Quick Menu

Preparation of Aldehydes and Ketones

Under construction

This page is a draft outline. It is visible to admins and still needs a full lesson.

Klein 19.2. The addition reactions are taught. This page collects where aldehydes and ketones come from.

  • Oxidation of primary alcohols to aldehydes, and of secondary alcohols to ketones, using the reagents from the alcohols chapter.
  • Ozonolysis of alkenes, once that page exists.
  • Hydration of alkynes, already taught, as a route to ketones and acetaldehyde.
  • Friedel-Crafts acylation as a route to aryl ketones.
  • A short "which starting material?" practice set.

Preparation of Aldehydes and Ketones

Having explored how aldehydes and ketones react as electrophiles, we now turn our attention backward to examine their synthesis. Carbonyl compounds serve as central synthetic hubs, and they can be prepared reliably from several core functional groups.

[ Insert Image: Master Synthesis Map for Aldehydes and Ketones ]
Figure 1. Primary synthetic pathways leading to aldehyde and ketone functional groups.

1. Oxidation of Alcohols

The most direct route to carbonyl compounds involves the oxidation of alcohol precursors:

  • Primary Alcohols to Aldehydes: Oxidation of a primary alcohol yields an aldehyde. To prevent over-oxidation to a carboxylic acid, mild, anhydrous reagents such as PCC (pyridinium chlorochromate) in dichloromethane must be used.
  • Secondary Alcohols to Ketones: Oxidation of a secondary alcohol yields a ketone. Because ketones cannot be oxidized further without breaking carbon-carbon bonds, standard oxidants like Jones reagent ($\text{CrO}_3, \text{H}_2\text{SO}_4$) or PCC work exceptionally well.
[ Insert Reaction: Alcohol Oxidation Pathways to Carbonyls ]
Figure 2. Controlled oxidation of primary and secondary alcohols.

2. Ozonolysis of Alkenes

As covered in our discussion of alkene cleavage, carbon-carbon double bonds can be completely split apart to reveal carbonyl groups:

  • Reductive Ozonolysis: Treating an alkene with ozone ($\text{O}_3$) followed by a mild reducing workup like dimethyl sulfide ($\text{Me}_2\text{S}$) cleaves the $\text{C}=\text{C}$ bond to generate aldehydes and/or ketones depending on the substitution pattern of the starting alkene.
[ Insert Reaction: Alkene Ozonolysis to Carbonyl Fragments ]
Figure 3. Cleavage of alkene double bonds into aldehyde and ketone fragments.

3. Hydration of Alkynes

Alkynes provide direct routes to specific carbonyl classes through Markovnikov or anti-Markovnikov water additions:

  • Markovnikov Hydration (Ketones): Treating an alkyne with aqueous acid and mercuric sulfate ($\text{HgSO}_4, \text{H}_2\text{SO}_4$) adds water in a Markovnikov fashion, forming an enol intermediate that tautomerizes rapidly into a ketone (or acetaldehyde if acetylene is used).
  • Anti-Markovnikov Hydration (Aldehydes): Hydroboration-oxidation of a terminal alkyne yields an enol that tautomerizes into an aldehyde.
[ Insert Reaction: Alkyne Hydration Pathways ]
Figure 4. Synthesis of methyl ketones and aldehydes via alkyne hydration.

4. Friedel-Crafts Acylation

To attach a carbonyl directly to an aromatic ring, we use Friedel-Crafts acylation:

  • Reagents: An acyl chloride ($\text{RCOCl}$) or acid anhydride in the presence of a Lewis acid catalyst such as aluminum chloride ($\text{AlCl}_3$).
  • Product: Forms an aryl ketone (phenone). Unlike alkylations, acylations do not undergo carbocation rearrangements and deactivate the ring against over-reaction due to carbonyl electron withdrawal.
[ Insert Mechanism: Friedel-Crafts Acylation of Benzene ]
Figure 5. Direct synthesis of aryl ketones via Friedel-Crafts acylation.

5. Practice: Which Starting Material?

Test your synthesis planning by identifying the appropriate starting material and reagents required to prepare each target carbonyl compound below:

  1. Target: Hexanal (an aldehyde)
    Starting Material / Reagents: 1-Hexanol oxidized with PCC in $\text{DCM}$ (prevents over-oxidation to carboxylic acid). Alternatively, reductive ozonolysis of an appropriate 1-alkene.
  2. Target: Acetophenone (an aryl ketone)
    Starting Material / Reagents: Benzene treated with acetyl chloride ($\text{CH}_3\text{COCl}$) and $\text{AlCl}_3$ via Friedel-Crafts acylation.
  3. Target: 2-Pentanone (a methyl ketone)
    Starting Material / Reagents: 2-Pentyne subjected to acid-catalyzed hydration using $\text{HgSO}_4$ and $\text{H}_2\text{SO}_4$, or oxidation of 2-pentanol using Jones reagent / PCC.
[ Insert Reaction: Practice Set Solutions for Carbonyl Preparations ]
Figure 6. Worked pathways for selecting starting materials to build target carbonyls.