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Organic Chemistry · Aldehydes and Ketones: Nucleophilic Addition Reactions

Preparing Aldehydes and Ketones

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On this page 9 sections
  1. In 30 seconds
  2. Why this matters
  3. The college version
  4. Eli explains
  5. Worked example
  6. Key takeaway
  7. Check yourself
  8. Study tools
  9. Sources & references

In 30 seconds

Most carbonyl compounds are built by oxidation — adding oxygen or removing hydrogen at the right level. The master plan: primary alcohols oxidize to aldehydes, secondary alcohols to ketones; the only question is whether you can stop at the aldehyde stage. This topic organizes the toolbox into four routes: (1) alcohol oxidation with controlled reagents (, Dess–Martin, Jones, Swern); (2) of alkenes; (3) addition of water to alkynes (Markovnikov hydration gives methyl ketones; gives aldehydes); and (4) acylation chemistry (, , organocuprate coupling, Grignard–nitrile addition). Choosing a route is strategy: the same target can come from an alcohol, an alkene, or an alkyne, and the exam question is usually "which reagent stops at the aldehyde?"

Why this matters

Aldehydes and ketones are the gateway to most functional-group chemistry: hydride reductions make alcohols, Grignard reagents extend carbon chains, amines make imines. Knowing how to make them determines whether you can build a target molecule at all. The same logic runs at scale: acetone and MEK are produced by alcohol dehydrogenation, citral (an aldehyde) feeds vitamin A and fragrance synthesis, and the liver's oxidation of ethanol to acetaldehyde is the same chemistry. For exams, the highest-yield contrast is PCC (stops at aldehyde) vs (goes to carboxylic acid), then ozonolysis and alkyne hydration.

The college version

Core Concepts

Oxidation of alcohols

A primary alcohol can be oxidized twice (alcohol → aldehyde → carboxylic acid), so stopping at the aldehyde needs anhydrous, mild conditions: PCC (pyridinium chlorochromate, in CH2Cl2) or Dess–Martin periodinane. With water present (Jones reagent, CrO3/H2SO4), the aldehyde hydrates and is oxidized again to the carboxylic acid. Secondary alcohols have no such problem — any chromium reagent, PCC, or KMnO4 gives the ketone cleanly, since a ketone cannot be oxidized further without breaking C–C bonds. The mechanism is a two-electron oxidation: the C–H α to oxygen becomes a C–O bond, the metal oxidant accepting the hydride.

Ozonolysis of alkenes

Ozone cleaves a C=C completely. With a reductive workup (dimethyl sulfide or Zn/H2O), each alkene carbon becomes a carbonyl:

  • An internal alkene gives two ketones (or aldehydes, depending on substitution): 2-methyl-2-butene → propanone + ethanal.
  • A terminal alkene gives an aldehyde + formaldehyde.
  • A cyclic alkene opens into a single molecule with two carbonyls (e.g., 1-methylcyclohexene → 6-oxoheptanal).

With an oxidative workup (H2O2), aldehydes oxidize further to carboxylic acids — the workup is a product-determining decision, not a detail.

From alkynes: Markovnikov vs anti-Markovnikov hydration

Terminal alkynes are superb carbonyl synthons. Acid-catalyzed hydration (HgSO4, H2SO4, water — the Kucherov reaction) adds water with Markovnikov regiochemistry; the tautomerizes to the more stable carbonyl, giving a methyl ketone: CH3C≡CH → propan-2-one (acetone). Hydroboration–oxidation (disiamylborane, then H2O2/NaOH) adds water anti-Markovnikov, giving the aldehyde: CH3C≡CH → propanal. Both routes run through an enol (C=C–OH) intermediate — a preview of enol chemistry later.

Aromatic and acyl routes

Arenes acylate directly: benzene + acetyl chloride + AlCl3 gives acetophenone (Friedel–Crafts acylation). Unlike alkylation, acylation does not rearrange, and the ketone product deactivates the ring (mono-acylation only). Acid chlorides open both families: Rosenmund reduction (H2/Pd on BaSO4) converts RCOCl to the aldehyde; a dialkylcuprate (R2CuLi) gives a ketone; and a Grignard reagent adds to a nitrile to give, after hydrolysis, a ketone. The acyl route is valuable because it installs the carbonyl at a fixed oxidation state without touching the rest of the molecule.

Choosing the route

  1. Ketone? Oxidize the secondary alcohol, hydrate the terminal alkyne, or ozonolyze an internal alkene.
  2. Aldehyde? Use PCC/Dess–Martin on the primary alcohol, Rosenmund reduction of the acid chloride, hydroboration–oxidation of the terminal alkyne, or ozonolysis with reductive workup.
  3. Aryl ketone? Friedel–Crafts acylation.
  4. Watch over-oxidation: aqueous Cr(VI) on a 1° alcohol gives the acid, not the aldehyde.

Common Confusions

Do not confuseWithDifference
PCC on a primary alcoholJones reagent on a primary alcoholPCC (anhydrous) stops at the aldehyde; Jones (aqueous) oxidizes through to the carboxylic acid
Ozonolysis, reductive workupOzonolysis, oxidative workupReductive (DMS/Zn) leaves aldehydes/ketones; oxidative (H2O2) converts aldehydes to acids
Alkyne hydration (Hg2+/H+, Markovnikov)Alkyne hydroboration–oxidation (anti-Markovnikov)Terminal alkyne: hydration → methyl ketone; hydroboration–oxidation → aldehyde
Friedel–Crafts acylationFriedel–Crafts alkylationAcylation doesn't rearrange and mono-acylates (ketone deactivates ring); alkylation rearranges and polyalkylates
Aldehyde from oxidation of a 1° alcoholAldehyde from oxidation of a 2° alcoholOnly primary alcohols give aldehydes; secondary alcohols give ketones (no aldehyde possible)
"Ozonolysis cleaves C=C""Ozonolysis removes C=C and nothing else changes"Both alkene carbons become carbonyls — mass and carbon count change
Eli, the EliExplains learning guide

Eli explains

The same idea, in plain words

Explain it like I’m 10

Making an aldehyde or ketone is like cooking: start with a simpler ingredient (an alcohol, alkene, or alkyne) and follow a reagent "recipe." A primary alcohol cooks twice — once to an aldehyde, once to an acid — so stopping at the aldehyde needs a "no-water" recipe (PCC). A secondary alcohol cooks only once, straight to a ketone. Cutting a double bond with ozone is like slicing a rope: each cut end becomes a carbonyl.

Worked example

Example 1: Synthesis strategy — make 2-methylbutanal from an alcohol

Problem. Propose a two-step synthesis of 2-methylbutanal, CH3CH2CH(CH3)CHO, starting from 2-methyl-1-butanol, CH3CH2CH(CH3)CH2OH.

Analysis. The target is an aldehyde; the starting material is its primary alcohol — the challenge is stopping at the aldehyde.

Reagent choice. PCC in CH2Cl2 (anhydrous). Do not use aqueous Cr(VI): the aldehyde would hydrate and oxidize to 2-methylbutanoic acid.

Equation.

CH3CH2CH(CH3)CH2OH PCC, CH2Cl2⟶ CH3CH2CH(CH3)CHO

Why it works. PCC lacks water, so the aldehyde cannot hydrate and survives.

Example 2: Ozonolysis product prediction

Problem. What forms when 2-methyl-2-butene, (CH3)2C=CHCH3, is treated with O3 then dimethyl sulfide?

Step 1 — cut the double bond. Each alkene carbon becomes a carbonyl carbon:

(CH3)2C=CHCH3 1) O3; 2) (CH3)2S⟶ (CH3)2C=O + CH3CHO

Step 2 — identify products. The doubly substituted carbon becomes propanone (acetone); the singly substituted one becomes ethanal.

Check. Five carbons in, 3 + 2 out; reductive workup keeps both carbonyls, oxidative workup would convert ethanal to acetic acid.

Example 3: Mass-based yield calculation

Problem. A student oxidizes 15.0 g of 2-propanol (MW 60.10) with acidic Cr(VI) and collects 10.9 g of acetone (MW 58.08). Calculate the percent yield.

Balanced transformation (1:1):

CH3CH(OH)CH3 ⟶ CH3COCH3

Moles of 2-propanol:

n = 15.0 g60.10 g mol-1 = 0.2496 mol

Theoretical mass of acetone (dimensional analysis):

mtheoretical = 0.2496 mol C3H8O × 1 mol C3H6O1 mol C3H8O × 58.08 g C3H6O1 mol = 14.5 g

Percent yield:

% yield = 10.9 g14.5 g × 100% = 75.2%

The 1:1 stoichiometry and ~25% loss (volatile acetone lost during workup) make this a realistic lab result.

Key takeaways

  • 1° alcohol + PCC (anhydrous) → aldehyde; + Jones reagent (aqueous CrO3/H2SO4) → carboxylic acid.
  • 2° alcohol + any standard oxidant → ketone (no over-oxidation without C–C cleavage).
  • Ozonolysis (reductive workup): alkene → carbonyl(s); cyclic alkene → ring-opened dicarbonyl.
  • Terminal alkyne: Hg2+/H+ hydration → methyl ketone; Sia2BH then H2O2/NaOH → aldehyde.
  • Friedel–Crafts acylation: arene + RCOCl/AlCl3 → aryl ketone (no rearrangement, mono-acylation).
  • Rosenmund reduction (H2/Pd–BaSO4): RCOCl → aldehyde.
  • General principle: chromium(VI) reagents are toxic and carcinogenic — handle with gloves, avoid dust, and follow your institution's waste-disposal rules; never neutralize them by guesswork.

Check yourself

6 review questions from the chapter. Try each one, then open the answer.

  1. Which reagent converts a primary alcohol to an aldehyde without further oxidation, and why does the reagent choice matter?

    Show answer

    PCC (or Dess–Martin) in anhydrous conditions. In water the aldehyde hydrates and oxidizes further to the acid; PCC keeps the system dry so the aldehyde survives.

  2. What products form from ozonolysis of 2-methyl-2-butene with reductive workup?

    Show answer

    Propanone (acetone) and ethanal (acetaldehyde): the disubstituted alkene carbon becomes the ketone, the monosubstituted one the aldehyde.

  3. A terminal alkyne is treated with HgSO4/H2SO4/H2O. What functional group results, and via what intermediate?

    Show answer

    A methyl ketone (e.g., propan-2-one from propyne). Water adds Markovnikov to give an enol, which tautomerizes to the more stable carbonyl.

  4. How would you convert benzene to acetophenone in one step?

    Show answer

    Friedel–Crafts acylation: benzene + acetyl chloride + AlCl3 → acetophenone.

  5. A student starts with 15.0 g of 2-propanol (MW 60.10) and obtains 10.9 g of acetone (MW 58.08). Is this result consistent with a 75% yield? Show the calculation.

    Show answer

    Moles of alcohol = 15.0/60.10 = 0.2496 mol; theoretical acetone = 0.2496 × 58.08 = 14.5 g; yield = (10.9/14.5) × 100% = 75.2%. Yes, consistent.

  6. Why can a secondary alcohol be oxidized to a ketone with any standard oxidant, while a primary alcohol needs a special reagent for the aldehyde?

    Show answer

    A ketone cannot be oxidized further without breaking C–C bonds, so even strong aqueous oxidants stop at the ketone. An aldehyde has a reactive C–H that aqueous oxidants convert to a C–OH, so stopping there requires anhydrous, mild conditions.

Keep learning

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Study tools & related lessonsKey vocabulary · Related

Key vocabulary

PCC
Pyridinium chlorochromate, a mild anhydrous Cr(VI) oxidant
Jones reagent
CrO3 in aqueous H2SO4/acetone
Ozonolysis
Cleavage of a C=C bond by ozone, followed by a workup
Enol
A C=C–OH isomer that tautomerizes to a carbonyl
Hydroboration–oxidation
Anti-Markovnikov addition of water to a π bond
Friedel–Crafts acylation
Electrophilic aromatic substitution with an acyl chloride/AlCl3
Rosenmund reduction
Catalytic hydrogenation of an acid chloride to an aldehyde

Sources & references

  1. openstax.org — Organic Chemistry

This lesson was adapted from the open educational references above; their licenses and attributions are preserved. See Copyright & Licensing.

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