Aldehydes And Ketones (Chemical Reactions)

Chemical Reactions

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Aldehydes and ketones undergo a variety of chemical reactions, primarily involving the carbonyl group and the alpha-hydrogens.

Nucleophilic Addition Reactions

Description: These are the characteristic reactions of aldehydes and ketones. The polar carbonyl group ($C=O$), with its electrophilic carbon ($\delta^+$) and nucleophilic oxygen ($\delta^-$), readily undergoes attack by nucleophiles ($Nu^-$).

Mechanism:

1. Nucleophilic Attack: The nucleophile attacks the electrophilic carbonyl carbon.
2. Protonation: The resulting alkoxide ion is protonated (usually by water or acid) to form the addition product.

General Reaction:

$$ R_2C=O + Nu^- \rightarrow [R_2C(O^-)Nu] \xrightarrow{H^+} R_2C(OH)Nu $$

Key Examples:

• Addition of Hydrogen Cyanide (HCN): Forms cyanohydrins.

$RCHO + HCN \rightarrow RCH(OH)CN$

• Addition of Ammonia and its Derivatives: Reaction with ammonia ($NH_3$), hydroxylamine ($NH_2OH$), phenylhydrazine ($C_6H_5NHNH_2$), semicarbazide ($NH_2CONHNH_2$) forms imines, oximes, phenylhydrazones, and semicarbazones, respectively.

$RCHO + NH_2OH \rightarrow RCH=NOH + H_2O$ (Oxime)

• Addition of Alcohols: Forms hemiacetals and acetals (reversible reaction).

$RCHO + ROH \rightleftharpoons RCH(OH)(OR)$ (Hemiacetal)

$RCH(OH)(OR) + ROH \rightleftharpoons RCH(OR)_2 + H_2O$ (Acetal)

• Addition of Sodium Bisulfite ($NaHSO_3$): Forms crystalline bisulfite adducts, which can be used to separate and purify aldehydes and ketones.

$RCHO + NaHSO_3 \rightleftharpoons RCH(OH)SO_3Na$

• Grignard Reagents and Other Organometallic Reagents: Reaction with $R'MgX$ or $RLi$ followed by hydrolysis yields alcohols.

$RCHO + R'MgX \xrightarrow{1. \text{ether}} \xrightarrow{2. H_3O^+} RCH(OH)R'$ (Secondary alcohol)

$R_2CO + R'MgX \xrightarrow{1. \ ether} \xrightarrow{2. H_3O^+} R_2C(OH)R'$ (Tertiary alcohol)

Reactivity Order: Aldehydes are generally more reactive than ketones towards nucleophilic addition due to:

• Steric hindrance: Less steric hindrance around the carbonyl carbon in aldehydes.
• Electronic effects: The alkyl groups in ketones are electron-donating, which reduces the partial positive charge on the carbonyl carbon, making it less electrophilic.

Reduction

1. Reduction to Alcohols:

• Aldehydes can be reduced to primary alcohols.
• Ketones can be reduced to secondary alcohols.
• Reducing Agents: Catalytic hydrogenation ($H_2$/Ni, Pt, Pd) or chemical reducing agents like Lithium Aluminum Hydride ($LiAlH_4$) or Sodium Borohydride ($NaBH_4$).

$RCHO \xrightarrow{[H]} RCH_2OH$

$R_2CO \xrightarrow{[H]} R_2CHOH$

2. Reduction to Hydrocarbons:

• Clemmensen Reduction: Reduction of the carbonyl group to a methylene group ($-CH_2-$) using amalgamated zinc ($Zn(Hg)$) and concentrated hydrochloric acid ($HCl$). This reaction is specific for ketones and aldehydes and converts them to alkanes.

$RCHO \text{ or } R_2CO \xrightarrow{Zn(Hg), HCl} RCH_2R'$

• Wolff-Kishner Reduction: Reaction with hydrazine ($NH_2NH_2$) followed by heating with a strong base (like $KOH$ in ethylene glycol) converts the carbonyl group to a methylene group.

$RCHO \text{ or } R_2CO \xrightarrow{1. NH_2NH_2, KOH, \ heat} RCH_2R'$

Oxidation

Aldehydes: Aldehydes are easily oxidized to carboxylic acids, even by mild oxidizing agents.

• Mild Oxidizing Agents: Tollens' reagent (ammoniacal silver nitrate solution, $[Ag(NH_3)_2]^+$) and Fehling's solution (alkaline solution of copper(II) tartrate complex) are used to test for aldehydes. Aldehydes reduce $Ag^+$ to $Ag$ (silver mirror) or $Cu^{2+}$ to $Cu^+$ (red precipitate of $Cu_2O$).

$RCHO + 2[Ag(NH_3)_2]^+ + 3OH^- \rightarrow RCOO^- + 4NH_3 + 2Ag(s) + 2H_2O$

$RCHO + 2Cu^{2+}(in \ Fehling's \ soln) + 5OH^- \rightarrow RCOO^- + Cu_2O(s) + 3H_2O$

• Strong Oxidizing Agents: Readily oxidized by $KMnO_4$ or $K_2Cr_2O_7$ in acidic medium.

Ketones: Ketones are generally resistant to oxidation by mild oxidizing agents. They are oxidized to carboxylic acids (usually involving cleavage of a $C-C$ bond) only by strong oxidizing agents under harsh conditions. The oxidation generally occurs at the more substituted $\alpha$-carbon (Krypton rule).

Cannizzaro Reaction: Aldehydes that do not have an $\alpha$-hydrogen atom (like formaldehyde and benzaldehyde) undergo disproportionation in the presence of concentrated alkali. One molecule is oxidized to carboxylic acid, and another is reduced to alcohol.

$2RCHO + Conc. NaOH \rightarrow RCH_2OH + RCOONa$

Crossed Cannizzaro Reaction: If an aldehyde without $\alpha$-hydrogens is treated with another aldehyde (which has $\alpha$-hydrogens), the aldehyde without $\alpha$-hydrogens undergoes Cannizzaro reaction, while the other aldehyde undergoes nucleophilic addition.

Reactions Due To $\alpha$-Hydrogen

Acidity of $\alpha$-Hydrogen: The hydrogen atoms attached to the carbon atom adjacent to the carbonyl group (called $\alpha$-hydrogens) are acidic.

Reason: The electron-withdrawing inductive effect of the carbonyl group and the resonance stabilization of the resulting enolate ion make the $\alpha$-hydrogens acidic.

$$ \underset{\alpha \rightarrow}{R-CH_2-CHO} \xrightarrow{-H^+} [\underset{\alpha \leftarrow}{R-CH=CH-O^-} \leftrightarrow R-CH^--CHO] $$

Reactions:

1. Aldol Condensation:

• Description: Aldehydes and ketones having $\alpha$-hydrogens undergo self-condensation in the presence of dilute acid or base catalysts to form $\beta$-hydroxy aldehydes or ketones (aldols), which then often undergo dehydration to form $\alpha, \beta$-unsaturated aldehydes or ketones (enals or enones).
• Reaction:

$2CH_3CHO \xrightarrow{dil. NaOH} CH_3CH(OH)CH_2CHO \text{ (Aldol)}$

$CH_3CH(OH)CH_2CHO \xrightarrow{-H_2O} CH_3CH=CHCHO \text{ (Crotonaldehyde)}$

• Crossed Aldol Condensation: Reaction between two different aldehydes or ketones, or between an aldehyde and a ketone.

2. Haloform Reaction:

• Description: Methyl ketones (ketones with $CH_3CO-$ group) and aldehydes with $\alpha$-hydrogens like acetaldehyde react with halogens (Cl$_2$, Br$_2$, I$_2$) in the presence of a base ($NaOH$, $KOH$) to form a haloform ($CHX_3$) and a carboxylate salt.
• Reaction:

$CH_3COCH_3 + 4I_2 + 6NaOH \rightarrow CHI_3(s) + CH_3COONa + 5NaI + 3H_2O$

• Only methyl ketones and acetaldehyde give this reaction.

Other Reactions

1. Reaction with Phosphorus Pentachloride ($PCl_5$): Carbonyl oxygen is replaced by chlorine.

$RCHO + PCl_5 \rightarrow RCHCl_2 + POCl_3$

$R_2CO + PCl_5 \rightarrow R_2CCl_2 + POCl_3$

2. Reaction with Phosphorus Pentachloride ($PCl_5$): Carbonyl oxygen is replaced by chlorine.

$RCHO + PCl_5 \rightarrow RCHCl_2 + POCl_3$

$R_2CO + PCl_5 \rightarrow R_2CCl_2 + POCl_3$

3. Reaction with $P_4O_{10}$ or $PCl_5$: These act as dehydrating agents.

4. Reaction with $SOCl_2$:

$RCHO + SOCl_2 \rightarrow RCHCl_2 + SO_2$