Source: Klein, Organic Chemistry, Chapter 20 (Sections 20.1–20.3)
Tags: aldehydes, ketones, carbonyl group, IUPAC nomenclature, naming aldehydes, naming ketones, formaldehyde, acetone, benzaldehyde, oxidation, ozonolysis, hydroboration-oxidation, Friedel-Crafts acylation, organic chemistry
Difficulty: Intermediate Prerequisites: IUPAC naming conventions for alkanes, alkenes, alkynes, and alcohols (Ch. 4–5). Grignard reagents (Section 13.6). Oxidation of alcohols (Section 13.10). Retrosynthetic analysis (Section 12.3).
Aldehydes and ketones are among the most central functional groups in organic chemistry. Both contain a C=O (carbonyl) group, and their reactions with nucleophiles form the backbone of countless synthetic and biological pathways. If you are coming in cold: you need to be comfortable naming simple organic compounds and drawing Lewis structures before tackling this material. This chapter builds directly on alcohol oxidation and Grignard chemistry from earlier in the course, and the reactions here will reappear in nearly every remaining chapter.
Aldehydes (RCHO) and ketones (R₂CO) both have a carbonyl group. They are named using IUPAC rules with the suffixes "-al" and "-one," respectively. Several preparation methods from earlier chapters (oxidation of alcohols, ozonolysis, hydroboration-oxidation, Friedel-Crafts acylation) feed into this chapter's reaction toolkit.
Carbonyl group
A C=O double bond. The defining feature of both aldehydes and ketones. In an aldehyde, one side of the carbonyl is bonded to a hydrogen; in a ketone, both sides are bonded to carbon atoms.
In simple terms, it is the "C double-bond O" you will see drawn over and over in this chapter and the rest of the course.
Aldehyde (RCHO)
A compound in which the carbonyl carbon is bonded to at least one hydrogen atom (and one R group, or two hydrogens in the case of formaldehyde).
Think of it as the carbonyl sitting at the end of a carbon chain.
Ketone (R₂CO)
A compound in which the carbonyl carbon is flanked by two carbon-containing groups.
Think of it as the carbonyl buried inside the chain rather than at the end.
PCC (pyridinium chlorochromate)
A mild oxidising agent that converts primary alcohols to aldehydes without over-oxidising to carboxylic acids. Used in CH₂Cl₂.
In simple terms, this is the "gentle" oxidiser you reach for when you want to stop at the aldehyde stage.
Aldehydes and ketones both possess a C=O bond, called the carbonyl group
The carbonyl group of an aldehyde is flanked by one carbon and one hydrogen; a ketone's carbonyl is flanked by two carbons
Aldehydes and ketones are responsible for many familiar flavours and odours:
Vanillin (vanilla), cinnamaldehyde (cinnamon), (R)-carvone (spearmint), benzaldehyde (almond)
Biologically important carbonyl compounds include progesterone and testosterone
Industrially, formaldehyde (preservative) and acetone (solvent, nail polish remover) are ubiquitous
The carbonyl group reacts with a wide variety of nucleophiles, which is what makes aldehydes and ketones so synthetically versatile
The same four-step IUPAC naming procedure applies: (1) identify the parent, (2) name substituents, (3) assign locants, (4) assemble alphabetically
The suffix "-al" indicates an aldehyde group
Butane becomes butanal
The parent chain must include the aldehydic carbon
The aldehydic carbon is always assigned position 1, so no locant is needed in the name
For cyclic compounds with an aldehyde directly on the ring, use the suffix "-carbaldehyde" (e.g. cyclohexanecarbaldehyde)
Chirality centres are indicated at the beginning of the name, e.g. (R)-2-chloro-3-phenylpropanal
Common names recognised by IUPAC: formaldehyde, acetaldehyde, benzaldehyde
The suffix "-one" indicates a ketone group
Butane becomes butanone
A locant is required for the ketone group's position; it can appear before the parent name or before the suffix:
3-heptanone or heptan-3-one (both acceptable)
Common names recognised by IUPAC: acetone, acetophenone, benzophenone
Simple ketones may also be named as "alkyl alkyl ketone" (e.g. ethyl propyl ketone for 3-hexanone), though this is rarely used
Compounds with two carbonyl groups are named as alkane diones, e.g. 2,3-butanedione (the artificial butter flavour in microwave popcorn, also a contributor to body odour)
To name a branched ketone: find the longest chain containing the carbonyl, number to give the carbonyl the lowest locant, identify and number substituents, assemble alphabetically, and assign stereochemistry. For instance, (R)-6-ethyl-4,4-dimethyl-3-nonanone.
Oxidation of primary alcohols (Section 13.10): Use PCC in CH₂Cl₂. A strong oxidiser would push past the aldehyde to a carboxylic acid, so a mild agent like PCC is necessary
Ozonolysis of alkenes (Section 9.19): O₃ followed by DMS cleaves a C=C double bond. If either carbon of the former double bond bears a hydrogen, an aldehyde is formed
Hydroboration-oxidation of terminal alkynes (Section 10.8): R₂BH then H₂O₂/NaOH gives anti-Markovnikov addition of water across the triple bond, followed by tautomerisation of the resulting enol to form an aldehyde
Oxidation of secondary alcohols (Section 13.10): Strong or mild oxidising agents (e.g. Na₂Cr₂O₇ / H₂SO₄) work because the ketone product does not undergo further oxidation
Ozonolysis of alkenes (Section 9.19): Tetrasubstituted alkenes are cleaved to form ketones
Acid-catalysed hydration of terminal alkynes (Section 10.8): H₂SO₄/H₂O with HgSO₄ gives Markovnikov addition of water, followed by tautomerisation to a methyl ketone
Friedel-Crafts acylation (Section 19.6): An acid halide reacts with an aromatic ring (not too strongly deactivated) in the presence of AlCl₃ to produce an aryl ketone
General aldehyde structure: RCHO (R-C(=O)-H)
General ketone structure: R₂CO (R-C(=O)-R)
Key preparation summary:
Primary alcohol + PCC → Aldehyde
Secondary alcohol + Na₂Cr₂O₇ → Ketone
Alkene + O₃, then DMS → Aldehyde and/or ketone (depending on substitution)
Terminal alkyne + R₂BH, then H₂O₂/NaOH → Aldehyde
Terminal alkyne + H₂SO₄/HgSO₄/H₂O → Methyl ketone
Aromatic ring + RCOCl/AlCl₃ → Aryl ketone
Vanillin, cinnamaldehyde, and benzaldehyde are responsible for the flavours of vanilla, cinnamon, and almonds, respectively. Acetone is one of the most widely used industrial solvents. Formaldehyde is used as a preservative and disinfectant. The carbonyl group's versatility is exploited extensively in pharmaceutical synthesis.
Students often think the aldehydic carbon needs a locant in the name. It does not: the "-al" suffix already implies position 1.
Students sometimes choose the longest overall carbon chain as the parent, forgetting that the parent chain must include the carbonyl carbon. Always build the parent through the C=O.
Students may confuse PCC (mild, stops at aldehyde) with strong oxidisers like Na₂Cr₂O₇ (which push primary alcohols all the way to carboxylic acids). The choice of oxidising agent determines whether you get an aldehyde or an acid.
When naming ketones, students occasionally forget that the carbonyl should receive the lowest possible locant, not the substituents.
⚠️ Nomenclature of aldehydes and ketones is a staple of exam questions. Be comfortable with the four-step naming procedure and the "-al" / "-one" suffixes.
⚠️ Know the distinction between mild oxidation (PCC, stops at aldehyde) and strong oxidation (goes to carboxylic acid). This is a common exam trap.
⚠️ Be able to identify which preparation method produces an aldehyde versus a ketone, particularly ozonolysis (substitution pattern of the alkene determines the product).
⚠️ Common names (formaldehyde, acetaldehyde, benzaldehyde, acetone, acetophenone, benzophenone) appear frequently on exams; memorise them.
True or False: The suffix "-al" is used for ketones. ___
Fill in the blank: The parent chain of an aldehyde must include the ________ carbon.
True or False: PCC will oxidise a primary alcohol to a carboxylic acid. ___
Fill in the blank: Ozonolysis of a tetrasubstituted alkene produces two ________.
True or False: The IUPAC name "pentan-1-al" is preferred over "pentanal." ___
Answers: 1. False ("-al" is for aldehydes; "-one" is for ketones). 2. Aldehydic (carbonyl). 3. False (PCC stops at the aldehyde). 4. Ketones. 5. False (no locant is needed because the aldehyde is always at C1; "pentanal" is correct).
Q: Provide the systematic (IUPAC) name for a nine-carbon ketone with the carbonyl at position 3, a 6-ethyl substituent, 4,4-dimethyl substituents, and an (R) chirality centre.
A: (R)-6-ethyl-4,4-dimethyl-3-nonanone.
Q: What reagent would you use to convert a primary alcohol into an aldehyde without further oxidation?
A: PCC (pyridinium chlorochromate) in CH₂Cl₂.
Q: Starting from a terminal alkyne, describe two methods for producing different carbonyl products.
A: Hydroboration-oxidation (R₂BH then H₂O₂/NaOH) gives an aldehyde (anti-Markovnikov). Acid-catalysed hydration (H₂SO₄/HgSO₄/H₂O) gives a methyl ketone (Markovnikov).
Q: What is the IUPAC-recognised common name for the simplest aldehyde (HCHO)?
A: Formaldehyde.
Q: A cyclic compound has an aldehyde group directly attached to a cyclohexane ring. What suffix is used?
A: "-carbaldehyde" (the compound is named cyclohexanecarbaldehyde).
This material connects directly to alcohol chemistry (Ch. 13), since many aldehyde and ketone preparations start from alcohols. The ozonolysis and hydroboration reactions link back to alkene and alkyne chemistry (Chs. 9–10). The reactivity of the carbonyl group introduced here is the foundation for carboxylic acid derivative chemistry in the next chapter (Ch. 21) and for enolate chemistry (Ch. 22).
Related Terms / Search Tags: carbonyl, C=O bond, aldehyde naming, ketone naming, IUPAC nomenclature organic, "-al" suffix, "-one" suffix, formaldehyde, acetaldehyde, benzaldehyde, acetone, acetophenone, benzophenone, PCC oxidation, ozonolysis products, hydroboration-oxidation aldehyde, Friedel-Crafts acylation ketone, methyl ketone, aryl ketone, carbonyl preparation methods