Difficulty: Intermediate | Prerequisites: Functional groups, alcohols, alkenes, alkynes, oxidation/reduction basics (Chapters 8–12 recommended).
This is the first half of Chapter 17, covering what aldehydes and ketones are, how to name them, and how to make them. The carbonyl group (C=O) is the single most important functional group in organic chemistry: it appears in aldehydes, ketones, carboxylic acids, esters, amides, and more. If you have not yet reviewed alcohol oxidation (Chapter 12) and alkene/alkyne reactions (Chapters 8–9), do that first. Everything here builds on those foundations and feeds directly into the nucleophilic addition reactions covered in Part 2.
Aldehydes have a carbonyl bonded to at least one hydrogen; ketones have the carbonyl flanked by two carbon groups. You name them with the suffixes -al (aldehydes) and -one (ketones). Both can be synthesised from alcohols, alkenes, alkynes, esters, acid chlorides, and cuprate reagents, with the choice of reagent controlling whether you stop at the aldehyde or push through to the carboxylic acid.
Carbonyl group
A C=O double bond. The carbon is sp2-hybridised and electrophilic (partial positive charge, delta+) because oxygen is more electronegative. In simple terms, this is the reactive heart of every aldehyde and ketone.
Aldehyde (RCHO)
A carbonyl group bonded to at least one hydrogen atom. The suffix in IUPAC naming is -al. Think of it as the "exposed" version of the carbonyl, with that hydrogen making it more reactive and easier for nucleophiles to reach.
Ketone (RCOR')
A carbonyl group bonded to two carbon-containing groups (R and R'). The suffix is -one. In simple terms, the two R groups crowd the carbonyl and stabilise it, making ketones less reactive than aldehydes toward nucleophilic addition.
PCC (Pyridinium Chlorochromate)
A mild oxidising agent (used in CH2Cl2) that converts primary alcohols to aldehydes without over-oxidising to carboxylic acids. Think of it as the "gentle" oxidant: it stops at the aldehyde stage.
Jones' reagent
A strong chromium-based oxidant (CrO3 / H2SO4 in acetone) that oxidises secondary alcohols to ketones. It would push a primary alcohol all the way to the carboxylic acid, so it is not used when you want to stop at the aldehyde.
Ozonolysis
Cleavage of a C=C double bond using ozone (O3) followed by a reductive workup (Zn, HOAc). Produces aldehydes, ketones, or both, depending on the substitution of the alkene.
DIBAL-H (Diisobutylaluminium hydride)
A bulky reducing agent that partially reduces esters to aldehydes at low temperature (in toluene), followed by acid workup. In simple terms, it delivers just one hydride equivalent, stopping the reduction halfway.
Tollens reagent (Ag2O, NH4OH)
A very mild oxidant that selectively oxidises aldehydes to carboxylic acids but leaves ketones untouched. This is also the basis of the silver-mirror test to distinguish aldehydes from ketones.
Friedel-Crafts acylation
An electrophilic aromatic substitution that installs an acyl group (RCO-) onto a benzene ring using an acid chloride and AlCl3. This is a key route to aromatic ketones.
Hydrate (gem-diol)
The product formed when water adds across a carbonyl: two OH groups on the same (geminal) carbon. The equilibrium position depends on steric and electronic effects at the carbonyl.
The C=O bond is polarised: carbon carries a partial positive charge (delta+), oxygen carries a partial negative charge (delta-).
This polarisation is what makes the carbonyl carbon electrophilic and susceptible to attack by nucleophiles.
Resonance contributors show one structure with a full C=O double bond and another with a single C–O bond plus positive charge on carbon and negative charge on oxygen.
Aldehydes: R–CHO. The H and R groups attached to the carbonyl are poor leaving groups (H:- and R:-), so aldehydes undergo addition rather than substitution.
Ketones: R–CO–R'. Same logic: both substituents are poor leaving groups.
Carboxylic acid derivatives (acids, acid chlorides, esters, amides) have substituents that can act as leaving groups (HO-, Cl-, R'O-, R'2N-), so they undergo substitution (addition-elimination) instead of simple addition.
Aldehydes (-al): Find the longest chain containing the CHO group. The carbonyl carbon is always C-1. Examples: butanal, 2,4-dimethylpentanal, 5-hydroxy-4-methylhexanal.
Ketones (-one): Find the longest chain containing the C=O group. Number to give the carbonyl the lowest possible locant. Examples: 2-propanone (acetone), 4-chloro-5-hydroxy-3-hexanone.
Common names still appear frequently: formaldehyde (methanal), acetaldehyde (ethanal), acetone (propan-2-one), benzaldehyde (phenyl methanal).
From primary alcohols (oxidation with PCC): PCC in CH2Cl2 oxidises a primary alcohol (RCH2OH) to an aldehyde (RCHO) and stops there. This is the go-to mild oxidation.
From alkenes (ozonolysis): Treatment with O3 in CH2Cl2 followed by Zn/HOAc cleaves the double bond. If the alkene carbon bore a hydrogen, the product at that position is an aldehyde.
From terminal alkynes (hydroboration-oxidation): A terminal alkyne treated with disiamylborane then H2O2/HO- gives an aldehyde (anti-Markovnikov addition). The borane adds to the terminal carbon, and oxidation yields the CHO.
From esters (partial reduction with DIBAL-H): An ester (RCO2R') treated with DIBAL-H in toluene at low temperature, then acid workup, gives an aldehyde. This is partial reduction: only one equivalent of hydride adds.
Caution with LiAlH4: LiAlH4 is too powerful for stopping at the aldehyde. It reduces esters all the way to primary alcohols (two hydride equivalents delivered). An intermediate aldehyde forms but is immediately reduced further.
From secondary alcohols (oxidation): Jones' reagent (CrO3, H2SO4, acetone/water) or PCC in CH2Cl2. Both convert secondary alcohols to ketones. Secondary alcohols cannot be over-oxidised, so either reagent works.
From alkenes (ozonolysis): Same conditions as for aldehydes. If both carbons of the broken double bond bear carbon substituents (no hydrogens on the alkene carbons), you get ketones. A mixed substitution pattern gives one aldehyde and one ketone.
From terminal alkynes (Markovnikov hydration): H2O, H2SO4, HgSO4 adds water across a terminal alkyne following Markovnikov's rule, giving a methyl ketone (RCOCH3) via an enol intermediate that tautomerises.
Friedel-Crafts acylation: An aromatic ring plus an acid chloride (RCOCl) and AlCl3 gives an aryl ketone. The acylium cation is the electrophile.
Organo-copper (cuprate) coupling: An acid chloride (RCOCl) treated with a lithium dialkylcuprate (R'2CuLi) in Et2O, then H2O workup, yields a ketone (RCOR'). The cuprate is selective: it reacts with acid chlorides but not with the ketone product.
Aldehydes are easily oxidised to carboxylic acids by CrO3/H2SO4 (Jones' conditions) or by Tollens reagent (Ag2O, NH4OH).
The mechanism proceeds through the hydrate: the aldehyde first adds water to form a gem-diol, and the chromium oxidant then removes the C–H from that intermediate.
Ketones are generally resistant to oxidation under these mild conditions. Tollens reagent does not oxidise ketones, which is the basis for distinguishing the two functional groups (the silver-mirror test).
Methyl ketones (RCOCH3) can be oxidised under forcing conditions (CrO3, H2SO4) but this is not a standard synthetic tool.
Primary alcohol to aldehyde: PCC / CH2Cl2
Secondary alcohol to ketone: Jones' reagent (CrO3, H2SO4, H2O, acetone) or PCC / CH2Cl2
Alkene to aldehyde/ketone: 1) O3, CH2Cl2; 2) Zn, HOAc
Terminal alkyne to aldehyde: 1) disiamylborane; 2) H2O2, HO-
Terminal alkyne to methyl ketone: H2O, H2SO4, HgSO4
Ester to aldehyde: 1) DIBAL-H, toluene; 2) H3O+
Acid chloride to ketone: 1) R'2CuLi, Et2O; 2) H2O
Aromatic ring to aryl ketone: RCOCl, AlCl3 (Friedel-Crafts acylation)
Aldehyde to carboxylic acid: CrO3/H2SO4 or Tollens reagent (Ag2O, NH4OH)
The Tollens test (silver-mirror reaction) was historically used to coat the insides of glass flasks and mirrors with metallic silver. Modern glucose test strips exploit the same principle: glucose is an aldehyde-containing sugar that reduces silver or copper reagents, producing a colour change.
Ozonolysis is widely used in forensic and structural chemistry to determine the position of double bonds in unknown molecules: you cleave the bond and identify the carbonyl fragments.
Students often think PCC and Jones' reagent are interchangeable for primary alcohols. They are not. PCC stops at the aldehyde; Jones' reagent pushes through to the carboxylic acid.
Students sometimes assume LiAlH4 can be used for partial reduction of esters to aldehydes. It cannot in practice: it is too reactive and reduces the intermediate aldehyde further to the primary alcohol. DIBAL-H is the reagent for partial reduction.
Ozonolysis is sometimes confused with oxidative cleavage using KMnO4. Both break double bonds, but their products differ: ozonolysis gives aldehydes and/or ketones, while KMnO4 under acidic conditions gives carboxylic acids and/or ketones.
Students forget that the Tollens reagent only works on aldehydes. It will not oxidise ketones, and that selectivity is the entire point.
⚠️ Expect questions asking you to choose between PCC and Jones' reagent for a specific transformation. Know which one stops at the aldehyde.
⚠️ Synthesis problems frequently ask you to convert an ester to an aldehyde. The answer is DIBAL-H, not LiAlH4.
⚠️ Ozonolysis product prediction is a classic exam question: given an alkene, draw the carbonyl fragments. Work backwards too, given the fragments, draw the original alkene.
⚠️ The distinction between aldehydes and ketones in terms of reactivity toward oxidation (Tollens test) is commonly tested.
True or false: PCC oxidises primary alcohols to carboxylic acids.
Fill in the blank: The IUPAC suffix for an aldehyde is ____; for a ketone it is ____.
True or false: LiAlH4 can be used to reduce an ester to an aldehyde in one step.
Fill in the blank: Hydration of a terminal alkyne with H2O/H2SO4/HgSO4 gives a ____ ketone.
True or false: Tollens reagent oxidises both aldehydes and ketones.
Answers: 1) False (stops at aldehyde). 2) -al; -one. 3) False (reduces all the way to alcohol; use DIBAL-H). 4) Methyl. 5) False (only aldehydes).
Q: What reagent converts a primary alcohol to an aldehyde without further oxidation?
A: PCC (pyridinium chlorochromate) in CH2Cl2.
Q: An ester is treated with 1) DIBAL-H in toluene, then 2) H3O+. What is the product?
A: An aldehyde. DIBAL-H partially reduces the ester, delivering one hydride equivalent.
Q: Draw the ozonolysis products of 2-methylbut-2-ene (treat with O3/CH2Cl2 then Zn/HOAc).
A: Acetone (propan-2-one) and acetaldehyde (ethanal). The double bond is cleaved, and each fragment becomes a carbonyl compound based on its substitution.
Q: Why does Tollens reagent not oxidise ketones?
A: Ketones lack the C–H bond at the carbonyl that is present in aldehydes. The oxidation proceeds through a hydrate intermediate, and ketone hydrates do not have the requisite hydrogen for the chromium (or silver) oxidant to remove.
Q: What is the product of treating benzene with CH3COCl and AlCl3?
A: Acetophenone (methyl phenyl ketone). This is a Friedel-Crafts acylation.
This material connects directly to alcohol chemistry (Chapter 12), since many aldehyde and ketone syntheses start from alcohols. It also builds on alkene and alkyne addition reactions (Chapters 8–9), particularly hydration and hydroboration. The nucleophilic addition reactions covered in Part 2 of these notes are the logical next step: once you can make aldehydes and ketones, you need to know what reactions they undergo.
carbonyl group, aldehyde, ketone, RCHO, RCOR, C=O, PCC oxidation, Jones reagent, Jones oxidation, ozonolysis, reductive ozonolysis, DIBAL-H, diisobutylaluminium hydride, partial reduction ester to aldehyde, Friedel-Crafts acylation, acid chloride, cuprate coupling, R2CuLi, lithium dialkylcuprate, Tollens reagent, silver mirror test, aldehyde oxidation, gem-diol, hydrate, nomenclature aldehydes ketones, -al suffix, -one suffix, organic chemistry chapter 17