FAD1018 W8-W10 — Carbonyl Compounds
Weeks 8–10 lectures covering aldehydes and ketones. Lecturer: Mrs Azlina Puang. Source files: W8-01.png through W8-19.png (Lecture 1), plus W9.pdf, W10 (1).pdf, W10 (2).pdf, W10 (3).pdf, W10 (4).pdf.
Summary
Comprehensive study of carbonyl compounds (aldehydes and ketones) including structure, nomenclature, preparation, reactions, and identification tests.
Key Concepts
- Carbonyl Compounds — Compounds with C=O functional group
- Aldehydes — Carbonyl at carbon chain end (-CHO)
- Ketones — Carbonyl within carbon chain (>C=O)
- Nucleophilic Addition — Characteristic carbonyl reaction
- Oxidation/Reduction — Redox reactions of carbonyls
- Addition-Elimination — Reactions with nitrogen nucleophiles
W8 — Lecture 1: Introduction, Nomenclature, Preparation & Physical Properties
1.1 Introduction
The simplest carbonyl compounds are aldehydes (RCHO) and ketones (RCOR). They contain the carbonyl group as the functional group and have a general formula of:
$$C_nH_{2n}O$$
In the simplest aldehyde (formaldehyde), the carbonyl carbon is bonded to two hydrogens; in other aldehydes it is bonded to one hydrogen and one alkyl/aryl group (R). In ketones, the carbonyl carbon is bonded to two alkyl/aryl groups.
Both are similar in structure and properties, but differ in reactivity in oxidation and nucleophilic addition reactions. Aldehydes are more reactive than ketones.
Key structures:
C=O
CC=O
CC(=O)C
c1ccccc1C=O
C1CCCCC1=O
CC(=O)c1ccccc1
1.2 Nomenclature
Aldehydes
- The longest chain containing the functional group is determined.
- The chain is numbered beginning from the functional group (carbon of carbonyl always has the 1-position).
- Parent name is terminated with suffix -al.
- The location of substituents is denoted with the corresponding number.
- Cyclic / aromatic aldehydes: If attached to a ring, add ‘carbaldehyde’ to the cyclic compound name. For aromatic aldehydes, benzaldehyde is more commonly used than benzenecarbaldehyde.
[!note] Functional group priority If a compound has two functional groups, the one with lower priority is indicated by its prefix (as substituent). Therefore, the ketone oxygen in a compound that also contains an aldehyde is indicated by the prefix ‘oxo’ (e.g., 4-oxohexanal).
Aldehyde examples:
| Systematic name | Common name | SMILES |
|---|---|---|
| methanal | formaldehyde | C=O |
| ethanal | acetaldehyde | CC=O |
| 2-bromopropanal | α-bromopropionaldehyde | CC(Br)C=O |
| 3-chlorobutanal | β-chlorobutyraldehyde | CC(Cl)CC=O |
| 3-methylbutanal | isovaleraldehyde | CC(C)CC=O |
| hexanedial | — | O=CCCCCC=O |
| trans-2-methylcyclohexanecarbaldehyde | — | CC1CCCCC1C=O |
| benzenecarbaldehyde | benzaldehyde | c1ccccc1C=O |
Ketones
- The longest chain containing the functional group is determined.
- The chain is numbered beginning from the terminal closer to the functional group.
- Parent name is terminated with suffix -one.
- The location of substituents is denoted with the corresponding number.
- In cyclic ketones, the carbonyl is assumed to be at the 1-position.
[!note] Minimum position For acyclic ketones, the carbonyl should be given the lowest possible locant (minimum at the 2nd carbon if possible).
Ketone examples:
| Systematic name | Common / derived name | SMILES |
|---|---|---|
| propanone | acetone / dimethyl ketone | CC(=O)C |
| 3-hexanone | ethyl propyl ketone | CCC(=O)CCC |
| 6-methyl-2-heptanone | isohexyl methyl ketone | CC(=O)CCCC(C)C |
| cyclohexanone | — | O=C1CCCCC1 |
| butanedione | — | CC(=O)C(=O)C |
| 2,4-pentanedione | acetylacetone | CC(=O)CC(=O)C |
| 4-hexen-2-one | — | CC(=O)CC=CC |
Phenyl-substituted Ketones
Common names are also used: the number of carbons (other than those of the phenyl group) is indicated by the common name of the corresponding carboxylic acid, substituting “-ophenone” for “-ic acid”.
| Systematic name | Common name | SMILES |
|---|---|---|
| phenylethanone | acetophenone / methyl phenyl ketone | CC(=O)c1ccccc1 |
| 1-phenyl-1-butanone | butyrophenone / phenyl propyl ketone | CCCC(=O)c1ccccc1 |
| 1-phenyl-2-butanone | — | CC(=O)Cc1ccccc1 |
1.3 Synthesis / Preparation
Aldehydes & Aliphatic Ketones from Alcohols
Primary alcohol → Aldehyde
Use a mild oxidizing agent; PCC (Pyridinium chlorochromate) in CH₂Cl₂ is the only reagent that stops at the aldehyde.
Strong oxidizing agents (K₂Cr₂O₇ / H₂SO₄, CrO₃ / H₂SO₄, KMnO₄ / H₂SO₄) will over-oxidize primary alcohols to carboxylic acids.
Secondary alcohol → Ketone
Any of the above oxidizing agents (including PCC) can be used. Secondary alcohols oxidize to ketones without over-oxidation.
[!example] Example: Oxidation of secondary alcohol 3-tert-butylcyclohexanol → 3-tert-butylcyclohexanone (
CC(C)(C)C1CCCC(=O)C1) Reagent:Na₂Cr₂O₇ / H⁺
Ozonolysis of Alkenes
Cleavage of the C=C bond in an alkene and insertion of oxygen atoms to obtain carbonyl products.
$$R'CH=CR_2 \xrightarrow[\text{H}_2\text{O / Zn}]{\text{O}_3} R'CHO + R_2C=O$$
[!example] Example
CH₃CH=C(CH₃)₂(5 C) →CH₃CHO(2 C) +(CH₃)₂C=O(3 C)
A good method for preparing aromatic ketones:
Benzene reacts with an acyl chloride in the presence of AlCl₃ (Lewis acid catalyst) to give an aromatic ketone, HCl, and regenerate AlCl₃.
Reference comparison (comparable MW):
b) Addition of Grignard Reagents
- Formation of alcohols
- Primary, secondary, tertiary alcohol formation
c) Addition of Alcohols
- Acetal and ketal formation
- Protecting groups
- Cyclic acetals
d) Addition of Bisulfite
- Bisulfite addition compounds
- Purification applications
5. Addition-Elimination Reactions
With Nitrogen Nucleophiles:
- Oxime formation (hydroxylamine)
- Hydrazone formation (hydrazine)
- Phenylhydrazone formation
- 2,4-DNP test (orange/red precipitate)
- Imine/Schiff base formation (primary amines)
6. Oxidation and Reduction
Oxidation:
- Aldehydes → Carboxylic acids
- Ketones — resistant (except under vigorous conditions)
- Tollens' test (silver mirror)
- Fehling's/Benedict's test (brick-red precipitate)
Reduction:
- Aldehydes → Primary alcohols
- Ketones → Secondary alcohols
- Clemmensen reduction
- Wolff-Kishner reduction
7. Aldol Condensation
- Self-condensation of aldehydes/ketones
- Crossed aldol condensation
- Dehydration to α,β-unsaturated carbonyls
CC=O.CC=O.[OH-]>>CC(O)CC=O
CC(O)CC=O>>CC=CC=O.O
c1ccccc1C=O.CC=O.[OH-]>>c1ccccc1C=CC=O.O
8. Cannizzaro Reaction
- Disproportionation of non-enolizable aldehydes
- Base-catalyzed redox reaction
c1ccccc1C=O.c1ccccc1C=O.[OH-]>>c1ccccc1C(=O)[O-].c1ccccc1CO
9. Haloform Reaction
- Iodoform test for methyl ketones
- Yellow precipitate of CHI₃
CC(=O)C.I[I-]I.[OH-]>>CC(=O)[O-].[I-].CI(I)I
10. Identification Tests Summary
| Test | Aldehydes | Ketones |
|---|---|---|
| Tollens' | Silver mirror | Negative |
| Fehling's | Brick-red Cu₂O | Negative |
| 2,4-DNP | Orange/red ppt | Orange/red ppt |
| Iodoform | Positive if CH₃CHO | Positive if methyl ketone |
| Schiff's | Pink color | Negative |
Related Topics
- Alcohol & Phenol — Precursors to carbonyls
- Carboxylic Acids & Derivatives — Oxidation products
- Amines & Amino Acids — React with carbonyls
Study Notes
[!important] High-frequency organic topic Aldehydes & Ketones appear consistently in finals with ~7% average mark weight. Know your identification tests!