and Ketones ketones or a mixture of both depending on the substitution pattern of the alkene (Unit , Class XI). ( ii ) By hydration of alkynes: Addition of water to ethyne in the presence of H SO and HgSO gives acetaldehyde. All other alkynes give ketones in this reaction (Unit , Class XI). .
From acyl chloride (acid chloride) Acyl chloride (acid chloride) is hydrogenated over catalyst, palladium on barium sulphate. This reaction is called 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.
This reaction is called Stephen reaction . Alternatively, nitriles are selectively reduced by diisobutylaluminium hydride, (DIBAL-H) to imines followed by hydrolysis to aldehydes: . . Preparation of Similarly, esters are also reduced to aldehydes with DIBAL-H.
. From hydrocarbons Aromatic aldehydes (benzaldehyde and its derivatives) are prepared from aromatic hydrocarbons by the following methods: (i) 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) Use of chromyl chloride (CrO Cl ): Chromyl chloride oxidises methyl group to a chromium complex, which on hydrolysis gives corresponding benzaldehyde. This reaction is called Etard reaction . (b) Use of chromic oxide (CrO ): 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.
(iii) 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. (ii) 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. This reaction is known as Gatterman-Koch reaction.
. From acyl chlorides Treatment of acyl chlorides with dialkylcadmium, prepared by the reaction of cadmium chloride with Grignard reagent, gives ketones. . .
Preparation of Ketones . From nitriles Treating a nitrile with Grignard reagent followed by hydrolysis yields a ketone. Give names of the reagents to bring about the following transformations: (i) Hexan- -ol to hexanal Cyclohexanol to cyclohexanone (iii) p -Fluorotoluene to Ethanenitrile to ethanal p -fluorobenzaldehyde (v) Allyl alcohol to propenal (vi) But- -ene to ethanal (i) C H NH + CrO Cl - (PCC) Anhydrous CrO (iii) CrO in the presence (Diisobutyl)aluminium of acetic anhydride/ hydride (DIBAL-H) . CrO Cl .
HOH (v) PCC (vi) O /H O-Zn dust