Preparation of Aldehydes & Ketones

Preparation of Aldehydes & Ketones — From Alcohols and Hydrocarbons

From alcohols (oxidation/dehydrogenation): a 1° alcohol gives an aldehyde (use a mild oxidant like PCC to stop at the aldehyde; strong oxidants over-oxidise to the acid). A 2° alcohol gives a ketone. Passing alcohol vapour over hot copper at 573 K dehydrogenates 1° → aldehyde and 2° → ketone.

From alkenes (ozonolysis): an alkene treated with O$_3$ then Zn/H$_2$O is cleaved at the double bond into carbonyl compounds (aldehydes and/or ketones), depending on the substitution at each end.

From alkynes (hydration): adding water across a triple bond (dil. H$_2$SO$_4$ / HgSO$_4$) gives a carbonyl. Ethyne → ethanal; any other alkyne → a ketone (Markovnikov addition of water, then tautomerism of the enol).

Key Point: 1° alcohol → aldehyde (PCC), 2° alcohol → ketone; ozonolysis cleaves C=C into carbonyls; alkyne hydration gives ethanal (from ethyne) or a ketone (from others).

Preparation of Aldehydes — From Acyl Chlorides and Nitriles

Rosenmund reduction (acyl chloride → aldehyde): an acyl chloride is hydrogenated over palladium on barium sulphate (Pd-BaSO$_4$, a poisoned catalyst) to give an aldehyde. The poison stops the reduction at the aldehyde stage (otherwise it would go to the alcohol):

R-COCl + H$_2$ →(Pd-BaSO$_4$)→ R-CHO + HCl

Stephen reaction (nitrile → aldehyde): a nitrile is reduced with SnCl$_2$/HCl to an imine, which on hydrolysis gives an aldehyde: R-C≡N → R-CHO. (Alternatively, partial reduction with DIBAL-H gives the aldehyde.)

From nitriles with Grignard (→ ketone): a nitrile reacts with a Grignard reagent and the imine salt is hydrolysed to a ketone: R-C≡N + R'MgX → R-CO-R'.

Key Point: Rosenmund (R-COCl + H$_2$/Pd-BaSO$_4$) and Stephen (R-CN + SnCl$_2$/HCl) both give aldehydes; a nitrile + Grignard gives a ketone.

Preparation of Aromatic Carbonyls — From Arenes

Etard reaction (toluene → benzaldehyde): toluene is oxidised by chromyl chloride (CrO$_2$Cl$_2$); the chromium complex formed is hydrolysed to benzaldehyde. The mild conditions stop oxidation at the aldehyde.

Gattermann-Koch reaction: benzene (or a derivative) is treated with CO + HCl in the presence of anhydrous AlCl$_3$ (or CuCl) to give benzaldehyde directly — a formylation.

Friedel-Crafts acylation (→ aryl ketone): an arene reacts with an acyl chloride/anhydride and anhydrous AlCl$_3$ to give an aromatic ketone (e.g. benzene + CH$_3$COCl → acetophenone).

Key Point: Etard (toluene + CrO$_2$Cl$_2$) and Gattermann-Koch (CO/HCl/AlCl$_3$) give benzaldehyde; Friedel-Crafts acylation gives an aryl ketone.

Solved Examples

Example 1: 1° vs 2° alcohol oxidation

What carbonyl forms when (a) propan-1-ol and (b) propan-2-ol are oxidised by PCC?

Solution: (a) Propan-1-ol (1°) → propanal (PCC stops at the aldehyde). (b) Propan-2-ol (2°) → propanone (acetone).

Example 2: Rosenmund reduction

Give the product and explain the role of the catalyst when CH$_3$COCl is treated with H$_2$/Pd-BaSO$_4$.

Solution: Product is ethanal (CH$_3$CHO). The BaSO$_4$ poisons the Pd so the reduction stops at the aldehyde and does not continue to ethanol.

Example 3: Ozonolysis

What are the products of ozonolysis (O$_3$, then Zn/H$_2$O) of 2-methylbut-2-ene?

Solution: The double bond cleaves between the two carbons. (CH$_3$)$_2$C=CHCH$_3$ gives propanone (acetone) from the (CH$_3$)$_2$C= end and ethanal from the =CHCH$_3$ end.

Example 4: Alkyne hydration

What is formed when (a) ethyne and (b) propyne are hydrated (dil. H$_2$SO$_4$/HgSO$_4$)?

Solution: (a) Ethyne → ethanal (acetaldehyde) — the only alkyne giving an aldehyde. (b) Propyne → propanone (acetone) by Markovnikov addition.

Example 5: Stephen reaction

How is CH$_3$CH$_2$CN converted to propanal?

Solution: Stephen reaction: reduce the nitrile with SnCl$_2$/HCl to the imine (R-CH=NH), then hydrolyse to give propanal (CH$_3$CH$_2$CHO).

Example 6: Nitrile + Grignard → ketone

What ketone forms from CH$_3$CN + C$_2$H$_5$MgBr followed by hydrolysis?

Solution: The Grignard adds to the nitrile carbon; hydrolysis of the imine salt gives butan-2-one (CH$_3$COCH$_2$CH$_3$).

Example 7: Etard reaction

How is benzaldehyde prepared from toluene by the Etard reaction?

Solution: Toluene is treated with chromyl chloride (CrO$_2$Cl$_2$) in CS$_2$; the brown chromium complex is hydrolysed to give benzaldehyde. The mild oxidation stops at -CHO.

Example 8: Gattermann-Koch

Give the reagents for converting benzene to benzaldehyde by the Gattermann-Koch reaction.

Solution: Treat benzene with carbon monoxide and hydrogen chloride in the presence of anhydrous AlCl$_3$ (or CuCl) → benzaldehyde.

Example 9: Friedel-Crafts acylation

Predict the product of benzene + CH$_3$COCl with anhydrous AlCl$_3$.

Solution: Friedel-Crafts acylation introduces the acetyl group onto the ring → acetophenone (C$_6$H$_5$COCH$_3$).

Example 10: Choosing the route to an aldehyde

You need benzaldehyde from benzoyl chloride without forming the alcohol. Which reaction?

Solution: Use Rosenmund reduction: C$_6$H$_5$COCl + H$_2$ over Pd-BaSO$_4$ gives benzaldehyde, the poisoned catalyst preventing over-reduction to benzyl alcohol.