Aldehydes
Aldehydes undergo nucleophilic addition with cyanide to form hydroxynitriles, extending the organic route map beyond halogenoalkanes.
- Functional group
- CHO
- Also known as
- alkanals, aldehyde, carbonyl compounds
Overview
Carbonyl compounds with a terminal C=O group.
Aldehydes undergo nucleophilic addition with cyanide to form hydroxynitriles, extending the organic route map beyond halogenoalkanes.
An aldehyde has a carbonyl carbon bonded to at least one hydrogen. It can be reduced to a primary alcohol or oxidised to a carboxylic acid. Nucleophilic addition changes the planar carbonyl centre into a tetrahedral centre; a new stereocentre may give a racemic product when neither reagents nor surroundings are chiral.
Identifiers
- Functional group
- CHO
- Also known as
- alkanals, aldehyde, carbonyl compounds
Chemical identifiers
- SMILES
- CC=O
Properties
- Reaction choice
- Reduction retains carbon count; cyanide addition extends it by one carbon.
- Test interpretation
- 2,4-DNPH identifies an aldehyde/ketone candidate; a separate oxidation test helps distinguish them.
Diagnostic tests
2,4-DNP carbonyl test
- Reagent
- 2,4-DNP
- Conditions
- room temperature
- Positive observation
- orange precipitate
- Negative observation
- no orange precipitate
- Interpretation
- A precipitate supports an aldehyde or ketone functional group; a C=O bond in an ester, acid or amide does not by itself predict this result.
- Scope
- Aldehydes and ketones form 2,4-dinitrophenylhydrazones.
- Limitations
- Aldehydes and ketones give positive responses; compare derivative melting points or spectra.
- Evidence
- textbook core
Tollens' reagent aldehyde test
- Reagent
- Tollens' reagent
- Conditions
- warm gently
- Positive observation
- silver mirror
- Negative observation
- ketones usually give no silver mirror under these conditions
- Interpretation
- Aldehydes reduce Tollens' reagent and are oxidised to carboxylate products.
- Scope
- Used to distinguish aldehydes from ketones in the aliphatic carbonyl set.
- Limitations
- Reducing sugars and other reducing substances can also give positive responses; compare carbonyl and structural evidence.
- Evidence
- textbook core
Fehling's solution aldehyde test
- Reagent
- Fehling's solution
- Conditions
- warm
- Positive observation
- brick-red precipitate
- Negative observation
- ketones usually give no brick-red precipitate under these conditions
- Interpretation
- Aliphatic aldehydes reduce Fehling's solution and are oxidised to carboxylate products.
- Scope
- Typical qualitative test for aliphatic aldehydes.
- Limitations
- Aromatic aldehydes commonly give negative responses and reducing sugars can be positive; compare Tollens and spectra.
- Evidence
- textbook core
Related compounds
- Methanal
Methanal is the simplest molecule-level member of the aldehyde family.
- Ethanal
Ethanal is a two-carbon molecule-level member of the aldehyde family.
- Propanal
A three-carbon aldehyde linking oxidation and carbonyl addition.
- Butanal
A four-carbon aldehyde with a terminal electrophilic carbonyl group.
- Pentanal
Pentanal is an organic compound with the molecular formula C5H10O.
- Hexanal
An unbranched six-carbon aldehyde with a single terminal carbonyl.
- Benzaldehyde
An aromatic aldehyde that can oxidise to benzoic acid.
- Vanillin
Vanillin is an organic compound with the molecular formula C8H8O3.
- Cinnamaldehyde
Cinnamaldehyde is an organic compound with the molecular formula C9H8O.
- Glyoxal
Glyoxal is an organic compound with the molecular formula C2H2O2.
- 2-Methylpropanal
A branched four-carbon aldehyde and a constitutional isomer of butanal.
Connected reactions
- Cyanohydrin formation: Aldehydes → Hydroxynitriles
Cyanide addition to an aldehyde forms a hydroxynitrile, giving a carbonyl-route analogue of the nitrile chain-extension motif.
- Oxidation: Alcohols → Aldehydes
Controlled oxidation of a primary alcohol gives an aldehyde.
- Controlled oxidation: Primary alcohols → Aldehydes
Controlled oxidation of a primary alcohol gives an aldehyde.
- Reduction: Aldehydes → Primary alcohols
Reduction of an aldehyde gives a primary alcohol.
- Oxidation: Aldehydes → Carboxylic acids
Oxidation of an aldehyde gives a carboxylic acid.
- Grignard addition co reactant: Aldehydes → Secondary alcohols
Aldehydes plus Grignard reagents give secondary alcohols after acidic work-up.
References
- Organic Chemistry: Nucleophilic Addition Reactions of Aldehydes and KetonesJohn McMurry · OpenStax Organic Chemistry · 2023
Supports functional-group chemistry used to curate the linked examples. Checked 2026-09-16; named examples are applications of textbook scope, without claimed experimental yields.
- Pearson Edexcel International Advanced Level Chemistry Practical GuidePearson Education Limited · Pearson qualifications · 2018
Official Pearson practical guidance for organic qualitative tests, including bromine water, acidified dichromate, silver nitrate, carbonyl reagents and the iodoform reaction.
- Pearson Edexcel Level 3 Advanced GCE in Chemistry specificationPearson Education Limited · Pearson qualifications · 2024
Supports the textbook-core organic reaction routes used for displayed route and mechanism content.
- PubChem Compound Summary for FormaldehydeNational Center for Biotechnology Information · PubChem · 2026
Used for stable identifiers and basic compound metadata.
- PubChem Compound Summary for AcetaldehydeNational Center for Biotechnology Information · PubChem · 2026
Used for stable identifiers and basic compound metadata.
- PubChem Compound Summary: Propanal (CID 527)National Center for Biotechnology Information · PubChem
Identity, molecular formula, molecular weight and aliases retrieved via PUG REST on 2026-09-08.
- PubChem Compound Summary: Butanal (CID 261)National Center for Biotechnology Information · PubChem
Identity, molecular formula, molecular weight and aliases retrieved via PUG REST on 2026-09-08.
- PubChem Compound Summary: Pentanal (CID 8063)National Center for Biotechnology Information · PubChem
Identity, molecular formula, molecular weight and aliases retrieved via PUG REST on 2026-09-08.
- PubChem Compound Summary: Hexanal (CID 6184)National Center for Biotechnology Information · PubChem
Identity, molecular formula, molecular weight and aliases retrieved via PUG REST on 2026-09-08.
- PubChem Compound Summary: Benzaldehyde (CID 240)National Center for Biotechnology Information · PubChem
Identity, molecular formula, molecular weight and aliases retrieved via PUG REST on 2026-09-08.
- PubChem Compound Summary: Vanillin (CID 1183)National Center for Biotechnology Information · PubChem
Identity, molecular formula, molecular weight and aliases retrieved via PUG REST on 2026-09-08.
- PubChem Compound Summary: Cinnamaldehyde (CID 637511)National Center for Biotechnology Information · PubChem
Identity, molecular formula, molecular weight and aliases retrieved via PUG REST on 2026-09-08.
- PubChem Compound Summary: Glyoxal (CID 7860)National Center for Biotechnology Information · PubChem
Identity, molecular formula, molecular weight and aliases retrieved via PUG REST on 2026-09-08.
- PubChem Compound Summary: 2-Methylpropanal (CID 6561)National Center for Biotechnology Information · PubChem
Identity, molecular formula, molecular weight and selected English aliases retrieved via PUG REST on 2026-09-16. Chinese search names are curated nomenclature translations. Unspecified stereochemistry remains unspecified.