Aldehydes, Ketones and Carboxylic Acids · Lesson 1 of 15
Carbonyl Structure and Nomenclature
“Connect carbonyl bonding, functional-group identity and systematic names with molecular structure.”
• Distinguish aldehydes, ketones and carboxylic acid derivatives by connectivity. • Explain carbonyl geometry, polarity and resonance. • Name carbonyl compounds and construct structures from names. • Locate α-carbons and distinguish a prefix from the principal suffix.
One C=O bond, several chemical families
A carbon–oxygen double bond is a carbonyl group. Its surroundings decide its family: R–C(=O)–H is an aldehyde, R–C(=O)–R′ is a ketone, and R–C(=O)–OH is a carboxylic acid. R and R′ represent carbon groups, which need not be identical. Methanal, H–C(=O)–H, is the aldehyde exception with two carbonyl-bound hydrogens and no carbon group. A formula such as C₃H₆O does not specify connectivity: propanal and propanone share it but differ in their reactions.
| Family | Connectivity at the acyl carbon | Example |
|---|---|---|
| Aldehyde | Hydrogen and carbon, or two hydrogens | CH₃CHO |
| Ketone | Two carbon groups | CH₃COCH₃ |
| Carboxylic acid | –OH attached directly to C=O | CH₃COOH |
| Ester | –OR′ attached directly to C=O | CH₃COOCH₃ |
| Amide | Nitrogen attached directly to C=O | CH₃CONH₂ |
| Acyl halide | Halogen attached directly to C=O | CH₃COCl |
| Anhydride | Two acyl groups joined through oxygen | CH₃CO–O–COCH₃ |
These families occur in familiar materials. Vanillin has aldehyde, methoxy and phenolic groups; salicylaldehyde is 2-hydroxybenzaldehyde; cinnamaldehyde is 3-phenylprop-2-enal. The presence of a fragrance does not define the family: the location of bonds does. Acetone is a solvent because of its molecular interactions and volatility, not simply because its name ends in “-one”.
Why the carbonyl carbon is attacked
The carbonyl carbon uses three sp² orbitals to form three σ bonds in an approximately trigonal planar arrangement, with angles near 120°. Its remaining p orbital overlaps sideways with an oxygen p orbital to form the π bond, above and below the σ-bond plane. Oxygen also has two lone pairs. The geometry concerns the carbonyl centre; it does not require every atom in a large molecule to be coplanar.
Oxygen attracts bonding electrons more strongly than carbon. Carbon therefore carries partial positive charge, δ+, and oxygen partial negative charge, δ−. The charge-separated contributor expresses this uneven distribution. A nucleophile can donate an electron pair to carbon; oxygen can accept a proton through a lone pair. The π bond can open as a new bond forms, so carbon does not acquire five ordinary bonds. Compare an alkene, whose electron-rich π system commonly reacts with electrophiles.
Build systematic names from a numbered skeleton
For an open-chain aldehyde choose the longest appropriate parent chain containing –CHO. The aldehyde carbon is carbon 1 and is part of the count. Replace the alkane ending “-e” by “-al”: CH₃CH₂CHO is propanal. For a ketone choose a parent containing the carbonyl carbon and number to give the principal carbonyl the lowest locant: CH₃COCH₂CH₂CH₃ is pentan-2-one. Multiple ketone groups use “-dione”, as in pentane-2,4-dione. Place substituent names alphabetically with their own locants.
The principal suffix receives numbering priority over a C=C or C≡C bond. Thus CH₃COCH=CHCH₃ is pent-3-en-2-one, while CH₃CH=CHCHO is but-2-enal. If both aldehyde and ketone groups occur, the aldehyde supplies the suffix and the other carbonyl is “oxo”: OHCCH₂CH₂COCH₃ is 4-oxopentanal. Hydroxy, alkoxy and halo prefixes keep their positions explicit. Do not choose a shorter chain merely to make an attractive substituent name.
A ring carbon cannot itself become the terminal –CHO carbon without changing the skeleton. Use “carbaldehyde” when –CHO is attached to a ring: cyclohexanecarbaldehyde. Number the ring attachment carbon as 1. Benzaldehyde is an accepted retained name for C₆H₅CHO; 3-bromobenzaldehyde and 4-nitrobenzaldehyde locate ring substituents. Benzene with two –CHO groups is benzene-1,2-dicarbaldehyde or benzene-1,4-dicarbaldehyde according to attachment. In cyclic ketones the carbonyl carbon belongs to the ring and is carbon 1.
| Structure or description | Systematic name | Common name or convention |
|---|---|---|
| HCHO | Methanal | Formaldehyde |
| CH₃CHO | Ethanal | Acetaldehyde |
| (CH₃)₂CHCHO | 2-Methylpropanal | Isobutyraldehyde |
| CH₃CH(OCH₃)CHO | 2-Methoxypropanal | α-Methoxypropionaldehyde |
| CH₃(CH₂)₃CHO | Pentanal | Valeraldehyde |
| CH₂=CHCHO | Prop-2-enal | Acrolein |
| CH₃COCH₃ | Propanone | Acetone; dimethyl ketone |
| C₆H₅COCH₃ | 1-Phenylethan-1-one | Acetophenone |
| C₆H₅COCH₂CH₃ | 1-Phenylpropan-1-one | Propiophenone |
| C₆H₅COC₆H₅ | Diphenylmethanone | Benzophenone |
| (CH₃)₂CHCOCH(CH₃)₂ | 2,4-Dimethylpentan-3-one | Diisopropyl ketone |
| (CH₃)₂C=CHCOCH₃ | 4-Methylpent-3-en-2-one | Mesityl oxide |
Greek letters describe distance from a functional group
The carbon adjacent to a carbonyl carbon is α, the next β, then γ and δ. The carbonyl carbon itself is not α. Ketones can have α positions on both sides, sometimes distinguished by primes. Thus α-bromobutyraldehyde is CH₃CH₂CHBrCHO, systematically 2-bromobutanal. Common ketone names state the two carbon groups attached to C=O: methyl n-propyl ketone is pentan-2-one. Historical names are useful only when tied to an unambiguous structure.
Problem
Name CH₃CH(CH₃)CH₂CH₂CHO.
- 1.Number from CHO: C1=CHO, C2=CH₂, C3=CH₂, C4=CH, C5=CH₃. The parent has five carbons, so it is pentanal.
- 2.The additional CH₃ is on C4. The name is 4-methylpentanal; reversing the chain would improperly give the aldehyde a high locant.
Problem
Construct 3-hydroxybutanal and 4-oxopentanal.
- 1.For 3-hydroxybutanal build a four-carbon aldehyde chain and place OH on C3: CH₃CH(OH)CH₂CHO. OH is on the β-carbon, not on the aldehyde carbon.
- 2.For 4-oxopentanal build OHC–CH₂–CH₂–CH₂–CH₃ and replace the C4 methylene by C=O: OHC–CH₂–CH₂–CO–CH₃. The molecule has one aldehyde and one ketone.
Problem
Name CH₃CH₂COCH(C₂H₅)CH₂CH₂Cl.
- 1.A six-carbon chain through C=O gives the principal carbonyl locant 3 when numbered from the left. The alternative reverse numbering would give 4.
- 2.The substituents are ethyl at 4 and chloro at 6. Alphabetical order gives 6-chloro-4-ethylhexan-3-one; the carbonyl priority determines the numbering even though chlorine could otherwise receive 1.
| Additional structure | Name | How numbering works |
|---|---|---|
| 3-Methylcyclohexanecarbaldehyde | Ring C1 bears external CHO; methyl is on C3 | CHO carbon is not a ring member |
| 2-Methylcyclohexanone | Carbonyl ring carbon is C1 | Adjacent methyl receives C2 |
| OHCCH₂CHO | Propane-1,3-dial | Both terminal CHO carbons belong to the three-carbon parent |
| OHCCH₂CH₂CH₂CHO | Pentane-1,5-dial | Both aldehyde ends are expressed by dial |
| CH₃COCH₂CH₂CHO | 4-Oxopentanal | Aldehyde suffix takes precedence over ketone |
| 3-Oxopentanal, OHCCH₂COCH₂CH₃ | An aldehyde plus an internal ketone | Number every group from the aldehyde end |
Quick Quiz
Quiz
Which compound is a ketone?
Explanation: The carbonyl carbon in CH₃COCH₂CH₃ has two carbon neighbours.
What is the geometry at a carbonyl carbon?
Explanation: Three σ-bond directions lie approximately in one plane; carbon is sp².
Which atom is the main electrophilic site of CH₃CHO?
Explanation: C=O polarisation places partial positive charge on carbon.
What is the correct name of CH₃COCH=CHCH₃?
Explanation: The ketone suffix takes the lower locant before the double bond.
Which carbon is α in CH₃CH₂CHO?
Explanation: Greek letters begin at the carbon adjacent to the carbonyl carbon.
Practice Problems
- Write structures for α-methoxypropionaldehyde and 4-fluoroacetophenone.
- Name CH₃CO(CH₂)₄CH₃ and CH₃CH₂CHBrCH₂CH(CH₃)CHO.
- Construct di-sec-butyl ketone and 2-hydroxycyclopentanecarbaldehyde.
- Why does C₃H₆O not identify an aldehyde uniquely?
- Name and draw p,p′-dihydroxybenzophenone.
The first is CH₃CH(OCH₃)CHO: propanal has CHO as C1 and methoxy replaces a C2 hydrogen. The second is p-FC₆H₄COCH₃: fluorine is para to the acetyl substituent. It remains a ketone because C=O is bonded to phenyl and methyl.
Key Takeaways
• Connectivity distinguishes the carbonyl families. • A carbonyl carbon is approximately planar and sp² hybridised. • C=O polarity makes carbon electrophilic and oxygen basic. • Include an aldehyde carbon in an open-chain parent count. • Give the principal carbonyl suffix numbering priority. • Use carbaldehyde when CHO is external to a ring. • The α-carbon is adjacent to the carbonyl carbon.
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Next · Lesson 2
Preparing Aldehydes Selectively