Fehling’s solution is a complex solution containing Cu²⁺ ions. Aldehydes are reducing agents, but ketones are not. In the presence of aldehydes, the blue Cu²⁺ is reduced to the red copper (I) oxide, Cu₂O
Carboxylic acids can be formed from the oxidation of primary alcohols or aldehydes by refluxing with excess potassium dichromate (VI) and dilute sulfuric (VI) acid.