Why does Rosenmund reduction stop at the aldehyde and not over-reduce to an alcohol?

The palladium catalyst is deliberately poisoned by depositing it on barium sulphate and adding a trace of sulphur or quinoline. This lowers the activity of the catalyst just enough to reduce the reactive acyl chloride to an aldehyde while leaving the much less reactive aldehyde untouched, so the reaction halts cleanly at RCHO instead of running on to RCH2OH.

Which preparation methods give an aldehyde and which give a ketone?

Aldehyde-only routes: oxidation or dehydrogenation of a primary alcohol, Rosenmund reduction of an acyl chloride, Stephen reaction and DIBAL-H on a nitrile or ester, Etard reaction, Gattermann-Koch and side-chain chlorination of toluene. Ketone-only routes: oxidation or dehydrogenation of a secondary alcohol, Friedel-Crafts acylation, acyl chloride with dialkylcadmium, and nitrile with a Grignard reagent. Ozonolysis and alkyne hydration can give either, depending on the substrate.

Why does ethyne give an aldehyde on hydration while all other alkynes give ketones?

Hydration of ethyne with H2SO4 and HgSO4 adds water across the triple bond to form an enol that tautomerises to acetaldehyde, because ethyne is symmetrical and has a terminal CH. In any other alkyne the Markovnikov addition places the OH on an internal carbon, so the enol tautomerises to a ketone rather than an aldehyde.

Why does Friedel-Crafts acylation give only aromatic ketones and never an aldehyde?

Friedel-Crafts acylation uses an acyl chloride RCOCl with AlCl3 to attach an RCO group to the ring, so the carbonyl carbon ends up bonded to the ring on one side and an alkyl or aryl group on the other — that is a ketone. There is no simple formyl chloride (HCOCl) that survives to introduce a CHO group, which is why benzaldehyde must be made by the special Gattermann-Koch, Etard or side-chain methods instead.

How does DIBAL-H stop a nitrile or ester at the aldehyde stage?

Diisobutylaluminium hydride (DIBAL-H) is a bulky, mild hydride that delivers only one hydride to the substrate at low temperature, forming a stable tetrahedral aluminium intermediate (an imine from a nitrile or a hemiacetal-type species from an ester). This intermediate survives until aqueous work-up, where it hydrolyses to the aldehyde, so over-reduction to an amine or alcohol is avoided.

Preparation of Aldehydes & Ketones — NEET Chemistry

Q: What is the difference between the Etard reaction and Gattermann-Koch reaction?

The Etard reaction oxidises the methyl group of toluene with chromyl chloride (CrO2Cl2) to a chromium complex that is hydrolysed to benzaldehyde. The Gattermann-Koch reaction is a formylation: benzene is treated with carbon monoxide and hydrogen chloride in the presence of anhydrous AlCl3 (with CuCl) to introduce a CHO group directly, giving benzaldehyde. Etard starts from toluene, Gattermann-Koch starts from benzene.

Why the Aldehyde-vs-Ketone Distinction Matters

An aldehyde carries the carbonyl carbon at the end of a chain, bonded to at least one hydrogen ($\ce{R-CHO}$). A ketone has that carbon flanked by two carbon groups ($\ce{R-CO-R'}$). The preparation methods divide along exactly this structural line: a given reagent set will deliver one or the other, and many NEET items reward the student who can predict which without drawing the full mechanism.

The NCERT text (§8.2.1–8.2.3) sorts the methods into three families — from alcohols, from carbonyl-equivalent functional groups (acyl chlorides, nitriles, esters), and from hydrocarbons. NIOS §27.1.2 condenses the same content. We keep that grouping but add a running label on every route so the aldehyde-or-ketone verdict is never in doubt.

Before the routes, fix the reactivity logic that underpins the selective ones: the carbonyl carbon is electrophilic because oxygen pulls the π-electrons toward itself, and an aldehyde — having only one electron-donating group — is more electrophilic and less hindered than a ketone. That is precisely why "controlled" reagents must be used to halt at an aldehyde, which we explore in the nucleophilic addition subtopic.

From Alcohols: Oxidation & Dehydrogenation

The most direct entry to the carbonyl group is the alcohol one oxidation state below it. A primary alcohol oxidises to an aldehyde; a secondary alcohol oxidises to a ketone. The danger with a primary alcohol is over-oxidation to the carboxylic acid, so a mild, anhydrous oxidant such as pyridinium chlorochromate (PCC) is used to stop at the aldehyde.

$$\ce{R-CH2-OH ->[PCC][CH2Cl2] R-CHO}\qquad(\text{1$^\circ$ alcohol} \rightarrow \text{aldehyde})$$

$$\ce{R-CH(OH)-R' ->[\text{oxidation}] R-CO-R'}\qquad(\text{2$^\circ$ alcohol} \rightarrow \text{ketone})$$

For volatile alcohols an industrial alternative is catalytic dehydrogenation: alcohol vapours are passed over a heated copper or silver catalyst at 573 K. Two hydrogens leave as $\ce{H2}$, and once again the primary alcohol gives an aldehyde while the secondary gives a ketone.

$$\ce{CH3CH2OH ->[Cu][573\,K] CH3CHO + H2}$$

$$\ce{CH3CH(OH)CH3 ->[Cu][573\,K] CH3COCH3 + H2}$$

NEET Trap

PCC vs strong oxidant on a primary alcohol

A common mismatch is selecting an aldehyde product when the reagent is hot acidic $\ce{KMnO4}$ or $\ce{K2Cr2O7}$. Those strong oxidants drive a primary alcohol straight to the carboxylic acid. The aldehyde is the answer only when the reagent is mild and anhydrous (PCC, or Cu/573 K dehydrogenation).

Mild + anhydrous → stops at aldehyde. Strong + aqueous → goes to acid.

From Acyl Chlorides: Rosenmund & Cadmium

An acyl chloride ($\ce{R-COCl}$) sits one oxidation level above the carbonyl, so reaching an aldehyde from it requires a controlled reduction. The Rosenmund reduction hydrogenates the acyl chloride over palladium supported on barium sulphate; the catalyst is partially poisoned so the reaction halts at the aldehyde rather than running on.

$$\ce{R-COCl ->[\text{H2, Pd-BaSO4}] R-CHO + HCl}\qquad(\rightarrow \text{aldehyde})$$

To reach a ketone from an acyl chloride, treat it instead with a dialkylcadmium, $\ce{R2Cd}$ (made from a Grignard reagent and $\ce{CdCl2}$). The cadmium reagent is mild enough to deliver only one alkyl group, giving the ketone cleanly.

$$\ce{2 R'-COCl + R2Cd -> 2 R'-CO-R + CdCl2}\qquad(\rightarrow \text{ketone})$$

Figure 1 · Scheme

One acyl chloride, two destinations: a poisoned Pd catalyst (Rosenmund) gives the aldehyde; a mild dialkylcadmium gives the ketone.

From Nitriles & Esters: Stephen, DIBAL-H, Grignard

Nitriles ($\ce{R-CN}$) and esters ($\ce{R-COOR'}$) are also one oxidation level above the carbonyl. The trick, as with Rosenmund, is to add hydride or metal only once and trap a stable intermediate that hydrolyses to the carbonyl on work-up.

Stephen reaction. A nitrile is reduced by stannous chloride ($\ce{SnCl2}$) in the presence of $\ce{HCl}$ to an imine salt; aqueous hydrolysis then delivers the aldehyde.

$$\ce{R-C#N ->[\text{SnCl2, HCl}] R-CH=NH ->[H3O+] R-CHO}$$

DIBAL-H. Diisobutylaluminium hydride selectively reduces a nitrile or an ester to the aldehyde, again by stopping at a single-addition intermediate that hydrolyses on work-up.

$$\ce{R-C#N ->[\text{(i) DIBAL-H}][\text{(ii) H2O}] R-CHO}$$

$$\ce{R-COOR' ->[\text{(i) DIBAL-H}][\text{(ii) H2O}] R-CHO}$$

Grignard on a nitrile. If, instead, a nitrile is treated with a Grignard reagent and the resulting imine salt is hydrolysed, the product is a ketone — the carbon framework gains the Grignard's alkyl group on the carbonyl carbon.

$$\ce{R-C#N ->[\text{(i) R'MgX}][\text{(ii) H3O+}] R-CO-R'}\qquad(\rightarrow \text{ketone})$$

NEET Trap

Same nitrile, different product

Reading "$\ce{R-CN}$" and jumping to a fixed answer is a frequent error. With $\ce{SnCl2/HCl}$ (Stephen) or DIBAL-H the nitrile yields an aldehyde; with a Grignard reagent the same nitrile yields a ketone; and with full hydrolysis it gives a carboxylic acid. The reagent, not the substrate, decides.

Nitrile + SnCl₂/HCl or DIBAL-H → aldehyde · Nitrile + R′MgX → ketone.

Keep going

Once you have made the carbonyl, the next exam target is how it reacts and how aldehydes are told apart from ketones — see oxidation & reduction of carbonyls.

From Hydrocarbons: Ozonolysis & Alkyne Hydration

Two hydrocarbon routes turn unsaturation directly into a carbonyl. Ozonolysis of an alkene — ozone followed by reductive work-up with zinc dust and water — cleaves the double bond. Whether each fragment is an aldehyde or a ketone depends on the substitution at that carbon: a $\ce{=CH-}$ end gives an aldehyde, a $\ce{=CR2}$ end gives a ketone.

$$\ce{R-CH=CH-R' ->[\text{(i) O3}][\text{(ii) Zn, H2O}] R-CHO + R'-CHO}$$

$$\ce{(CH3)2C=CH2 ->[\text{(i) O3}][\text{(ii) Zn, H2O}] (CH3)2C=O + HCHO}$$

Hydration of alkynes adds water across the triple bond in the presence of $\ce{H2SO4}$ and $\ce{HgSO4}$. Ethyne is the lone exception that gives an aldehyde (acetaldehyde); every other alkyne gives a ketone.

$$\ce{CH#CH + H2O ->[\text{Hg^2+, H2SO4}] CH3CHO}\qquad(\text{ethyne only} \rightarrow \text{aldehyde})$$

$$\ce{R-C#CH + H2O ->[\text{Hg^2+, H2SO4}] R-CO-CH3}\qquad(\rightarrow \text{ketone})$$

Hydrocarbon route	Reagents	Product
Ozonolysis, =CH– carbon	`O3`; then `Zn / H2O`	Aldehyde fragment
Ozonolysis, =CR₂ carbon	`O3`; then `Zn / H2O`	Ketone fragment
Hydration of ethyne	`H2O / H2SO4 / HgSO4`	Acetaldehyde
Hydration of higher alkyne	`H2O / H2SO4 / HgSO4`	Ketone

Aromatic Carbonyls: Friedel-Crafts, Gattermann-Koch, Etard

The benzene ring needs its own toolkit. For an aromatic ketone, Friedel-Crafts acylation attaches an acyl group ($\ce{RCO-}$) using an acyl chloride or anhydride with anhydrous $\ce{AlCl3}$. The carbonyl carbon ends up between the ring and an alkyl/aryl group — always a ketone, never an aldehyde, because there is no usable formyl chloride.

$$\ce{C6H6 + CH3COCl ->[\text{anhyd. AlCl3}] C6H5-CO-CH3 + HCl}\qquad(\rightarrow \text{aromatic ketone})$$

Reaching benzaldehyde needs the special aromatic-aldehyde methods of §8.2.2. The Gattermann-Koch reaction is a formylation: benzene is treated with carbon monoxide and hydrogen chloride over anhydrous $\ce{AlCl3}$ (aided by $\ce{CuCl}$), installing a $\ce{-CHO}$ group directly.

$$\ce{C6H6 + CO + HCl ->[\text{anhyd. AlCl3 / CuCl}] C6H5-CHO}$$

The Etard reaction takes the other approach — partial oxidation of the toluene methyl group. Chromyl chloride ($\ce{CrO2Cl2}$) converts $\ce{-CH3}$ to a chromium complex that resists further oxidation; acidic hydrolysis then frees benzaldehyde. A parallel route uses side-chain chlorination ($\ce{Cl2}$, light) to benzal chloride, hydrolysed to benzaldehyde — the commercial method.

$$\ce{C6H5CH3 ->[\text{(i) CrO2Cl2}][\text{(ii) H3O+}] C6H5CHO}\qquad(\text{Etard})$$

$$\ce{C6H5CH3 ->[Cl2, h\nu] C6H5CHCl2 ->[H2O] C6H5CHO}$$

Figure 2 · Scheme

Benzaldehyde from two directions: Gattermann-Koch builds the CHO onto benzene; Etard trims toluene's methyl group down to CHO. Friedel-Crafts, by contrast, can only deliver ketones.

The Complete Route Map

The table below collapses every method into a single aldehyde-or-ketone verdict — the form most useful in the last minute before an exam. Routes that can give either product are flagged as substrate-dependent.

Starting material	Reagents / name reaction	Carbonyl formed
1° alcohol	PCC (mild oxidation) / Cu, 573 K (dehydrogenation)	Aldehyde
2° alcohol	Oxidation / Cu, 573 K	Ketone
Acyl chloride	H₂, Pd-BaSO₄ (Rosenmund)	Aldehyde
Acyl chloride	R₂Cd (dialkylcadmium)	Ketone
Nitrile	SnCl₂/HCl then H₃O⁺ (Stephen)	Aldehyde
Nitrile or ester	DIBAL-H then H₂O	Aldehyde
Nitrile	R′MgX then H₃O⁺	Ketone
Alkene	O₃; then Zn, H₂O (ozonolysis)	Aldehyde and/or ketone
Ethyne / higher alkyne	H₂O, H₂SO₄, HgSO₄	Acetaldehyde / ketone
Benzene + RCOCl	Anhyd. AlCl₃ (Friedel-Crafts acylation)	Aromatic ketone
Benzene + CO/HCl	Anhyd. AlCl₃, CuCl (Gattermann-Koch)	Benzaldehyde
Toluene	CrO₂Cl₂ then H₃O⁺ (Etard) / Cl₂,hν then H₂O	Benzaldehyde

Three "controlled-reduction" reactions — Rosenmund, Stephen and DIBAL-H — all share one theme: a higher-oxidation substrate is tamed so it stops at the aldehyde. That conceptual thread is worth more marks than rote recall of conditions. To see how these carbonyls then build into other functional groups, the parent chapter hub links the full reaction set, and the structure & nomenclature subtopic anchors the naming of every product above.

Quick Recap