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Unit 8: Aldehydes, Ketones and Carboxylic Acids

8.1 Aldehydes and Ketones

Aldehydes have the carbonyl group (\(-C=O\)) bonded to a carbon and hydrogen, whereas in ketones, it is bonded to two carbon atoms. Both involve an \(sp^2\) hybridized carbon atom bonded to oxygen through a double bond (one \(\sigma\) and one \(\pi\) bond). Because oxygen is much more electronegative than carbon, the carbonyl group is highly polarized with a partial positive charge on carbon (\(\delta^+\)) and a partial negative charge on oxygen (\(\delta^-\)).

Methods of Preparation

  1. From Alcohols:
    • Oxidation of primary alcohols gives aldehydes (using PCC/CrO\(_3\)).
    • Oxidation of secondary alcohols gives ketones.
    • Dehydrogenation over heated copper at 573 K.
  2. From Hydrocarbons:
    • Ozonolysis of alkenes.
    • Hydration of alkynes in the presence of Hg\(^{2+}\) and dil. H\(_2\)SO\(_4\).
  3. From Acid Chlorides (Rosenmund Reduction): Hydrogenation of acyl chlorides over Pd on \(BaSO_4\) yields aldehydes.
  4. From Nitriles:
    • Stephen reaction: Reduction of nitriles with \(SnCl_2/HCl\).
    • Reaction with Grignard reagents gives ketones.
  5. From Benzene: Friedel-Crafts acylation to prepare aromatic ketones. Etard reaction to prepare benzaldehyde from toluene.

Physical and Chemical Properties

  • Boiling Points: Higher than hydrocarbons and ethers of comparable molecular masses due to weak molecular association arising from dipole-dipole interactions. Lower than alcohols (as they do not form intermolecular hydrogen bonds).
  • Solubility: Lower members are miscible with water because they can form hydrogen bonds with water molecules.

Mechanism of Nucleophilic Addition

The electrophilic carbonyl carbon is attacked by a nucleophile from a direction perpendicular to the plane of \(sp^2\) hybridized orbitals of carbonyl carbon. The carbon changes from \(sp^2\) to \(sp^3\) creating a tetrahedral alkoxide intermediate.

  • Addition of HCN: Gives cyanohydrins.
  • Addition of \(NaHSO_3\): Gives bisulphite addition product.
  • Addition of Alcohols: Aldehydes form acetals; ketones form ketals.
  • Addition of Ammonia Derivatives: Forms imine derivatives with elimination of water (e.g., hydrazine gives hydrazone).

Relative Reactivity: Aldehydes are generally more reactive than ketones in nucleophilic addition reactions due to steric and electronic (inductive) reasons. The two alkyl groups in ketones hinder the approach of nucleophiles and reduce the positive charge on the carbonyl carbon (due to their +I effect).

Reactivity of Alpha (\(\alpha\)) Hydrogen

The \(\alpha\)-hydrogen of aldehydes and ketones is acidic due to the electron-withdrawing effect of the carbonyl group and resonance stabilization of the resulting enolate anion.

  1. Aldol Condensation: Aldehydes/ketones having at least one \(\alpha\)-hydrogen undergo a reaction in the presence of dilute alkali to form \(\beta\)-hydroxy aldehydes (aldol) or \(\beta\)-hydroxy ketones (ketol). Heating results in dehydration to form \(\alpha,\beta\)-unsaturated carbonyl compounds.
  2. Cannizzaro Reaction: Aldehydes which do not have an \(\alpha\)-hydrogen (e.g., formaldehyde, benzaldehyde) undergo self-oxidation and reduction (disproportionation) on treatment with concentrated alkali. \[ 2HCHO + NaOH \rightarrow CH_3OH + HCOONa \]

8.2 Carboxylic Acids

Carbon compounds containing a carboxyl functional group \(-COOH\).

Methods of Preparation

  1. From primary alcohols and aldehydes: Using strong oxidizing agents like alkaline \(KMnO_4\) or acidified \(K_2Cr_2O_7\).
  2. From alkylbenzenes: Vigorous oxidation of alkylbenzenes with chromic acid or alkaline/acidic \(KMnO_4\) gives benzoic acid.
  3. From nitriles and amides: Hydrolysis catalyzed by \(H^+\) or \(OH^-\).
  4. From Grignard Reagents: Reaction with solid \(CO_2\) (dry ice) followed by acid hydrolysis.

Chemical Properties and Acidic Nature

Carboxylic acids are weaker than mineral acids but stronger than alcohols and phenols. \[ R-COOH \rightleftharpoons R-COO^- + H^+ \] The carboxylate ion is highly stabilized by two equivalent resonance structures where the negative charge is delocalized over two highly electronegative oxygen atoms.

  • Effect of substituents on acidity: Electron-withdrawing groups (EWG) like \(-CF_3, -NO_2, -CN, -Cl\) stabilize the carboxylate ion through the -I / -R effect and increase acidity. Electron-donating groups (EDG) like alkyl groups destabilize the carboxylate ion and decrease acidity.

Important Reactions of Carboxylic Acids

  1. Formation of Anhydrides: Heating with \(P_2O_5\) or concentrated \(H_2SO_4\).
  2. Esterification: Reaction with alcohols/phenols in the presence of a mineral acid catalyst.
  3. Reactions with \(PCl_5, PCl_3, SOCl_2\): The -OH group is replaced by -Cl to form acyl chlorides.
  4. Reduction: Reduced to primary alcohols by \(LiAlH_4\) (but not by \(NaBH_4\)).
  5. Decarboxylation: Heating sodium salts with soda lime (NaOH + CaO) yields hydrocarbons.
  6. Hell-Volhard-Zelinsky (HVZ) Reaction: Carboxylic acids having an \(\alpha\)-hydrogen are halogenated at the \(\alpha\)-position on treatment with chlorine or bromine in the presence of small amounts of red phosphorus.

Competency-Based Questions (CBQs)

Q1. (CBSE 2022) Arrange the following compounds in increasing order of their reactivity in nucleophilic addition reactions: Ethanal, Propanal, Propanone, Butanone.


Answer: Increasing order of reactivity: Butanone < Propanone < Propanal < Ethanal

Reason: Nucleophilic addition to a carbonyl compound depends on:

  1. Steric Hindrance: Bulky groups hinder the attack of the nucleophile. Ketones have two alkyl groups (bulkier) compared to aldehydes, making ketones less reactive. Butanone has one methyl and one ethyl group; Propanone has two methyl groups. Ethanal has a methyl group, and propanal has an ethyl group.
  2. Electronic Factor (+I effect): Alkyl groups are electron-donating. More alkyl groups reduce the electrophilicity (positive charge) of the carbonyl carbon, making it less susceptible to nucleophilic attack. Ketones possess two alkyl groups, reducing the partial positive charge on carbon more than aldehydes do. Combining both effects, larger ketones are the least reactive, and smaller aldehydes are the most reactive.

Q2. (Sample Paper 2024) Predict the products of the Aldol condensation of Ethanal (Acetaldehyde) and name them. Give the overall chemical equation.


Answer: Ethanal (\(CH_3CHO\)) possesses three \(\alpha\)-hydrogen atoms. In the presence of a dilute base (e.g., dilute NaOH), it undergoes self-condensation.

Step 1: Aldol formation. \[ CH_3-CHO + CH_3-CHO \xrightarrow{\text{dil. NaOH}} CH_3-CH(OH)-CH_2-CHO \] The product is 3-hydroxybutanal (an aldol).

Step 2: Dehydration. Upon heating, an \(\alpha, \beta\)-elimination of water occurs due to the stability gained by forming a conjugated double bond. \[ CH_3-CH(OH)-CH_2-CHO \xrightarrow{\Delta} CH_3-CH=CH-CHO + H_2O \] The final product is But-2-enal (Crotonaldehyde).

Q3. (CBSE 2023) An organic compound (A) with molecular formula \(C_8H_8O\) forms an orange-red precipitate with 2,4-DNP reagent and gives yellow precipitate on heating with iodine in the presence of sodium hydroxide. It neither reduces Tollen’s or Fehling’s reagent, nor does it decolourize bromine water or Baeyer’s reagent. On drastic oxidation with chromic acid, it gives a carboxylic acid (B) having molecular formula \(C_7H_6O_2\). Identify the compounds (A) and (B) and write the reactions involved.


Answer:

  1. Forms an orange-red ppt with 2,4-DNP: It contains a carbonyl group (aldehyde or ketone).
  2. Gives yellow ppt with \(I_2\) and NaOH (Iodoform test): It is a methyl ketone containing the \(CH_3CO-\) group.
  3. Does NOT reduce Tollen’s or Fehling’s reagents: It is not an aldehyde. Thus, it must be a ketone.
  4. Does NOT decolorize bromine water: It does not have an isolated carbon-carbon double/triple bond (degree of unsaturation belongs to an aromatic ring).
  5. Oxidation gives \(C_7H_6O_2\): Compound B is Benzoic Acid (\(C_6H_5COOH\)).

Combining these observations, Compound (A) is Acetophenone (\(C_6H_5COCH_3\)). Molecular formula check for A: \(C_6 + C_2 = 8\) carbons, \(H_5 + H_3 = 8\) hydrogens, 1 oxygen. Perfect match.

Reactions:

  1. Iodoform Reaction: \[ C_6H_5COCH_3 + 3I_2 + 4NaOH \rightarrow C_6H_5COONa + CHI_3 \downarrow (\text{yellow ppt, iodoform}) + 3NaI + 3H_2O \]
  2. Oxidation: \[ C_6H_5COCH_3 \xrightarrow{[O] / \text{Chromic acid}, \Delta} C_6H_5COOH , (\text{Compound B, Benzoic acid}) + CO_2 + H_2O \]