Simulation · Chemistry · Class 12
Where do H and OH land?
From the lesson Preparing alcohols in Alcohols, Phenols and Ethers. Change the values and watch what happens.
Where do H and OH land?Chemistry · Class 12
The idea behind it
NCERT §7.4.1
- Acid-catalysed hydration: an alkene reacts with water in the presence of acid to give an alcohol. With an unsymmetrical alkene the addition follows Markovnikov's rule.
- Hydration runs in three steps: the alkene takes a proton from H₃O⁺ to form a carbocation; water attacks the carbocation; the protonated alcohol then loses H⁺ to give the alcohol.
- Hydroboration–oxidation: diborane, (BH₃)₂, adds across the C=C to give a trialkylborane, which aqueous sodium hydroxide and hydrogen peroxide oxidise to the alcohol. The yield is excellent.
- In hydroboration, boron bonds to the double-bond carbon carrying more hydrogens, so the net result resembles anti-Markovnikov addition of water: propene gives propan-1-ol. H.C. Brown reported the method in 1959; he and G. Wittig were jointly awarded the 1979 Nobel Prize in Chemistry.
- Aldehydes and ketones are reduced to alcohols by hydrogen over finely divided Pt, Pd or Ni, or by sodium borohydride (NaBH₄) or lithium aluminium hydride (LiAlH₄). Aldehydes give primary alcohols; ketones give secondary alcohols.
- Carboxylic acids are reduced to primary alcohols by LiAlH₄, a strong reducing agent. LiAlH₄ is costly, so industry first turns the acid into an ester and then reduces the ester with hydrogen over a catalyst.
- Grignard reagents add to the C=O of aldehydes and ketones to give an adduct, which is hydrolysed to the alcohol.
- With a Grignard reagent, methanal gives a primary alcohol, any other aldehyde gives a secondary alcohol and a ketone gives a tertiary alcohol.