CBSE Class 12 Chemistry Notes: Aldehydes, Ketones and Carboxylic Acids

Key Definitions

Carbonyl Group

A functional group consisting of a carbon atom double-bonded to an oxygen atom (C=O), found in aldehydes, ketones, carboxylic acids, and their derivatives.

Aldehyde

An organic compound containing a carbonyl group bonded to at least one hydrogen atom and one alkyl/aryl group (R-CHO).

Ketone

An organic compound containing a carbonyl group bonded to two alkyl or aryl groups (R-CO-R').

Carboxylic Acid

An organic compound containing a carboxyl group (-COOH), which is a carbonyl group bonded to a hydroxyl group.

Nucleophilic Addition Reaction

A characteristic reaction of aldehydes and ketones where a nucleophile attacks the electrophilic carbon of the carbonyl group, followed by protonation of the oxygen.

Aldol Condensation

A reaction between two molecules of an aldehyde or ketone (or one of each) containing α-hydrogens, leading to the formation of a β-hydroxy carbonyl compound, which can then undergo dehydration to form an α,β-unsaturated carbonyl compound.

Cannizzaro Reaction

A disproportionation reaction undergone by aldehydes that lack α-hydrogens, in the presence of strong base, to produce a primary alcohol and a carboxylic acid salt.

Tollen's Reagent

An ammoniacal silver nitrate solution ([Ag(NH₃)₂]⁺OH⁻), used to detect aldehydes (forms a silver mirror) as it oxidizes them to carboxylic acids.

Must Remember: Key Concepts

• The carbonyl carbon is sp² hybridized, trigonal planar, and electrophilic due to oxygen's electronegativity.
• Aldehydes are generally more reactive than ketones towards nucleophilic addition due to lesser steric hindrance and greater electrophilicity (less electron-donating alkyl groups).
• The presence of α-hydrogens (hydrogens on the carbon adjacent to the carbonyl group) is crucial for reactions like Aldol Condensation.
• Tollen's and Fehling's tests are specific for aldehydes (reducing sugars also give positive tests) and not for ketones.
• Carboxylic acids are more acidic than alcohols and phenols due to the resonance stabilization of the carboxylate anion (R-COO⁻).
• Electron-withdrawing groups (EWGs) increase acidity of carboxylic acids, while electron-donating groups (EDGs) decrease acidity.
• HVZ reaction (Hell-Volhard-Zelinsky) is specific for carboxylic acids having α-hydrogens, allowing halogenation at the α-position.
• Common reducing agents for aldehydes/ketones: NaBH₄, LiAlH₄ (to alcohols). For carboxylic acids: LiAlH₄ (to primary alcohols).

Nucleophilic Addition: The Core Reaction of Carbonyl Compounds

Aldehydes and ketones are characterized by their carbonyl group (C=O), which dictates much of their chemical behavior. The carbon-oxygen double bond is highly polar because oxygen is significantly more electronegative than carbon. This creates a partial positive charge (δ+) on the carbon and a partial negative charge (δ-) on the oxygen.

Due to this electrophilic nature of the carbonyl carbon, these compounds readily undergo nucleophilic addition reactions. A nucleophile, which is an electron-rich species, attacks the electron-deficient carbonyl carbon. This attack causes the pi (π) bond between carbon and oxygen to break, and the electron pair shifts onto the oxygen, forming a tetrahedral intermediate with a negative charge on the oxygen. Subsequently, this negatively charged oxygen usually gets protonated by an available proton source (e.g., H₂O, H⁺), leading to the formation of an addition product. The overall reaction proceeds via a two-step mechanism.

The reactivity order for nucleophilic addition is generally aldehydes > ketones. This is attributed to two main factors:

1. Steric Hindrance: Aldehydes have at least one small hydrogen atom attached to the carbonyl carbon, allowing easier access for the nucleophile. Ketones have two bulkier alkyl or aryl groups, which sterically hinder the approach of the nucleophile to the carbonyl carbon.
2. Electronic Effects: Alkyl groups are electron-donating. Ketones have two alkyl groups pushing electron density towards the carbonyl carbon, which reduces its partial positive charge and thus makes it less electrophilic compared to aldehydes that have only one alkyl group (or none, in formaldehyde).

Distinguishing Aldehydes and Ketones

Worked Examples

• {"title":"1. Aldol Condensation","bodyMarkdown":"Q: Write the product of aldol condensation of ethanal (CH₃CHO) followed by heating.\n\nA:\n1. Aldol formation: 2 CH₃CHO --OH⁻/Δ--> CH₃CH(OH)CH₂CHO (3-Hydroxybutanal)\n2. Dehydration: CH₃CH(OH)CH₂CHO --Δ, -H₂O--> CH₃CH=CHCHO (But-2-enal)"}
• {"title":"2. Cannizzaro Reaction","bodyMarkdown":"Q: What are the products when benzaldehyde (C₆H₅CHO) undergoes Cannizzaro reaction?\n\nA: Benzaldehyde lacks α-hydrogens. In the presence of concentrated NaOH:\n2 C₆H₅CHO --Conc. NaOH--> C₆H₅CH₂OH (Benzyl alcohol) + C₆H₅COO⁻Na⁺ (Sodium benzoate)"}
• {"title":"3. Acidity Order","bodyMarkdown":"Q: Arrange the following in increasing order of acidity: Benzoic acid, 4-Nitrobenzoic acid, 4-Methoxybenzoic acid.\n\nA: 4-Methoxybenzoic acid < Benzoic acid < 4-Nitrobenzoic acid\n -NO₂ group is a strong electron-withdrawing group, stabilizing the carboxylate ion, thus increasing acidity.\n -OCH₃ group is an electron-donating group (via resonance, despite -I effect), destabilizing the carboxylate ion, thus decreasing acidity."}

Exam Tip: Mastering Conversions & Mechanisms

Organic conversions are high-scoring questions. For this chapter, focus on interconverting functional groups. For instance, how to go from an alcohol to an aldehyde/ketone, then to a carboxylic acid, and vice-versa. Pay close attention to specific reagents and reaction conditions (e.g., PCC for mild oxidation of primary alcohols to aldehydes, KMnO₄ for strong oxidation to carboxylic acids). Practice mechanisms for nucleophilic addition, Aldol, and Cannizzaro reactions; they often appear directly or indirectly. Don't forget distinguishing tests – a common question type that requires knowing both positive and negative results for different compound classes.

Practice Questions with Solutions

• Q: Why is formaldehyde more reactive than acetaldehyde towards nucleophilic addition? A: Formaldehyde (HCHO) has two hydrogen atoms attached to the carbonyl carbon, leading to minimal steric hindrance and maximal electrophilicity compared to acetaldehyde (CH₃CHO) which has one electron-donating methyl group and more steric hindrance.
• Q: Name a reagent that can selectively reduce a carbonyl group in the presence of an ester group. A: Sodium borohydride (NaBH₄) selectively reduces aldehydes and ketones to alcohols, but does not usually reduce ester groups.
• Q: How would you convert propan-1-ol to propanoic acid? A: Propan-1-ol (primary alcohol) can be oxidized to propanoic acid using strong oxidizing agents like acidified potassium permanganate (KMnO₄/H⁺) or potassium dichromate (K₂Cr₂O₇/H⁺).
• Q: What is the iodoform test and which compounds give a positive result? A: The iodoform test uses I₂ and NaOH. Compounds containing a CH₃CO- group (methyl ketones) or a CH₃CH(OH)- group (secondary alcohols that can be oxidized to methyl ketones) give a positive result, forming a yellow precipitate of iodoform (CHI₃).

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