--- title: "Mercury(II) Sulfate | Organic Chemistry" description: "Mercury(II) sulfate is a Hg2+ catalyst used in alkene and alkyne hydration, especially to convert alkynes into ketones through acid-catalyzed addition." canonical: "https://fiveable.me/organic-chem/key-terms/mercuryii-sulfate" type: "key-term" subject: "Organic Chemistry" unit: "Unit 9" --- # Mercury(II) Sulfate | Organic Chemistry ## Definition Mercury(II) sulfate is an inorganic mercury salt used in Organic Chemistry as a catalyst for alkyne hydration. It helps add water across a triple bond, usually turning an alkyne into a ketone. ## What It Is Mercury(II) sulfate, HgSO4, is the mercury salt you see in the classic hydration of alkynes reaction in Organic Chemistry. It is not the carbon-based reactant itself, but the Lewis acidic catalyst that makes the triple bond reactive enough for water to add across it. In the usual setup, HgSO4 is used with dilute sulfuric acid and water. The mercury(II) ion coordinates to the alkyne π bond, which weakens the triple bond and makes one carbon easier for water to attack. That coordination step is the whole reason this reaction works smoothly enough to be useful in synthesis. What you get from mercury(II)-catalyzed hydration is not usually an alcohol that stays that way. The first product is an enol, which is a molecule with an alkene and an OH group on the same carbon framework. Enols are usually unstable, so they quickly undergo keto-enol tautomerization to form a carbonyl compound, most often a ketone. For terminal alkynes, this matters because the reaction gives a Markovnikov product. The OH ends up on the more substituted carbon of the original triple bond, and after tautomerization you get a methyl ketone. That outcome is the big difference between mercury-catalyzed hydration and hydroboration-oxidation, which gives the anti-Markovnikov aldehyde from terminal alkynes. A useful way to think about HgSO4 is as the activator that turns a relatively unreactive alkyne into a substrate that can undergo electrophilic addition. The sulfuric acid provides the acidic medium, water is the nucleophile, and mercury(II) is the catalyst that gets the addition started. Without that activation step, the hydration of an alkyne would be much less straightforward. ## Why It Matters Mercury(II) sulfate shows up whenever your Organic Chemistry class compares ways to make carbonyl compounds from alkynes. If you know what HgSO4 does, you can predict whether an alkyne will become a ketone, an aldehyde, or something else after workup. It also helps you read reaction conditions correctly. A line like HgSO4, H2SO4, H2O is not just a random list of reagents. It tells you the reaction is a mercury-catalyzed hydration, so you should be thinking electrophilic addition, enol formation, and tautomerization to a carbonyl product. This term is especially useful for mechanism questions. You may need to show the alkyne π bond coordinating to Hg2+, then water attacking, then the organomercury intermediate being converted into the enol. If you skip that sequence, the product still might be right, but the mechanism would be incomplete. HgSO4 also helps you avoid one of the most common mistakes in alkyne chemistry, which is mixing up hydration conditions with hydroboration-oxidation. Both reactions add water overall, but they do not give the same regiochemistry. Knowing the catalyst lets you choose the right product faster and explain why that product forms. ## Connections ### Hydration Reaction Mercury(II) sulfate is used specifically in the hydration of alkynes, where water adds across a carbon-carbon triple bond. The catalyst makes that addition happen under acidic conditions, and the immediate product is usually an enol that tautomerizes to a ketone. If you see hydration reagents, you should think about where the OH and H end up. ### [Alkyne](/organic-chem/key-terms/alkyne) HgSO4 is used because alkynes have a π system that can be activated by Hg2+. Terminal alkynes are especially common in this reaction because they often give methyl ketones after hydration. The structure of the alkyne determines the product, so you need to look at the triple bond before predicting the result. ### [Hydroboration-Oxidation](/organic-chem/key-terms/hydroboration-oxidation) This is the main comparison reaction for alkyne hydration. Hydroboration-oxidation gives a different regiochemistry from the mercury(II)-catalyzed route, especially for terminal alkynes, where it leads to aldehydes instead of ketones. If a problem asks you to choose between the two, the reagents decide the product. ### [Enol](/organic-chem/key-terms/enol) The first stable-looking product of HgSO4-catalyzed alkyne hydration is an enol, not the final carbonyl compound. Enols usually do not stay around long under these conditions because they tautomerize to a more stable ketone or aldehyde. Recognizing the enol intermediate helps you explain why the product changes after the reaction step. ## On the AP Exam A reaction problem will usually give you HgSO4, H2SO4, and H2O, then ask for the product of an alkyne hydration. Your job is to identify that this is mercury-catalyzed hydration, not hydroboration-oxidation, and then place the carbonyl in the right spot. For a terminal alkyne, that usually means a methyl ketone after tautomerization. On mechanism questions, you may need to show the alkyne being activated by Hg2+, followed by nucleophilic attack by water and then conversion of the enol to the carbonyl compound. In a lab or homework context, you might also explain why the reaction is Markovnikov and why the enol is not the final isolated product. If the question compares two reagent sets, use the catalyst identity to justify the regiochemistry. ## Mercury(II) Sulfate vs Hydroboration-Oxidation These are both alkyne hydration methods, but they do not give the same product. Mercury(II) sulfate in acidic water gives Markovnikov hydration, which usually leads to ketones from terminal alkynes after tautomerization. Hydroboration-oxidation gives anti-Markovnikov addition, which makes aldehydes from terminal alkynes. ## Key Takeaways - Mercury(II) sulfate is the Hg2+ catalyst used in the classic hydration of alkynes in Organic Chemistry. - It works by activating the alkyne triple bond so water can add across it under acidic conditions. - The first product is usually an enol, and that enol quickly tautomerizes to a ketone or, for some substrates, an aldehyde. - For terminal alkynes, HgSO4 hydration usually gives a Markovnikov ketone product. - If you see HgSO4, H2SO4, and H2O together, think mercury-catalyzed alkyne hydration, not hydroboration-oxidation. ## FAQs ### What is Mercury(II) sulfate in Organic Chemistry? Mercury(II) sulfate is an inorganic mercury salt used as a catalyst in alkyne hydration reactions. In acidic water, Hg2+ activates the triple bond so water can add across it. The reaction usually ends with a carbonyl product after enol tautomerization. ### What does HgSO4 do in alkyne hydration? HgSO4 coordinates to the alkyne and makes it more reactive toward water. That catalyst step is what allows the addition to happen under mild conditions. Without it, hydration of an alkyne is much less direct. ### Does Mercury(II) sulfate give an aldehyde or ketone? For terminal alkynes, it usually gives a ketone, often a methyl ketone, after the enol intermediate tautomerizes. Internal alkynes also form carbonyl compounds, but the exact ketone depends on the alkyne structure. This is different from hydroboration-oxidation, which can give aldehydes from terminal alkynes. ### How is Mercury(II) sulfate different from hydroboration-oxidation? They are both used to transform alkynes into carbonyl compounds, but they give different regiochemistry. Mercury(II) sulfate in acidic water gives Markovnikov hydration, while hydroboration-oxidation gives anti-Markovnikov addition. The reagent set tells you which product to draw. ## Related Study Guides - [9.4 Hydration of Alkynes](/organic-chem/unit-9/hydration-alkynes/study-guide/nN3WWfsvmgPFiE8Z) ## About This Document Canonical Fiveable pages are available as Markdown at the same path plus `.md`. - [llms.txt](https://fiveable.me/llms.txt): index of Fiveable's sections and URL patterns - [llms-full.txt](https://fiveable.me/llms-full.txt): complete subject and unit listing - [MCP server](https://fiveable.me/mcp): call Fiveable as tools instead of fetching pages (`https://fiveable.me/api/mcp`) - [MCP server for AP teachers](https://fiveable.me/mcp/teachers): a teacher's classes, assignments and AP-rubric grading (`https://fiveable.me/api/mcp/teacher`) ## Structured Data ```json {"@context":"https://schema.org","@graph":[{"@type":"LearningResource","@id":"https://fiveable.me/organic-chem/key-terms/mercuryii-sulfate#resource","name":"Mercury(II) Sulfate | Organic Chemistry","url":"https://fiveable.me/organic-chem/key-terms/mercuryii-sulfate","learningResourceType":"Concept explainer","educationalLevel":"AP® / High School","about":{"@id":"https://fiveable.me/organic-chem/key-terms/mercuryii-sulfate#term"},"audience":{"@type":"EducationalAudience","educationalRole":"student"},"dateModified":"2026-07-03T02:23:40.731Z","isPartOf":{"@type":"Collection","name":"Organic Chemistry Key Terms","url":"https://fiveable.me/organic-chem/key-terms"},"author":{"@type":"Organization","@id":"https://fiveable.me/editorial#content-team","name":"Fiveable Content Team","url":"https://fiveable.me/editorial","parentOrganization":{"@id":"https://fiveable.me/#organization"},"member":[{"@type":"Person","name":"Amanda DoAmaral","sameAs":["https://www.linkedin.com/in/adoamaral"]},{"@type":"Person","name":"Becky Bahr","sameAs":["https://www.linkedin.com/in/becky-bahr-b52743132"]}]},"publisher":{"@type":"Organization","name":"Fiveable","url":"https://fiveable.me"}},{"@type":"DefinedTerm","@id":"https://fiveable.me/organic-chem/key-terms/mercuryii-sulfate#term","name":"Mercury(II) Sulfate","description":"Mercury(II) sulfate is an inorganic mercury salt used in Organic Chemistry as a catalyst for alkyne hydration. It helps add water across a triple bond, usually turning an alkyne into a ketone.","url":"https://fiveable.me/organic-chem/key-terms/mercuryii-sulfate","inDefinedTermSet":{"@type":"DefinedTermSet","name":"Organic Chemistry Key Terms","url":"https://fiveable.me/organic-chem/key-terms"}},{"@type":"FAQPage","mainEntity":[{"@type":"Question","name":"What is Mercury(II) sulfate in Organic Chemistry?","acceptedAnswer":{"@type":"Answer","text":"Mercury(II) sulfate is an inorganic mercury salt used as a catalyst in alkyne hydration reactions. In acidic water, Hg2+ activates the triple bond so water can add across it. The reaction usually ends with a carbonyl product after enol tautomerization."}},{"@type":"Question","name":"What does HgSO4 do in alkyne hydration?","acceptedAnswer":{"@type":"Answer","text":"HgSO4 coordinates to the alkyne and makes it more reactive toward water. That catalyst step is what allows the addition to happen under mild conditions. Without it, hydration of an alkyne is much less direct."}},{"@type":"Question","name":"Does Mercury(II) sulfate give an aldehyde or ketone?","acceptedAnswer":{"@type":"Answer","text":"For terminal alkynes, it usually gives a ketone, often a methyl ketone, after the enol intermediate tautomerizes. Internal alkynes also form carbonyl compounds, but the exact ketone depends on the alkyne structure. This is different from hydroboration-oxidation, which can give aldehydes from terminal alkynes."}},{"@type":"Question","name":"How is Mercury(II) sulfate different from hydroboration-oxidation?","acceptedAnswer":{"@type":"Answer","text":"They are both used to transform alkynes into carbonyl compounds, but they give different regiochemistry. Mercury(II) sulfate in acidic water gives Markovnikov hydration, while hydroboration-oxidation gives anti-Markovnikov addition. The reagent set tells you which product to draw."}}]},{"@type":"BreadcrumbList","itemListElement":[{"@type":"ListItem","position":1,"name":"Organic Chemistry","item":"https://fiveable.me/organic-chem"},{"@type":"ListItem","position":2,"name":"Key Terms","item":"https://fiveable.me/organic-chem/key-terms"},{"@type":"ListItem","position":3,"name":"Unit 9","item":"https://fiveable.me/organic-chem/unit-9"},{"@type":"ListItem","position":4,"name":"Mercury(II) Sulfate"}]}]} ```