Lesson 2 of 13 · 5 min
Preparing aldehydes and ketones
NCERT §8.2.1–8.2.2
The factory buys its ethanal, but a supplier's sheet says it was made from an alkyne and another batch from ethanol vapour over hot copper. Tara lists every route the sheet mentions.
The lesson in notes
In short
Oxidising alcohols: primary alcohols give aldehydes and secondary alcohols give ketones (Unit 7).
Dehydrogenating alcohols: vapour of a volatile alcohol is passed over heated Ag or Cu. This industrial route turns 1° alcohols into aldehydes and 2° alcohols into ketones.
Ozonolysis: an alkene treated with ozone and then zinc dust and water splits at the C=C to give aldehydes, ketones or both, depending on the groups around the double bond.
Hydration of alkynes: water adds to ethyne in the presence of H₂SO₄ and HgSO₄ to give acetaldehyde. Every other alkyne gives a ketone this way.
Rosenmund reduction: an acyl chloride is hydrogenated over palladium on barium sulphate, and the reaction stops at the aldehyde.
Stephen reaction: a nitrile is reduced by stannous chloride and hydrochloric acid to an imine, and hydrolysing the imine gives the aldehyde.
DIBAL-H, diisobutylaluminium hydride, reduces nitriles selectively to imines, which hydrolyse to aldehydes. Esters are also reduced to aldehydes by DIBAL-H.
Worked reagents: hexan-1-ol → hexanal with PCC; cyclohexanol → cyclohexanone with anhydrous CrO₃; ethanenitrile → ethanal with DIBAL-H; allyl alcohol → propenal with PCC; but-2-ene → ethanal with O₃ then H₂O–Zn dust.