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Aldehydes & Ketones revision notes

Every idea in the Aldehydes & Ketones chapter for JEE Main, with the usual mistakes and the fast methods. Topic 1 is open to everyone; sign in free to read all 16 and download the PDF.

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How we count: 7× asked means 7 past JEE Main questions in the PrepWiser bank needed that concept, and "last" is the latest year one appeared. ★ marks ideas asked 8 or more times.

Preparation of aldehydes and ketones

Ozonolysis and alkyne hydration
5× asked, last 2026
RX2C=CHRX′→(ii) Zn/HX2O(i) OX3RX2C=O+RX′CHO,RC≡CH→333 KHgX2+,dil. HX2SOX4RCOCHX3\ce{R2C=CHR' ->[{(i)} O3][{(ii)} Zn/H2O] R2C=O + R'CHO},\qquad \ce{RC#CH ->[Hg^{2+}, {dil.} H2SO4][333 K] RCOCH3}
Each C=C carbon becomes C=O; Zn/H₂O stops RCHO going on to acid. Hydration is Markovnikov: only ethyne gives an aldehyde.
2-methylbut-2-ene
(i) OX3\ce{O3}
(ii) Zn/HX2O\ce{H2O}
O
propanone
+
O
ethanal
1-methylcyclohexene
(i) OX3\ce{O3}
(ii) Zn/HX2O\ce{H2O}
O O
CH₃CO(CH₂)₄CHO: one chain, C7 kept
ethyne
HgX2+/HX+\ce{Hg^{2+}/H+}
 
O
ethanal (the only aldehyde)
propyne
HgX2+/HX+\ce{Hg^{2+}/H+}
 
O
propanone
  • Quick method. A ring alkene opens to ONE chain with C=O at both ends. To find the alkene, join the two C=O carbons back by C=C: CHX3CO(CHX2)X4CHO\ce{CH3CO(CH2)4CHO} comes from 1-methylcyclohexene.
From acyl chlorides, nitriles and esters
9× asked, last 2026
RCOCl→HX2,Pd−BaSOX4RCHO,RCN→(ii) HX3OX+(i) SnClX2/HClRCHORCN→(ii) HX2O(i) DIBAL−HRCHO,RCN→(ii) HX3OX+(i) RX′MgXRCORX′\begin{gathered}\ce{RCOCl ->[H2, Pd-BaSO4] RCHO},\qquad \ce{RCN ->[{(i)} SnCl2/HCl][{(ii)} H3O+] RCHO}\\ \ce{RCN ->[{(i)} DIBAL-H][{(ii)} H2O] RCHO},\qquad \ce{RCN ->[{(i)} R'MgX][{(ii)} H3O+] RCOR'}\end{gathered}
Rosenmund Pd is poisoned (BaSO₄, quinoline): RCHO survives. Rosenmund, Stephen, DIBAL-H keep the C count; RCN + R′MgX adds R′.
Start (R = C₂H₅)ReagentProductC change
Cl O
H₂, Pd–BaSO₄ (Rosenmund)
O
0
N
SnCl₂/HCl, then H₃O⁺ (Stephen)
O
0
N
DIBAL-H, then H₂O
O
0
O O
DIBAL-H (1 equiv., cold), then H₂O
O
0
N
CH₃MgBr, then H₃O⁺
O
+1
O O
LiAlH₄ (overshoots)
O H
0
Products made from each structure; only the Grignard step adds a carbon (R′ joins the old nitrile C). Ester OCH₃ leaves as CH₃OH and is not counted.
  • DIBAL-H (1 equiv., cold) also stops an ester at the aldehyde: RCOORX′→(ii) HX2O(i) DIBAL−HRCHO+RX′OH\ce{RCOOR' ->[{(i)} DIBAL-H][{(ii)} H2O] RCHO + R'OH}; LiAlHX4\ce{LiAlH4} takes the same ester to RCHX2OH\ce{RCH2OH}.
  • Not one step from RCOOH. Make the acyl chloride with SOClX2\ce{SOCl2}, then do Rosenmund. LiAlHX4\ce{LiAlH4} overshoots to RCHX2OH\ce{RCH2OH}, and NaBHX4\ce{NaBH4} ignores acids and esters.
  • Not an amine from RCN+RX′MgX\ce{RCN + R'MgX}. The imine salt hydrolyses to a ketone, with R' on the old nitrile carbon: CHX3CHX2CN\ce{CH3CH2CN} + CHX3MgBr\ce{CH3MgBr} gives butan-2-one.
Aromatic aldehydes and ketones
9× asked, last 2026
CX6HX5CHX3→(ii) HX3OX+(i) CrOX2ClX2,CSX2CX6HX5CHO,CX6HX6→anhyd. AlClX3/CuClCO,HClCX6HX5CHOCX6HX6→anhyd. AlClX3RCOClCX6HX5COR,2 RCOCl+RX2′Cd→2 RCORX′+CdClX2\begin{gathered}\ce{C6H5CH3 ->[{(i)} CrO2Cl2, CS2][{(ii)} H3O+] C6H5CHO},\qquad \ce{C6H6 ->[CO, HCl][{anhyd.} AlCl3/CuCl] C6H5CHO}\\ \ce{C6H6 ->[RCOCl][{anhyd.} AlCl3] C6H5COR},\qquad \ce{2RCOCl + R'2Cd -> 2RCOR' + CdCl2}\end{gathered}
Etard or CrO₃/(CH₃CO)₂O stops a ring CH₃ at CHO; KMnO₄ gives ArCOOH. –CHO/–COR deactivate, direct meta: one acyl only.
O O H O
benzene + succinic anhydride, AlCl₃
benzene
CHX3COCl\ce{CH3COCl}
anhyd. AlClX3\ce{AlCl3}
O
acetophenone: ring now deactivated
CHX3COCl\ce{CH3COCl}
AlClX3\ce{AlCl3}
no second acyl
  • Also: CX6HX5CHX3→ClX2,hνCX6HX5CHClX2→373 KHX2OCX6HX5CHO\ce{C6H5CH3 ->[Cl2, h\nu] C6H5CHCl2 ->[H2O][373 K] C6H5CHO}. From alcohols (Alcohols sheet): PCC stops a 1° alcohol at RCHO; Cu at 573 K dehydrogenates 1° and 2° alcohols.

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13 more concepts in 4 topics

Carbonyl group and nucleophilic addition; Oxidation, reduction and the haloform reaction; α-Hydrogen and aldol condensation; and Cannizzaro reaction and distinguishing tests. Read the first part free. Sign in free with Google to read the rest and download the PDF.

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Carbonyl group and nucleophilic addition

  • Reactivity towards nucleophiles6×
  • Addition of HCN, NaHSO₃ and water5×
  • Hemiacetals and acetals6×
  • Grignard addition4×
  • Ammonia derivatives: the C=N bond6×

Oxidation, reduction and the haloform reaction

  • Oxidation: Tollens' and Fehling's8×
  • Haloform (iodoform) reaction12×
  • Reduction: to alcohol or to CH₂8×

α-Hydrogen and aldol condensation

  • Acidity of α-H; keto–enol tautomerism4×
  • Aldol and cross-aldol condensation11×
  • Intramolecular aldol3×

Cannizzaro reaction and distinguishing tests

  • Cannizzaro and crossed Cannizzaro5×
  • Distinguishing tests (summary)7×

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