Hydration of Alkynes
Last Update: 8 February 2026
This is an extension of the HSC Chemistry course under the topic of Products of Reactions Involving Hydrocarbons.
HSC Chemistry Syllabus
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investigate, write equations and construct models to represent the reactions of unsaturated hydrocarbons when added to a range of chemicals, including but not limited to:
Hydration of Alkynes (Video)
Hydration involves the addition of water across unsaturated bonds like double and triple bonds. The hydration of alkenes leads to the formation of alcohols. The hydration of alkynes leads to the formation of aldehydes or ketones.
Introduction
Alkynes contain a characteristic triple bond, making them strong nucleophiles that readily engage in addition reactions. Their high electron density allows them to seek out positively charged nuclei.
Structural formula of propyne
Reaction between alkynes and water forms either a ketone or aldehyde depending on the exact structure of the alkyne.
Ketones tend to be the major product of alkyne hydration while aldehydes are usually the minor ones. This can be predicted using Markovnikov's Rule, which states that the major products form when the electrophile adds to the more substituted carbon.
Hence, ketones are generally the major products as their formation involves addition of water (electrophile) to the more substituted carbon (later becomes the secondary carbonyl carbon). In contrast, aldehydes are minor products because their formation involves addition of water to the less substituted terminal carbon. The terminal carbon later becomes the carbonyl carbon of the aldehyde functional group.
The exact mechanism of alkyne hydration is detailed below. This is not explicitly outlined in the HSC Chemistry syllabus.
Ketone Formation Mechanism (EXTENSION)
1. Nucleophilic Attack:
The alkyne's electron-rich triple bond initiates a nucleophilic attack on a hydrogen ion, commonly produced from the self-ionisation of water or an acid catalyst. This forms an electrophilic carbocation at the 2-position
2. Addition of Water
Water acts as a nucleophile, attacking the carbonation, leading to the formation of a positively charged oxonium ion.
3. Formation of an Enol Intermediate
The conjugate base, originating from the deprotonation of the catalyst, deprotonates the oxonium ion to form an enrol intermediate.
4. Tautomerisation
The enrol undergoes tautomerisation to become a more stable carbonyl compound, due to the greater electronegativity of the oxygen atom.
5. Ketone Formation
The tautomerisation step causes the oxonium ion to deprotonate and finally form a ketone.
Aldehyde Formation Mechanism (EXTENSION)
1. Nucleophilic attack of `H^+`
The first step is identical to ketone formation, except that the carbocation is at the 1-position. The subsequent addition of water, oxonium ion formation, and enol intermediate formation steps are also the same.
2. Addition of water
3. Formation of enrol intermediate
4. Tautomerisation
5. Aldehyde formation
