produce turbidity at room temperature. . Reaction with phosphorus trihalides: Alcohols are converted to alkyl bromides by reaction with phosphorus tribromide (Refer Unit , Class XII). .
Dehydration: Alcohols undergo dehydration (removal of a molecule of water) to form alkenes on treating with a protic acid e.g., concentrated H SO or H PO , or catalysts such as anhydrous zinc chloride or alumina. Ethanol undergoes dehydration by heating it with concentrated H SO at K. Aspirin possesses analgesic, anti- inflammatory and antipyretic properties. Secondary and tertiary alcohols are dehydrated under milder conditions.
For example Thus, the relative ease of dehydration of alcohols follows the following order: Tertiary Secondary Primary > > The mechanism of dehydration of ethanol involves the following steps: Mechanism Step : Formation of protonated alcohol. Step : Formation of carbocation: It is the slowest step and hence, the rate determining step of the reaction. Step : Formation of ethene by elimination of a proton. The acid used in step is released in step .
To drive the equilibrium to the right, ethene is removed as it is formed. . Oxidation: Oxidation of alcohols involves the formation of a carbon- oxygen double bond with cleavage of an O-H and C-H bonds. Such a cleavage and formation of bonds occur in oxidation reactions.
These are also known as dehydrogenation reactions as these involve loss of dihydrogen from an alcohol molecule. Depending on the oxidising agent used, a primary alcohol is oxidised to an aldehyde which in turn is oxidised to a carboxylic acid. Tertiary carbocations are more stable and therefore are easier to form than secondary and primary carbocations; tertiary alcohols are the easiest to dehydrate. Strong oxidising agents such as acidified potassium permanganate are used for getting carboxylic acids from alcohols directly.
CrO in anhydrous medium is used as the oxidising agent for the isolation of aldehydes. CrO R H R CHO A better reagent for oxidation of primary alcohols to aldehydes in good yield is pyridinium chlorochromate (PCC), a complex of chromium trioxide with pyridine