Methods of Preparation

CHENomenclature & Preparation

This subtopic covers the standard routes to aldehydes and ketones from alcohols, alkynes, acyl chlorides, nitriles and arenes. Each method comes with a specific reagent and condition, and named reactions (Rosenmund, Gattermann-Koch, Etard, Stephen) are frequent exam targets.

Hydration of an alkyne (Markovnikov)

HC{\equiv}CH + H_2O \xrightarrow{H_2SO_4,\ HgSO_4} CH_3CHO

ethyne is the only alkyne giving an aldehyde; higher alkynes give ketones

Rosenmund reduction

RCOCl + H_2 \xrightarrow{Pd\text{-}BaSO_4} RCHO + HCl

catalyst poisoned with

S

/quinoline to prevent over-reduction to

RCH_2OH

Gattermann-Koch formylation

C_6H_6 + CO + HCl \xrightarrow{AlCl_3,\ CuCl} C_6H_5CHO

introduces

\text{-}CHO

directly onto the benzene ring

Etard reaction

C_6H_5CH_3 \xrightarrow{CrO_2Cl_2} \text{chromium complex} \xrightarrow{H_3O^+} C_6H_5CHO

controlled side-chain oxidation of the

\text{-}CH_3

stops at the aldehyde

From alcohols: $1^\circ$ alcohol $\xrightarrow{\text{PCC}}$ aldehyde (mild, stops at $\text{-}CHO$ ); $2^\circ$ alcohol on oxidation gives a ketone. Strong $KMnO_4/K_2Cr_2O_7$ over-oxidises a $1^\circ$ alcohol to a carboxylic acid.
From alkynes (hydration, Markovnikov, $H_2SO_4/HgSO_4$ ): only ethyne gives an aldehyde (ethanal); every other alkyne gives a ketone.
Rosenmund reduction: acyl chloride $\xrightarrow{H_2/Pd\text{-}BaSO_4}$ aldehyde; the Pd is supported on $BaSO_4$ and poisoned with sulphur or quinoline so the $H_2$ does not reduce the aldehyde further to a $1^\circ$ alcohol.
Stephen reduction: nitrile $\xrightarrow{SnCl_2/HCl}$ imine salt $\xrightarrow{H_3O^+}$ aldehyde; e.g. propanenitrile gives propanal. DIBAL-H gives the same partial reduction.
Gattermann-Koch: benzene with $CO$ , $HCl$ and $AlCl_3/CuCl$ gives benzaldehyde (formylation of the ring).
Etard reaction: toluene $\xrightarrow{CrO_2Cl_2}$ chromium complex $\xrightarrow{H_3O^+}$ benzaldehyde (controlled side-chain oxidation of the methyl group).
From nitriles to acids: full hydrolysis $CH_3CH_2CN \xrightarrow{H_3O^+}$ propanoic acid; partial (Stephen) reduction instead gives the aldehyde - choose the route by the target oxidation level.
Friedel-Crafts acylation makes aryl ketones: $C_6H_6 + CH_3COCl \xrightarrow{AlCl_3}$ acetophenone (one general way to install a $\text{C=O}$ on an aromatic ring).
Dry distillation of a calcium carboxylate gives carbonyls: calcium acetate alone $\rightarrow$ propanone; a calcium acetate + calcium formate mixture $\rightarrow$ ethanal (a classic preparative route).
DIBAL-H ( $(i\text{-}Bu)_2AlH$ ) at low temperature partially reduces esters and nitriles to aldehydes - a milder alternative to Stephen for getting $\text{-}CHO$ .
Ozonolysis of an alkene splits $\text{C=C}$ into two carbonyls ( $O_3$ then $Zn/H_2O$ ); a terminal $\text{=CH}_2$ gives $HCHO$ - useful for making aldehydes/ketones from alkenes.
Choosing the oxidant is the key exam decision: PCC (or Cu at $573\,K$ ) for the aldehyde stage, acidic $KMnO_4/K_2Cr_2O_7$ when you deliberately want to push a $1^\circ$ alcohol all the way to the acid.

Where the marks go

Saying alkyne hydration gives an aldehyde for any alkyne - only ethyne does; propyne and higher give ketones by Markovnikov addition.
Omitting the catalyst poison in Rosenmund: without $S$ /quinoline the aldehyde is reduced further to the $1^\circ$ alcohol, so the named product is wrong.
Confusing Gattermann-Koch (ring formylation of benzene, gives benzaldehyde) with Etard (side-chain oxidation of toluene) - they start from different substrates.
Using hot acidic $KMnO_4$ when the question asks for an aldehyde: it over-oxidises the $1^\circ$ alcohol to the carboxylic acid; you must use PCC to stop at $\text{-}CHO$ .
Forgetting that the enol formed on alkyne hydration tautomerises to the carbonyl, or treating an unsymmetrical internal alkyne as giving a single ketone when two are possible.

How the board asks it

Conversionnamed-reaction preparation routes
How will you convert benzene into benzaldehyde? Convert ethanenitrile into ethanal and propan- $2$ -ol into propanone, giving the reagents and conditions in each case.
Predict the productreagent-controlled oxidation level
Write the structure and name of the organic product formed when (i) propyne is treated with dilute $H_2SO_4$ in the presence of $HgSO_4$ , and (ii) propan- $1$ -ol is warmed with $PCC$ .
Identify / classifychoosing the route by target oxidation level
Compound $A$ ( $C_3H_7Cl$ ) on heating with alcoholic $KCN$ gives $B$ ; $B$ on treatment with $SnCl_2/HCl$ followed by $H_3O^+$ gives $C$ , while $B$ on acidic hydrolysis gives $D$ . Identify $A$ , $B$ , $C$ and $D$ .
Give reasonscatalyst poison in rosenmund reduction
Account for the fact that the $Pd$ catalyst in Rosenmund reduction is poisoned with sulphur or quinoline. Give a reason why $PCC$ , and not acidified $KMnO_4$ , is used to prepare an aldehyde from a $1^\circ$ alcohol.
Distinguishgattermann-koch vs etard substrate and reagent
Distinguish between the Gattermann-Koch reaction and the Etard reaction with respect to the starting substrate and the reagent used, both of which give benzaldehyde.
Identify / classifynamed formylation and side-chain oxidation
Name the reaction by which toluene is converted to benzaldehyde using $CrO_2Cl_2$ , and identify the reagents used to carry out the Gattermann-Koch formylation of benzene.

Practise this topic

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Written for Sublevo. Question text quoted anywhere in these notes is the Council’s and carries its year and paper; the board’s own diagrams are not reproduced.