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CBSE Class 12th Chemistry Notes: Aldehydes, Ketones and Carboxylic Acids (Part - II)

Oct 26, 2016 17:00 IST

    In this article, students will get revision notes on Chapter 12: Aldehydes, Ketones and Carboxylic Acids based on NCERT Class 12 Chemistry textbook. This article is a continuation of the revision notes on Class 12 Chemistry, Chapter- Aldehydes, Ketones and Carboxylic Acids, Part-I.

    In Part-I, we have studied about the nomenclature, methods of preparation and physical properties aldehydes and ketones. In Part-II, we will study the various chemical reactions of aldehydes and ketones. These quick notes are prepared strictly according to the latest CBSE syllabus for Class 12th Chemistry.

    The main topics covered in this part are:

    o    Chemical Reactions of Aldehydes and Ketones

          •    Nucleophilic addition reactions

          •    Reduction

          •    Oxidation

          •    Aldol condensation

          •    Cross-Aldol condensation

          •    Cannizzaro reaction:

          •    Perkin’s condensation

          •    Electrophilic substitution reactions

    o    Test to Distinguish between Aldehyde and Ketones

    o    Uses of Aldehyde and Ketones

    Chemical Reactions of Aldehydes and Ketones

    (a) Nucleophilic addition reactions:

    Due to partial positive charge on carbonyl carbon, aldehydes and ketones pose more attraction to the coming nucleophile to add to the carbonyl carbon. Therefore the reactions of aldehydes and ketones are nucleophilic addition reactions.
    Aldehydes with one R group is more reactive towards nucleophilic addition reaction than ketones with two alkyl groups at carbonyl carbon. This is due to the +inductive effect of alkyl group.

    Some important reactions involving the nucleophilic attack are given below:

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    Addition of hydrogen cyanide (HCN) to form cyanohydrins:

    Addition of sodium hydrogensulphite (NaHSO3) to form bisulphate addition compound:

    This reaction is used for the separation and purification of aldehydes and ketones. This is because the addition compound formed above is water soluble and can be converted back to the original carbonyl compound by treating it with dilute mineral acid or alkali.

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    Addition of Grignard reagent (RMgX) to form alcohol:

    Addition of ammonia and its derivatives:

    Addition of alcohols:

    Aldehydes on addition of monohydric alcohol in presente of dry HC1 forms hemiacetal which further on reacting with one more molecule of alcohol turns to and acetal.

    Ketones do not react with monohydric alcohols. Ketones react with ethylene glycol under similar conditions to form cyclic products known as ethylene glycol ketals.

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    (b) Reduction of aldehydes and ketones:

    Reduction to alcohols: Aldehydes and ketones are reduced to primary and secondary alcohols respectively by sodium borohydride (NaBH4) or lithium aluminium hydride (LiAlH4)

    Reduction to alkanes:

    Clemmensen reduction:

    Wolff-Kishner reduction:

    (c) Oxidation of Aldehydes and Ketones:

    Due to the presence of hydrogen at carbonyl carbon, aldehydes are easily oxidisable while ketones are difficult to oxidise.

    Aldehydes are oxidized to acids in presence of common oxidising agents HNO3, K2Cr2O7, KMnO4.

    Ketones are oxidized under drastic conditions i.e. with powerful oxidising agents like HNO3, K2Cr2O7/H2SO4, KMnO4/H2SO4 at higher temperature to give a mixture of acids, each containing lesser number of carbon atoms than the parent ketones.

    Haloform reaction:

    Methyl ketones are oxidized with X2/NaOH to sodium salts of corresponding carboxylic acids having one carbon atom less than that of carbonyl compound.

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    (d) Reactions of aldehydes and ketones due to α -hydrogen:

    Aldol condensation:

    Aldehydes and ketones containing atleast one α -hydrogen undergo self condensation reactions in the presence of dilute alkali to form β-hydroxy aldehydes (aldol) or β-hydroxy ketones (ketol), respectively which on heating in the presence of H+ gives  α, β-unsaturated aldehydes or ketones.

    Cross aldol condensation:

    When aldol condensation is carried out between two different aldehydes and / or ketones, it is called cross aldol condensation. If both of them contain α-hydrogen atoms, it gives a mixture of four products

    (e) Canizzaro reaction:

    Aldehydes which do not have an α -hydrogen atom undergo self-oxidation and reduction (disproportionation) reaction on treatment with concentrated alkali to form alcohol and salt of acid.

    (f) Perkin's condensation:

    (g) Electrophilic substitution reaction:

    Aromatic aldehyde and ketones undergo electrophilic substitution at the meta position. Carbonyl group shows + R effect, therefore acts as a deactivating and meta directing group.

    Example of electrophilic susbstitution reaction:

    Test to Distinguish between AKdehyde and Ketones

    (i) Tollen's reagent test:

    Tollen's reagent is ammoniacal silver nitrate and is a mild oxidising agent. Aldehydes give silver mirror with Tollen's reagent.

    (ii) Fehling's solution test:

    Fehling's solution is an alkaline solution of CuSO4 (Fehling A) and sodium potassium tartrate, Rochelle salt (Fehling B).  Aldehyde gives reddish brown ppt. of brown precipitates of cuprous oxide (Cu2O). Aromatic aldehyde gives poor yield.

    Uses of aldehydes and ketones:

    •    Aldehyde is mostly used in the formation of resins when it is combined with melamine, urea, phenol etc.

    •    Formaldehyde in the form of formalin (40%) solution, is used to preserve biological specimens.

    •    Benzaldehyde is used in perfumery and in dye industries.

    •    Acetophenone is used as ingredients in flavours and deodorants.

    •    Ketones are used as solvents and intermediates in the chemical industry.

    CBSE Class 12th Chemistry Notes: Aldehydes, Ketones and Carboxylic Acids (Part - I)

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