Physical and Chemical Properties — Revision Notes

⚡ 30-Second Revision

Carbonyl Polarity: — $C=O$ is polar ($\\delta^+C-\delta^-O$).
Boiling Points: — Aldehydes/ketones > Alkanes (dipole-dipole), but < Alcohols (no intermolecular H-bonding).
Solubility: — Lower members water-soluble (H-bonding with water).
Nucleophilic Addition: — Characteristic reaction. Reactivity: Formaldehyde > Aldehydes > Ketones.
Oxidation: — Aldehydes $\xrightarrow{\text{mild oxid.}}$ Carboxylic acids (Tollens', Fehling's). Ketones are resistant.
Reduction to Alcohols: — Aldehydes $\xrightarrow{NaBH_4/LiAlH_4}$ Primary alcohols. Ketones $\xrightarrow{NaBH_4/LiAlH_4}$ Secondary alcohols.
Reduction to Hydrocarbons: — Clemmensen ($Zn-Hg/HCl$) or Wolff-Kishner ($NH_2NH_2/KOH$).
\$\alpha\$-Hydrogens: — Acidic, form enolates. Essential for Aldol Condensation.
Aldol Condensation: — Aldehydes/ketones with \\alpha\-H $\xrightarrow{dil. base}$ \\beta\-hydroxy carbonyl $\xrightarrow{\Delta}$ \\alpha,\beta\-unsaturated carbonyl.
Cannizzaro Reaction: — Aldehydes *without* \\alpha\-H $\xrightarrow{conc. base}$ Alcohol + Carboxylic acid salt.
Haloform Reaction: — $CH_3CO-$ or $CH_3CH(OH)-$ groups $\xrightarrow{X_2/NaOH}$ $CHX_3$ (haloform) + $RCOONa$.

2-Minute Revision

Aldehydes and ketones are defined by the carbonyl group ($C=O$). This group's polarity dictates their physical and chemical behavior. Physically, they have higher boiling points than alkanes due to dipole-dipole interactions but lower than alcohols because they cannot form intermolecular hydrogen bonds among themselves.

Smaller members are water-soluble as their carbonyl oxygen can hydrogen bond with water. Chemically, the electrophilic carbonyl carbon makes them highly susceptible to nucleophilic addition reactions.

Aldehydes are generally more reactive than ketones due to less steric hindrance and greater electrophilicity. They can be reduced to alcohols using $NaBH_4$ or $LiAlH_4$, or completely deoxygenated to hydrocarbons via Clemmensen (acidic) or Wolff-Kishner (basic) reductions.

A key distinction lies in oxidation: aldehydes are easily oxidized to carboxylic acids by mild agents like Tollens' and Fehling's reagents, while ketones are resistant. The presence of acidic \\alpha\-hydrogens enables reactions like aldol condensation.

Aldehydes lacking \\alpha\-hydrogens undergo the Cannizzaro reaction. Methyl ketones and acetaldehyde give a positive haloform test.

5-Minute Revision

Let's consolidate the key aspects of aldehydes and ketones. Their central feature, the carbonyl group, is highly polar, with a partial positive charge on carbon and a partial negative charge on oxygen.

This polarity leads to significant dipole-dipole interactions, making their boiling points higher than non-polar alkanes of similar molecular weight. However, unlike alcohols, they lack an O-H bond, so they cannot form intermolecular hydrogen bonds with each other, resulting in lower boiling points than alcohols.

Their solubility in water decreases with increasing carbon chain length, but smaller members are water-soluble due to hydrogen bonding between the carbonyl oxygen and water molecules.

Chemically, the electrophilic carbonyl carbon is the site for nucleophilic addition reactions, which are characteristic. Aldehydes are more reactive than ketones due to less steric hindrance and greater electrophilicity. Examples include addition of HCN (forming cyanohydrins) and Grignard reagents (forming alcohols). They also react with ammonia derivatives to form imines, oximes, etc.

Reduction can lead to alcohols (using $NaBH_4$ for aldehydes to primary alcohols, ketones to secondary alcohols) or complete deoxygenation to hydrocarbons using Clemmensen ($Zn-Hg/HCl$) or Wolff-Kishner ($NH_2NH_2/KOH$) reductions.

Oxidation provides a crucial distinction: Aldehydes are readily oxidized to carboxylic acids by mild agents like Tollens' reagent (silver mirror test) and Fehling's solution (red precipitate). Ketones are resistant to mild oxidation and require strong conditions, leading to C-C bond cleavage.

**Reactions involving \\alpha\-hydrogens are also vital. These hydrogens are acidic due to resonance stabilization of the resulting enolate ion. This enables Aldol Condensation**, where aldehydes/ketones with \\alpha\-hydrogens form \\beta\-hydroxy carbonyl compounds, which dehydrate upon heating to \\alpha,\beta\-unsaturated carbonyls.

Aldehydes *without* \\alpha\-hydrogens (e.g., formaldehyde, benzaldehyde) undergo the Cannizzaro reaction in concentrated base, disproportionating into an alcohol and a carboxylic acid salt. Finally, the Haloform reaction is a specific test for compounds containing a $CH_3CO-$ group or a $CH_3CH(OH)-$ group, producing a haloform ($CHX_3$) and a carboxylate salt.

Prelims Revision Notes

Physical Properties:

Boiling Points: — Higher than alkanes/ethers (due to dipole-dipole interactions). Lower than alcohols/carboxylic acids (no intermolecular H-bonding).
Solubility: — Lower members (up to 4 carbons) are water-soluble due to H-bonding with water. Solubility decreases with increasing alkyl chain length.
Odor: — Lower aldehydes (pungent), higher aldehydes/ketones (fragrant).

Chemical Properties (Reactivity):

I. Nucleophilic Addition Reactions (NAR):

Reactivity Order: — Formaldehyde > Aldehydes > Ketones (due to steric hindrance and electronic effects).
Examples:

* HCN addition: Forms cyanohydrins ($R_2C(OH)CN$). * **NaHSO$_3$ addition:** Forms bisulfite addition products ($R_2C(OH)SO_3Na$). * Alcohol addition: Aldehydes form hemiacetals (unstable) $\rightarrow$ acetals (stable).

Ketones form hemiketals $\rightarrow$ ketals (less favored). * Grignard reagent (RMgX) addition: Formaldehyde $\rightarrow$ 1° alcohol; Other aldehydes $\rightarrow$ 2° alcohol; Ketones $\rightarrow$ 3° alcohol.

* **Ammonia derivatives ($H_2N-Z$):** Forms imines, oximes, hydrazones, semicarbazones (nucleophilic addition-elimination, loss of $H_2O$).

II. Oxidation Reactions:

Aldehydes: — Easily oxidized to carboxylic acids ($RCOOH$).

* Tollens' Test: Aldehyde + Tollens' reagent ($[Ag(NH_3)_2]^+$) $\rightarrow$ Silver mirror ($Ag(s)$). (Positive for aldehydes). * Fehling's Test: Aldehyde + Fehling's solution ($Cu^{2+}$) $\rightarrow$ Red-brown precipitate ($Cu_2O$). (Positive for aldehydes, except aromatic aldehydes with strong EWG).

Ketones: — Resistant to mild oxidation. Strong oxidation causes C-C bond cleavage, forming carboxylic acids with fewer carbons (Popoff's rule).

III. Reduction Reactions:

To Alcohols:

* Aldehydes $\xrightarrow{NaBH_4 \text{ or } LiAlH_4}$ Primary alcohols ($RCH_2OH$). * Ketones $\xrightarrow{NaBH_4 \text{ or } LiAlH_4}$ Secondary alcohols ($R_2CHOH$).

To Hydrocarbons (Deoxygenation):

* Clemmensen Reduction: $C=O \xrightarrow{Zn-Hg, conc. HCl}$ $CH_2$. (Acid-stable compounds). * Wolff-Kishner Reduction: $C=O \xrightarrow{NH_2NH_2, KOH, \Delta}$ $CH_2$. (Base-stable compounds).

IV. Reactions due to \$\alpha\$-Hydrogens:

\$\alpha\$-Hydrogens: — Acidic due to resonance stabilization of enolate ion.
Aldol Condensation: — Aldehydes/ketones *with* \\alpha\-H $\xrightarrow{dil. base}$ \\beta\-hydroxy carbonyl (aldol/ketol) $\xrightarrow{\Delta}$ \\alpha,\beta\-unsaturated carbonyl.
Cannizzaro Reaction: — Aldehydes *without* \\alpha\-H (e.g., HCHO, $C_6H_5CHO$) $\xrightarrow{conc. base}$ Alcohol + Carboxylic acid salt (disproportionation).
Haloform Reaction: — Compounds with $CH_3CO-$ or $CH_3CH(OH)-$ groups $\xrightarrow{X_2/NaOH}$ Haloform ($CHX_3$) + $RCOONa$. (Test for methyl ketones and acetaldehyde/ethanol).

Vyyuha Quick Recall

Always Know Properties: Nucleophilic Oxidation Reduction Alpha-H.

Nucleophilic: NAR is key. Aldehydes > Ketones.
Oxidation: Aldehydes oxidize easily (Tollens', Fehling's). Ketones resist.
Reduction: To Alcohols ($NaBH_4$) or Hydrocarbons (Clemmensen/Wolff-Kishner).
Alpha-H: If present, Aldol. If absent, Cannizzaro. Methyl ketones do Haloform.