Hydration of Alkynes
This is an extension of the HSC Chemistry course under the topic of Products of Reactions Involving Hydrocarbons
HSC Chemistry Syllabus
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: (extension)
Hydration of Alkynes
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.
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.
Alkyne Hydration Mechanism
Ketone Formation Steps:
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.
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 Steps:
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
5. Aldehyde formation
Relevance of Products
Ketones tend to be the major product of alkyne hydration while aldehydes are usually the minor ones. This distribution 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 they involve secondary carbonyl carbons, compared to the terminal carbonyl carbons in aldehydes.