Aldehydes, Ketones and Carboxylic Acids — Revision Notes

⚡ 30-Second Revision

Aldehydes — $ext{RCHO}$, suffix '-al'. Oxidized by Tollen's ($ext{Ag}$ mirror), Fehling's ($ext{Cu}_2\text{O}$ red ppt.). Undergo NAR. With $alpha$-H: Aldol. Without $alpha$-H: Cannizzaro.
Ketones — $ext{RCOR}'$, suffix '-one'. Resistant to mild oxidation. Undergo NAR. With $alpha$-H: Aldol.
Carboxylic Acids — $ext{RCOOH}$, suffix '-oic acid'. Acidic due to resonance-stabilized carboxylate anion. Form esters, acid chlorides. Reduced by $ext{LiAlH}_4$.
Key Reagents

- $ext{PCC}$: Primary alcohol to aldehyde. - $ext{KMnO}_4$: Alcohol/aldehyde to carboxylic acid. - $ext{Pd}/\text{BaSO}_4$ (Rosenmund): $ext{RCOCl} \rightarrow \text{RCHO}$. - $ext{SnCl}_2/\text{HCl}$ (Stephen): $ext{RCN} \rightarrow \text{RCHO}$.

- $ext{NaBH}_4$: Aldehyde/ketone $ightarrow$ alcohol (selective). - $ext{LiAlH}_4$: Aldehyde/ketone/acid/ester $ightarrow$ alcohol (strong). - $ext{Zn-Hg}/\text{HCl}$ (Clemmensen): $ext{R}_2\text{C=O} \rightarrow \text{R}_2\text{CH}_2$.

- $ext{NH}_2\text{NH}_2/\text{KOH}$ (Wolff-Kishner): $ext{R}_2\text{C=O} \rightarrow \text{R}_2\text{CH}_2$. - $ext{NaOH}$ (dilute): Aldol condensation. - $ext{NaOH}$ (conc.): Cannizzaro reaction. - $ext{X}_2/\text{Red P}$ (HVZ): $ext{RCH}_2\text{COOH} \rightarrow \text{RCH(X)COOH}$.

- $ext{I}_2/\text{NaOH}$ (Iodoform): $ext{CH}_3\text{CO-R}$ or $ext{CH}_3\text{CH(OH)-R} \rightarrow \text{CHI}_3$ (yellow ppt.).

2-Minute Revision

Aldehydes and ketones are characterized by the carbonyl group ($>\text{C}=\text{O}$). Aldehydes ($ext{RCHO}$) have at least one hydrogen on the carbonyl carbon, making them more reactive in nucleophilic addition reactions (NAR) and easily oxidizable to carboxylic acids (positive Tollen's and Fehling's tests).

Ketones ($ext{RCOR}'$) have two alkyl/aryl groups, are less reactive in NAR, and resist mild oxidation. Both undergo NAR with reagents like $ext{HCN}$, alcohols (forming acetals/ketals), and ammonia derivatives.

They can be reduced to alcohols by $ext{NaBH}_4$ or $ext{LiAlH}_4$, or to alkanes by Clemmensen or Wolff-Kishner reductions. Aldehydes and ketones with $alpha$-hydrogens undergo Aldol condensation, while aldehydes without $alpha$-hydrogens undergo Cannizzaro reaction.

Methyl ketones give a positive Iodoform test.

Carboxylic acids ($ext{RCOOH}$) contain the carboxyl group, making them acidic due to resonance stabilization of the carboxylate anion. They are stronger acids than phenols and alcohols. They react with bases, metals, and carbonates.

They form derivatives like esters (esterification), acid chlorides ($ext{SOCl}_2$), and anhydrides. Carboxylic acids can be reduced to primary alcohols by $ext{LiAlH}_4$ and undergo decarboxylation.

The Hell-Volhard-Zelinsky (HVZ) reaction is specific for $alpha$-halogenation of carboxylic acids with $alpha$-hydrogens. Mastering name reactions, distinguishing tests, and reactivity trends is key for NEET.

5-Minute Revision

Let's consolidate the key aspects of aldehydes, ketones, and carboxylic acids for a quick yet comprehensive review. The central functional group is the carbonyl ($>\text{C}=\text{O}$), which is polar, making the carbon electrophilic. In aldehydes ($ext{RCHO}$), this carbon is bonded to at least one hydrogen, while in ketones ($ext{RCOR}'$), it's bonded to two alkyl/aryl groups. Carboxylic acids ($ext{RCOOH}$) feature a carboxyl group ($-\text{COOH}$), a combination of carbonyl and hydroxyl.

Preparation:

Aldehydes/Ketones: — Oxidation of primary/secondary alcohols (PCC for aldehydes, $ext{CrO}_3$ for ketones), ozonolysis of alkenes, hydration of alkynes (methyl ketones). Specific name reactions include Rosenmund reduction ($ext{RCOCl} \rightarrow \text{RCHO}$), Stephen reaction ($ext{RCN} \rightarrow \text{RCHO}$), and Gattermann-Koch (aromatic aldehydes). Friedel-Crafts acylation yields aromatic ketones.
Carboxylic Acids: — Oxidation of primary alcohols/aldehydes ($ext{KMnO}_4$), hydrolysis of nitriles/amides, Grignard reagents with $ext{CO}_2$, and oxidation of alkylbenzenes.

Physical Properties: Boiling points are higher than hydrocarbons due to dipole-dipole interactions. Carboxylic acids have the highest due to strong hydrogen bonding (dimers). Lower members are water-soluble due to H-bonding with water.

Chemical Reactions:

Aldehydes/Ketones:

* Nucleophilic Addition (NAR): Most characteristic. Aldehydes are more reactive than ketones. Examples: addition of $ext{HCN}$ (cyanohydrins), alcohols (acetals/ketals), ammonia derivatives (imines, oximes).

* Reduction: To alcohols ($ext{NaBH}_4$, $ext{LiAlH}_4$) or to alkanes (Clemmensen: $ext{Zn-Hg}/\text{HCl}$; Wolff-Kishner: $ext{NH}_2\text{NH}_2/\text{KOH}$). * Oxidation: Aldehydes are easily oxidized to carboxylic acids (Tollen's, Fehling's tests).

Ketones resist mild oxidation. * **$alpha$-Hydrogen Reactions:** Aldol condensation (with $alpha$-H, dilute base, forms $\beta$-hydroxy carbonyls). Cannizzaro reaction (without $alpha$-H, conc. base, disproportionation).

Haloform reaction (for methyl ketones/acetaldehyde, $ext{I}_2/\text{NaOH}$).

Carboxylic Acids:

* Acidity: Stronger than phenols/alcohols due to resonance-stabilized carboxylate anion. React with bases, metals, carbonates. * Derivative Formation: Esters (esterification with alcohol/acid), acid chlorides ($ext{SOCl}_2$), anhydrides. * Reduction: To primary alcohols ($ext{LiAlH}_4$). * Decarboxylation: Loss of $ext{CO}_2$ (e.g., with soda lime). * HVZ Reaction: $alpha$-halogenation of carboxylic acids with $alpha$-H ($ext{X}_2/\text{Red P}$).

NEET Focus: Prioritize name reactions, distinguishing tests, reagent specificity, and comparative acidity. Practice multi-step conversions and understand the underlying reasons for reactivity differences.

Prelims Revision Notes

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Functional Groups:

* Aldehyde: $ext{RCHO}$ (carbonyl carbon bonded to H and R/Ar). Suffix '-al'. * Ketone: $ext{RCOR}'$ (carbonyl carbon bonded to two R/Ar groups). Suffix '-one'. * Carboxylic Acid: $ext{RCOOH}$ (carboxyl group). Suffix '-oic acid'.

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Nomenclature: — IUPAC rules are crucial. Carbonyl C is C-1 for aldehydes/acids. Position for ketones is numbered.

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Preparation Methods:

* Aldehydes: Oxidation of $1^circ$ alcohols (PCC), Rosenmund ($ext{RCOCl} xrightarrow{\text{H}_2/\text{Pd}/\text{BaSO}_4} \text{RCHO}$), Stephen ($ext{RCN} xrightarrow{\text{SnCl}_2/\text{HCl}} \text{RCH=NH} xrightarrow{\text{H}_3\text{O}^+} \text{RCHO}$), Gattermann-Koch ($ext{C}_6\text{H}_6 xrightarrow{\text{CO, HCl, AlCl}_3} \text{C}_6\text{H}_5\text{CHO}$), DIBAL-H (from esters/nitriles).

* Ketones: Oxidation of $2^circ$ alcohols ($ext{CrO}_3$), hydration of alkynes (terminal alkynes $ightarrow$ methyl ketones), Friedel-Crafts acylation ($ext{ArH} + \text{RCOCl} xrightarrow{\text{AlCl}_3} \text{ArCOR}$), from nitriles with Grignard.

* Carboxylic Acids: Oxidation of $1^circ$ alcohols/aldehydes ($ext{KMnO}_4$), hydrolysis of nitriles/amides ($ext{RCN/RCONH}_2 xrightarrow{\text{H}_3\text{O}^+} \text{RCOOH}$), Grignard with $ext{CO}_2$ ($ext{RMgX} + \text{CO}_2 \rightarrow \text{RCOOMgX} xrightarrow{\text{H}_3\text{O}^+} \text{RCOOH}$), oxidation of alkylbenzenes.

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Physical Properties:

* Boiling Point: Carboxylic acids > Alcohols > Ketones $approx$ Aldehydes > Hydrocarbons (due to H-bonding and dipole-dipole interactions). * Solubility: Lower members (up to 4-5 C) are water-soluble due to H-bonding.

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Chemical Reactions (Aldehydes & Ketones):

* Nucleophilic Addition: Aldehydes > Ketones (less steric hindrance, more electrophilic C). Examples: $ext{HCN}$ (cyanohydrins), $ext{NaHSO}_3$ (bisulfite adducts), alcohols (acetals/ketals), $ext{NH}_2\text{Z}$ (imines, oximes, hydrazones).

* Reduction: To $1^circ$/$2^circ$ alcohols ($ext{H}_2/\text{Ni}$, $ext{NaBH}_4$, $ext{LiAlH}_4$). To alkanes (Clemmensen: $ext{Zn-Hg}/\text{HCl}$; Wolff-Kishner: $ext{NH}_2\text{NH}_2/\text{KOH}$).

* Oxidation: Aldehydes oxidized by Tollen's ($ext{Ag(NH}_3\text{)}_2^+$ $ightarrow$ $ext{Ag}$ mirror), Fehling's ($ext{Cu}^{2+}$ $ightarrow$ $ext{Cu}_2\text{O}$ red ppt.). Ketones resist mild oxidation.

* **$alpha$-Hydrogen Reactions:** * Aldol Condensation: Aldehydes/ketones with $alpha$-H, dilute base $ightarrow$ $\beta$-hydroxy carbonyls. * Cannizzaro Reaction: Aldehydes *without* $alpha$-H, conc.

base $ightarrow$ alcohol + acid salt. * Haloform Reaction: $ext{CH}_3\text{CO-R}$ or $ext{CH}_3\text{CH(OH)-R}$ with $ext{X}_2/\text{NaOH} \rightarrow \text{CHX}_3$ (yellow ppt. for $ext{I}_2$).

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Chemical Reactions (Carboxylic Acids):

* Acidity: $ext{RCOOH} > \text{ArOH} > \text{ROH}$. Electron-withdrawing groups increase acidity; electron-donating groups decrease it. * Reactions involving O-H bond: Esterification ($ext{RCOOH} + \text{R}'\text{OH} xrightarrow{\text{H}^+} \text{RCOOR}'$).

* Reactions involving C-OH bond: Formation of acid chlorides ($ext{SOCl}_2$, $ext{PCl}_3$, $ext{PCl}_5$), anhydrides. * Reactions involving COOH group: Reduction ($ext{LiAlH}_4 \rightarrow 1^circ$ alcohol), Decarboxylation ($ext{RCOONa} xrightarrow{\text{NaOH/CaO}} \text{RH}$).

* HVZ Reaction: $ext{RCH}_2\text{COOH} xrightarrow{\text{X}_2/\text{Red P}} \text{RCH(X)COOH}$. * Electrophilic Substitution (aromatic): $-\text{COOH}$ is deactivating and meta-directing.

Vyyuha Quick Recall

Always Know Carboxylic Acids:

Aldehydes: Always React (Tollen's, Fehling's), Always Light (Aldol) or Concentrated (Cannizzaro). Ketones: Keep Resisting (mild oxidation), Keep Always (Aldol). Carboxylic Acids: Can Always Share (acidic), Can Esterify (esters), Can Reduce (LiAlH4).