Aldehyde_and_ketone_formation

Alcohol Reactions: Aldehyde and Ketone Formation from Alcohols using PCC or DMP

Alcohol Oxidation in CH2Cl2: DMP vs. PCC

Dry dichloromethane levels the playing field for Dess-Martin periodinane (DMP) and pyridinium chlorochromate (PCC). Both reagents push primary alcohols one oxidation state up to aldehydes and secondary alcohols to ketones while leaving tertiary alcohols alone (no alpha-hydrogen). The mechanistic pathways diverge-hypervalent iodine vs. chromate ester-but the outcomes match under anhydrous conditions.


Introduction


  • Shared outcomes: In CH2Cl2 both reagents stop at aldehydes (primary) and ketones (secondary); tertiary substrates do not react.
  • Mechanistic contrast: DMP forms an alkoxyperiodinane that collapses via acetate. PCC builds a Cr(VI) chromate ester that undergoes E2-like beta-hydride elimination.
  • Why run both? DMP is neutral/mild and kind to acid-sensitive groups; PCC is cheaper, widely available, but acidic and generates Cr waste.
  • Anhydrous control: Keeping water out prevents aldehyde hydration; without the gem-diol, over-oxidation stalls.


Quick Summary


  • Reagents: DMP or PCC in dry CH2Cl2; mild base (NaHCO3) often buffers DMP.
  • Products: primary ROH -> R-CHO; secondary ROH -> R2C=O; tertiary ROH -> no oxidation (no alpha-H).
  • Arrow flow: Both sequences create a reagent-alcohol adduct then deliver a beta-hydride to iodine or chromium.
  • Mechanism panels: DMP is four frames; PCC is five frames (mirroring the updated diagrams below).
  • Stereochemistry: Oxidation flattens the alcohol carbon (sp3 -> sp2), erasing configuration.
  • Safety: Hypervalent iodine dust (DMP) and Cr(VI) sludge (PCC) demand PPE and regulated waste handling.
  • Swern alternative: For the DMSO/(COCl)2 route, see the Swern oxidation guide.


Mechanisms - DMP & PCC (4 vs. 5 Frames)


Dess-Martin periodinane (DMP)

Alcohol oxygen bonds to iodine while acetate departs, giving the alkoxyperiodinane.
Step D1 - Alkoxyperiodinane formation via ligand exchange.
Acetate removes the oxonium proton and the O–H electrons drop into iodine, neutralising the bound alkoxide.
Step D2 - Acetate deprotonates the bound alkoxyperiodinane.
Acetate pulls the alpha hydrogen; the C–H electrons form the carbonyl while a hydride migrates to iodine.
Step D3 - Internal hydride transfer creates the carbonyl.
Carbonyl product leaves and Dess-Martin byproducts are removed under basic workup.
Step D4 - Workup isolates the aldehyde or ketone.

Pyridinium chlorochromate (PCC)

Alcohol oxygen coordinates to chromium(VI), building the chromate ester in dry solvent.
Step P1 - Chromate ester formation.
Base removes the alpha hydrogen; electrons flow into the nascent carbonyl and toward chromium.
Step P2 - Beta-hydride transfer to chromium.
The negatively charged oxygen collapses into chromium as chloride prepares to disengage.
Step P3 - Chromate collapse toward chloride departure.
Chloride assists proton transfer, the alkoxide forms the C=O, and the O–Cr bond breaks.
Step P4 - Chloride-promoted cleavage liberates the carbonyl.
Carbonyl product is gathered while reduced chromium solids are filtered and quenched.
Step P5 - Workup / filtration removes chromium waste.


Worked Examples


Branched Primary Alcohol

DMP in CH2Cl2
Branched primary alcohol substrate Dess-Martin periodinane reagent icon Aldehyde product after DMP oxidation

DMP elevates the branched primary alcohol to the aldehyde with no over-oxidation in dry solvent.

PCC in dry CH2Cl2
Branched primary alcohol substrate PCC reagent icon Aldehyde product after PCC oxidation

Under anhydrous PCC conditions the same aldehyde is obtained; hydrate formation is avoided.

Cyclic Secondary Alcohol

DMP in CH2Cl2
Cyclic secondary alcohol substrate Dess-Martin periodinane reagent icon Cyclic ketone product after DMP oxidation

DMP converts the secondary alcohol to the corresponding ketone; the oxidised carbon becomes planar.

PCC in dry CH2Cl2
Cyclic secondary alcohol substrate PCC reagent icon Cyclic ketone product after PCC oxidation

PCC follows the chromate ester route to the same ketone; quench the chromium sludge carefully.

Tertiary Alcohol (No alpha-H)

DMP in CH2Cl2
Tertiary alcohol substrate lacking an alpha hydrogen Dess-Martin periodinane reagent icon No oxidation result

No beta-hydrogen means no oxidation—the tertiary alcohol remains unchanged with DMP.

PCC in dry CH2Cl2
Tertiary alcohol substrate lacking an alpha hydrogen PCC reagent icon No oxidation result

Chromate ester formation is possible, but the absence of a beta-hydrogen halts the elimination—the substrate is recovered.


Mechanistic Checklist


  • alpha-H required: No beta-hydride -> no oxidation (tertiary or neopentyl systems).
  • Adduct first: Build alkoxyperiodinane (DMP) or chromate ester (PCC) before elimination.
  • Concerted step: beta-H abstraction and O-I/O-Cr bond cleavage occur in a single arrow bundle (no carbocations).
  • Dry media: CH2Cl2 suppresses aldehyde hydrate formation, blocking further oxidation.
  • Stereochemical reset: Oxidation produces trigonal carbonyl; neighboring stereocenters remain unless epimerised downstream.


Edge Cases & Exam Traps


  • Water present? PCC plus water forms aldehyde hydrates that over-oxidise to acids-flagged in many problem sets.
  • Tertiary allylic systems: PCC under acidic conditions may trigger oxidative rearrangements (Babler oxidation); DMP rarely does.
  • Acid-sensitive motifs: Choose DMP for substrates bearing acetals, silyl ethers, or base-labile functional groups; PCC's acidity can unmask protections.
  • Diols: Both reagents can oxidise each OH one rung (dialdehydes/diketones) but will not cleave C-C (contrast with NaIO4).
  • Waste compliance: Cr(VI) residues require special disposal; DMP byproducts (iodobenzoates) still need hazardous waste handling.


Practical Tips


  • Pre-dry glassware and solvents; molecular sieves optional but helpful for PCC slurries.
  • Buffer DMP runs with NaHCO3 to neutralise acetic acid and maintain rate.
  • For PCC, stir until oxidation complete, then filter chromium solids and wash with dilute base.
  • Use HPLC/IR to confirm aldehyde stops-look for absence of broad O-H (hydrate) or carboxy peaks.
  • Document waste handling in lab notebooks; regulators scrutinise chromium disposal logs.


Exam-Style Summary


  • DMP / CH2Cl2: primary -> aldehyde, secondary -> ketone, tertiary -> no reaction; mild, neutral conditions.
  • PCC / CH2Cl2: primary -> aldehyde (if dry), secondary -> ketone; acidic, watch for hydrate over-oxidation when wet.
  • Key warning: No radicals, no rearrangements-show hydride arrows and E2-like geometry.
  • Decision point: Choose reagent based on functional group tolerance and waste profile.


Interactive Toolbox


  • Mechanism Solver — load DMP / CH2Cl2 or PCC / CH2Cl2 to regenerate the four- and five-frame SVGs.
  • Reaction Solver — test custom alcohols under DMP or PCC to predict aldehyde/ketone vs. no-reaction outcomes.
  • IUPAC Namer — confirm systematic names for the aldehydes and ketones generated in these examples.