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Aldehydes sit in an awkward middle place. Reduce an acid derivative too far and you get an alcohol; oxidise a methyl group too far and you get a carboxylic acid. The Rosenmund, Stephen and Etard reactions are three ways of stopping at the aldehyde, and conversion questions test whether you can pick the right one. This post sets out what each needs and why it stops where it does, then works through product, yield and route questions.
Where are these in the syllabus?
All three are in section 8.2.2 of the NCERT Class 12 unit Aldehydes, Ketones and Carboxylic Acids (reprint 2026-27), so they have not been dropped. The JEE (Advanced) 2026 brochure names preparation of aldehydes "from acid chlorides and nitriles" and "benzaldehyde from toluene and benzene". The JEE (Main) 2026 syllabus and the NEET (UG) 2026 syllabus do not name the reactions; they cover them under "general methods of preparation" in the unit on organic compounds containing oxygen. Check the current year's documents.
What does each reaction need?
Rosenmund reduction. An acyl chloride is hydrogenated over palladium on barium sulphate. NCERT's example is benzoyl chloride to benzaldehyde: C₆H₅COCl + H₂ (Pd–BaSO₄) → C₆H₅CHO + HCl. Karl Rosenmund reported the method in 1918 in the Berichte der deutschen chemischen Gesellschaft.
Stephen reaction. A nitrile is reduced with stannous chloride and hydrochloric acid to an imine, and the imine is hydrolysed: RCN + SnCl₂ + HCl → RCH=NH, then H₃O⁺ → RCHO. Henry Stephen published it in 1925 as A new synthesis of aldehydes in the Journal of the Chemical Society.
Etard reaction. Chromyl chloride, CrO₂Cl₂, in CS₂ converts the methyl group of toluene into a chromium complex, C₆H₅CH(OCrOHCl₂)₂, which hydrolyses to benzaldehyde.
Why does each one stop at the aldehyde?
The logic is the same in all three: the reaction passes through something that cannot easily go further until you choose to change it.
Rosenmund: the catalyst system, which NCERT gives as "palladium on barium sulphate", is chosen so that hydrogenation stops at the aldehyde instead of going on to the primary alcohol. Write the catalyst in full; "Pd" alone loses the point of the reaction.
Stephen: the product before hydrolysis is an imine, not an aldehyde, so the reducing agent has no carbonyl to attack. The aldehyde appears only in the separate hydrolysis step.
Etard: NCERT explains that strong oxidising agents take toluene all the way to benzoic acid, and that the useful reagents turn the methyl group into "an intermediate that is difficult to oxidise further". The chromium complex is that intermediate.
Worked example 1: a yield calculation
Sample numbers: 4.63 g of propanoyl chloride, CH₃CH₂COCl, undergoes Rosenmund reduction and 2.32 g of propanal is collected. Find the percentage yield.
Molar mass of CH₃CH₂COCl = 3(12.011) + 5(1.008) + 35.453 + 15.999 ≈ 92.5 g/mol.
Moles = 4.63 ÷ 92.5 = 0.0500 mol.
One mole of acyl chloride gives one mole of aldehyde. Molar mass of propanal, C₃H₆O = 58.08 g/mol, so the theoretical yield is 0.0500 × 58.08 = 2.90 g.
Yield = 2.32 ÷ 2.90 × 100 ≈ 80%.
Worked example 2: name the product
CH₃CN, then SnCl₂/HCl, then H₃O⁺: the imine CH₃CH=NH hydrolyses to ethanal, CH₃CHO. The carbon count does not change.
p-Nitrotoluene with CrO₂Cl₂, then H₃O⁺: p-nitrobenzaldehyde. Only the methyl group changes; the nitro group is a spectator.
C₆H₅COCl with H₂ over Pd–BaSO₄: benzaldehyde, NCERT's own example.
Worked example 3: choose a route
You have benzoic acid and need benzaldehyde. There is no direct reaction in this set for a carboxylic acid. Convert it first to the acid chloride, then use Rosenmund reduction. If you start with benzonitrile, use the Stephen reaction. If you start with toluene, use Etard. Our post on aldehyde and ketone reactions picks up from here with what the aldehyde does next.
Which mistakes cost marks?
Nitrile plus Grignard. NCERT's section 8.2.3 shows that a nitrile treated with a Grignard reagent and then hydrolysed gives a ketone, not an aldehyde. Stephen uses SnCl₂ and HCl.
Rosenmund on the acid. The substrate is the acyl chloride, not the carboxylic acid.
KMnO₄ for toluene. NCERT says strong oxidising agents give benzoic acid. Benzaldehyde needs chromyl chloride (Etard) or CrO₃ in acetic anhydride.
Forgetting the hydrolysis step in Stephen and Etard. Both need H₃O⁺ to release the aldehyde.
Mixing up Etard and Gatterman–Koch. Etard starts from toluene; Gatterman–Koch starts from benzene with CO and HCl.
If you keep reaching for the wrong reagent, the Fermi Problem Library has worked problems with a step-by-step solver that explains the reasoning rather than printing an answer.
A practice routine
Write each reaction from memory: substrate, reagent, intermediate, product.
Make a one-line card for every NCERT reagent in section 8.2.2 and test yourself on what it does.
Solve five two-step conversions that end in an aldehyde, naming the reaction at each step.
Do one yield calculation a week with fresh sample numbers.
Today, write the three reactions side by side and circle the intermediate in each.
