Chemistry Preparation of Aldehydes and Ketones

Topics Covered :

● Preparation of Aldehydes and Ketones
● Preparation of Aldehydes
● From Acyl Chloride
● From Nitriles and Esters
● From Hydrocarbons
● Preparation of Ketones

Preparation of Aldehydes and Ketones :

`color{green}(text(By Oxidation of Alcohols ))` : Aldehydes and ketones are generally prepared by oxidation of primary and secondary alcohols, respectively. See fig.1.

`color{green}(text(By Dehydrogenation of Alcohols ))` : This method is suitable for volatile alcohols and is of industrial application.

● In this method alcohol vapours are passed over heavy metal catalysts (`color{red}(Ag)` or `color{red}(Cu)`).

● Primary and secondary alcohols give aldehydes and ketones, respectively. See fig.2.

`color{green}(text(From Hydrocarbons ))` :

(i) `color{green}(text(By Ozonolysis of Alkenes )) ` : As we know, ozonolysis of alkenes followed by reaction with zinc dust and water gives aldehydes, ketones or a mixture of both depending on the substitution pattern of the alkene. See fig.3.

(ii) `color{green}(text(By Hydration of Alkynes ))` : Addition of water to ethyne in the presence of `color{red}(H_2SO_4)` and `color{red}(HgSO_4)` gives acetaldehyde. All other alkynes give ketones in this reaction. See fig.4.

Preparation of Aldehydes :

Aldehydes are prepared as follows :

From acyl chloride (acid chloride) :

Acyl chloride (acid chloride) is hydrogenated over catalyst, palladium on barium sulphate. This reaction is called `color{green}(text(Rosenmund))` reduction.

From nitriles and esters :

`=>` Nitriles are reduced to corresponding imine with stannous chloride in the presence of hydrochloric acid, which on hydrolysis give corresponding aldehyde.

`color{red}(RCN+SnCl_2+HCl → RCH = NH overset(H_3O)→ RCHO)` This reaction is called Stephen reaction.

`=>` Nitriles are selectively reduced by diisobutylaluminium hydride, (`color{red}(DIBAL-H)`) to imines followed by hydrolysis to aldehydes :

`color{red}(RCN underset(2.H_2O) overset(1.AlH(i-Bu)_2)→ R-CHO)`

`color{red}(CH_3-CH=CH-CH_2CH_2-CN underset(2.H_2O) overset(1.AlH(i-Bu)_2)→ CH_3-CH=CH-CH_2CH_2-CHO)`

`=>` Esters are also reduced to aldehydes with `color{red}(DIBAL-H)`.

`color{red}(CH_3(CH_2)_9 - overset (overset(O)(||))C-OC_2H_5 underset(2.H_2O) overset(1 . DIBAL-H)→ CH_3(CH_2)_9- overset(overset(O)(||))C-H)`

From hydrocarbons :

Aromatic aldehydes (benzaldehyde and its derivatives) are prepared from aromatic hydrocarbons by the following methods :

(i) `color{green}(text(By oxidation of methylbenzene ))` : Strong oxidising agents oxidise toluene and its derivatives to benzoic acids.

● However, it is possible to stop the oxidation at the aldehyde stage with suitable reagents that convert the methyl group to an intermediate that is difficult to oxidise further.

The following methods are used for this purpose.

(a) `color{green}(text(Use of chromyl chloride))` `color{red}((CrO_2Cl_2))` : Chromyl chloride oxidises methyl group to a chromium complex, which on hydrolysis gives corresponding benzaldehyde. This reaction is called `color{green}(text(Etard Reaction))`. See fig.1.

(b) `color{green}(text(Use of chromic oxide))` `color{red}((CrO_3))` : Toluene or substituted toluene is converted to benzylidene diacetate on treating with chromic oxide in acetic anhydride. The benzylidene diacetate can be hydrolysed to corresponding benzaldehyde with aqueous acid. See fig.2.

(ii) `color{green}(text(By side chain chlorination followed by hydrolysis ))` : Side chain chlorination of toluene gives benzal chloride, which on hydrolysis gives benzaldehyde. This is a commercial method of manufacture of benzaldehyde. See fig.3.

(iii) `color{green}(text(By Gatterman – Koch reaction ))` : When benzene or its derivative is treated with carbon monoxide and hydrogen chloride in the presence of anhydrous aluminium chloride or cuprous chloride, it gives benzaldehyde or substituted benzaldehyde. This reaction is known as Gatterman-Koch reaction. See fig.4.

Preparation of Ketones :

`color{green}(text(From Acyl Chlorides ))` : Treatment of acyl chlorides with dialkylcadmium, prepared by the reaction of cadmium chloride with Grignard reagent, gives ketones. See fig.1.

`color{red}(2R-Mg - X + CdCl_2 → R_2Cd + 2Mg(X) Cl_2)`

`color{green}(text(From Nitriles ))` : Treating a nitrile with Grignard reagent followed by hydrolysis yields a ketone. See fig.2.

`color{green}(text(From Benzene or Substituted Benzenes ))` : When benzene or substituted benzene is treated with acid chloride in the presence of anhydrous aluminium chloride, it affords the corresponding ketone. This reaction is known as `color{green}(text(Friedel-Crafts acylation))` reaction. See fig.3.
Q 3052167034

Give names of the reagents to bring about the following transformations:

(i) Hexan-1-ol to hexanal
(ii) Cyclohexanol to cyclohexanone
(iii) p-Fluorotoluene to p-fluorobenzaldehyde
(iv) Ethanenitrile to ethanal
(v) Allyl alcohol to propenal
(vi) But-2-ene to ethanal


(i) `C_5H_5NH^+CrO_3Cl-(PC C)`
(ii) `K_2Cr_2O_7` in acidic medium
(iii) `CrO_3` in the presence
(iv) (Diisobutyl)aluminium of acetic anhydride/ hydride (DIBAL-H) `1. CrO_2Cl_2 2. HOH`
(v) PCC
(vi) `O_3//H_2O-Zn` dust