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Aldehydes, Ketones and Carboxylic Acids - Preparation of Aldehydes and Ketones

Grade 12CBSEChemistry

Review the key concepts, formulae, and examples before starting your quiz.

🔑Concepts

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Oxidation of Alcohols: Primary alcohols (1∘1^\circ) are oxidized to aldehydes using mild oxidizing agents like Pyridinium chlorochromate (PCCPCC). Secondary alcohols (2∘2^\circ) are oxidized to ketones using CrO3CrO_3.

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Dehydrogenation of Alcohols: Passing vapors of alcohols over heated copper at 573 K573\,K. 1∘1^\circ alcohols give aldehydes, while 2∘2^\circ alcohols give ketones.

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Ozonolysis of Alkenes: Alkenes react with O3O_3 followed by Zn/H2OZn/H_2O to yield aldehydes or ketones depending on the substitution pattern of the alkene.

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Hydration of Alkynes: Addition of water to ethyne in the presence of H2SO4H_2SO_4 and HgSO4HgSO_4 gives acetaldehyde. Other alkynes yield ketones.

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Rosenmund Reduction: Acyl chlorides are hydrogenated over catalyst, palladium on barium sulphate (Pd/BaSO4Pd/BaSO_4), partially poisoned by sulfur or quinoline to yield aldehydes.

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Stephen Reaction: Nitriles are reduced to corresponding imines with stannous chloride (SnCl2SnCl_2) in the presence of hydrochloric acid (HClHCl), which on hydrolysis give corresponding aldehydes.

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Etard Reaction: Chromyl chloride (CrO2Cl2CrO_2Cl_2) oxidizes the methyl group of toluene to a chromium complex, which on hydrolysis gives benzaldehyde.

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Gatterman-Koch Reaction: Benzene or its derivative is treated with carbon monoxide (COCO) and hydrogen chloride (HClHCl) in the presence of anhydrous AlCl3AlCl_3 or CuClCuCl to give benzaldehyde.

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Preparation of Ketones from Nitriles: Treating a nitrile with a Grignard reagent followed by hydrolysis yields a ketone.

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Friedel-Crafts Acylation: Benzene reacts with acid chlorides in the presence of anhydrous AlCl3AlCl_3 to produce aromatic ketones.

📐Formulae

R−CH2OH→PCCR−CHOR-CH_2OH \xrightarrow{PCC} R-CHO

R−CH(OH)−R′→CrO3R−CO−R′R-CH(OH)-R' \xrightarrow{CrO_3} R-CO-R'

R−COCl+H2→Pd−BaSO4R−CHO+HClR-COCl + H_2 \xrightarrow{Pd-BaSO_4} R-CHO + HCl

R−CN+SnCl2+HCl→R−CH=NH→H3O+R−CHOR-CN + SnCl_2 + HCl \rightarrow R-CH=NH \xrightarrow{H_3O^+} R-CHO

C6H5CH3+2CrO2Cl2→C6H5CH(OCr(OH)Cl2)2→H2OC6H5CHOC_6H_5CH_3 + 2CrO_2Cl_2 \rightarrow C_6H_5CH(OCr(OH)Cl_2)_2 \xrightarrow{H_2O} C_6H_5CHO

CH≡CH+H2O→Hg2+/H+[CH2=CH−OH]→CH3CHOCH \equiv CH + H_2O \xrightarrow{Hg^{2+}/H^+} [CH_2=CH-OH] \rightarrow CH_3CHO

R−CN+R′MgX→etherR−C(R′)=NMgX→H3O+R−CO−R′R-CN + R'MgX \xrightarrow{ether} R-C(R')=NMgX \xrightarrow{H_3O^+} R-CO-R'

C6H6+RCOCl→Anhyd.AlCl3C6H5COR+HClC_6H_6 + RCOCl \xrightarrow{Anhyd. AlCl_3} C_6H_5COR + HCl

💡Examples

Problem 1:

Write the structure of the product formed when Benzoyl chloride is hydrogenated in the presence of Pd/BaSO4Pd/BaSO_4.

Solution:

C6H5CHOC_6H_5CHO (Benzaldehyde)

Explanation:

This is the Rosenmund reduction. Acyl chlorides (R−COClR-COCl) are reduced to aldehydes (R−CHOR-CHO) using H2H_2 in the presence of PdPd supported on BaSO4BaSO_4.

Problem 2:

Identify the products formed by the ozonolysis of 22-Methylbut-22-ene.

Solution:

Propan-2-one (CH3COCH3CH_3COCH_3) and Ethanal (CH3CHOCH_3CHO)

Explanation:

Ozonolysis breaks the C=CC=C bond. The alkene is CH3−C(CH3)=CH−CH3CH_3-C(CH_3)=CH-CH_3. Splitting at the double bond and adding oxygen to both carbons results in a ketone (from the branched carbon) and an aldehyde (from the unbranched carbon).

Problem 3:

How can you prepare Acetophenone from Benzene using an acylation reaction?

Solution:

C6H6+CH3COCl→Anhyd.AlCl3C6H5COCH3+HClC_6H_6 + CH_3COCl \xrightarrow{Anhyd. AlCl_3} C_6H_5COCH_3 + HCl

Explanation:

Benzene reacts with Acetyl chloride (CH3COClCH_3COCl) in the presence of a Lewis acid catalyst like anhydrous AlCl3AlCl_3 (Friedel-Crafts Acylation) to produce Acetophenone.

Preparation of Aldehydes and Ketones Revision - Class 12 Chemistry CBSE