Chapter 8 / 8.4 Chemical Reactions

Chemical Reactions of Aldehydes & Ketones 😊

General step: A nucleophile (Nu^–) attacks the electrophilic carbon of the carbon-oxygen double bond at ~90° to the plane, giving a tetrahedral alkoxide that quickly picks up H⁺ → overall addition of Nu^– and H⁺. :contentReference[oaicite:0]{index=0}
Reactivity trend: Aldehydes > ketones (steric crowding + two electron-releasing alkyl groups in ketones lower carbonyl electrophilicity).
Key examples 🌟
- HCN addition → cyanohydrin formation:
  \( \text{RCHO} + \text{HCN} \xrightarrow[\text{base}]{\,} \text{RCH(OH)CN} \) :contentReference[oaicite:1]{index=1}
- NaHSO₃ addition gives water-soluble adducts (useful for purification). :contentReference[oaicite:2]{index=2}
- Grignard reagents add (see Unit 7 for details). :contentReference[oaicite:3]{index=3}
- Alcohols (dry HCl) → hemiacetals → acetals; ketones + ethylene glycol → ketals.
- NH₃/derivatives \( \text{H}_2\text{N–Z} \) give imines, oximes, hydrazones, etc. (Table 8.2).

To alcohols: NaBH₄ or LiAlH₄ (or H₂/catalyst) reduce aldehydes → 1° alcohols, ketones → 2° alcohols.
To hydrocarbons 🚗
- Clemmensen (Zn-Hg/HCl): \( >\!C=O \rightarrow >\!CH_2 \)
- Wolff–Kishner (NH₂NH₂, base, heat) achieves the same in basic medium.

Aldehydes oxidise easily to carboxylic acids with KMnO₄, K₂Cr₂O₇, HNO₃, Tollens or Fehling reagents.
Ketones need strong conditions; C–C cleavage gives shorter-chain acids.
Diagnostic tests 🔍
- Tollens’ test: silver mirror (Ag⁰) for aldehydes.
- Fehling’s test: red Cu₂O precipitate for aliphatic aldehydes.
- Haloform (iodoform) reaction: methyl ketones give yellow CHI₃ + salt of acid with one fewer C. :contentReference[oaicite:4]{index=4}

Reason for acidity: –C=O pulls e^–s; conjugate base is resonance-stabilised.
Aldol condensation: dilute base gives β-hydroxy carbonyl (aldol/ketol); dehydration → α,β-unsaturated product.
Cross-aldol: two different carbonyl partners → mixture of products if both have α-H.

Cannizzaro reaction (no α-H): 2 aldehyde molecules → one alcohol + one carboxylate in hot concentrated alkali.
Electrophilic substitution on the ring: carbonyl is meta-directing & deactivating in aromatic aldehydes/ketones.

Mechanism and factors affecting nucleophilic addition to the carbonyl group. :contentReference[oaicite:5]{index=5}
Tollens vs. Fehling tests to distinguish aldehydes from ketones. :contentReference[oaicite:6]{index=6}
Aldol and Cross-aldol condensations, including dehydration step. :contentReference[oaicite:7]{index=7}
Cannizzaro reaction for aldehydes lacking α-H. :contentReference[oaicite:8]{index=8}
Clemmensen vs. Wolff–Kishner reductions—acidic vs. basic conditions. :contentReference[oaicite:9]{index=9}

Keep practicing these concepts—grasping the patterns makes reaction questions a breeze! ✨