{"id":7272,"date":"2013-05-14T08:35:41","date_gmt":"2013-05-14T12:35:41","guid":{"rendered":"https:\/\/www.masterorganicchemistry.com\/?p=7272"},"modified":"2026-05-01T14:27:58","modified_gmt":"2026-05-01T19:27:58","slug":"hydration-and-oxymercuration-of-alkynes","status":"publish","type":"post","link":"https:\/\/www.masterorganicchemistry.com\/2013\/05\/14\/hydration-and-oxymercuration-of-alkynes\/","title":{"rendered":"Hydration and Oxymercuration of Alkynes"},"content":{"rendered":"

Hydration and Oxymercuration of Alkynes Via Keto-Enol Tautomerism<\/strong><\/p>\n

Alkyne<\/em> chemistry bears many resemblances to alkene<\/em> chemistry, but in these first few posts on the subject, the purpose is to illustrate how one seemingly minor change – an extra \u03c0\u00a0bond – can lead to significant<\/strong> differences in chemical behavior.<\/p>\n

Previously, we saw that the sp-<\/em>\u00a0hybridization of alkynes leads to increased acidity, and the second \u03c0\u00a0bond of alkynes leads to the possibility for partial reduction to either cis or trans alkenes. In this post we’ll see again how the addition of that extra \u03c0\u00a0bond has a very important and surprising consequence.<\/p>\n

\"summary-hydration<\/a><\/p>\n

Table of Contents<\/strong><\/p>\n

    \n
  1. Hydration of Alkenes With Aqueous Acid Gives Alcohols<\/a><\/li>\n
  2. Hydration of Alkynes With Aqueous Acid Gives… Ketones??… What?!<\/a><\/li>\n
  3. The First Step In The Hydration of Alkynes Is Formation Of An “Enol”<\/a><\/li>\n
  4. The “Enol” Is Converted To A Ketone Through A Process Called “Tautomerization”<\/a><\/li>\n
  5. Alkynes Can Also Be “Hydrated” via Oxymercuration (HgSO4\/H2O)<\/a><\/li>\n
  6. Hydroboration Of Alkynes (R2BH) Occurs With Anti-Markovnikov Selectivity, Giving Aldehydes From Terminal Alkynes<\/a><\/li>\n
  7. Beware: Depending On The Alkyne, Mixtures Of Products Can Be Obtained<\/a><\/li>\n
  8. Notes<\/a><\/li>\n
  9. Quiz Yourself!<\/a><\/li>\n
  10. (Advanced) References and Further Reading<\/a><\/li>\n<\/ol>\n
    \n

    <\/a>1. Hydration of Alkenes Gives Alcohols<\/h2>\n

    Several posts ago we talked about the hydration of alkenes<\/strong>. This can be done either with aqueous acid, or with mercury and water (“oxymercuration” – see article – Oxymercuration of Alkenes<\/em><\/span><\/a>)<\/span>. Looking at the reaction with alkenes, the pattern is fairly straightforward: break a C-C \u03c0 bond, and form a C-H and C-OH bond. Also recall that the oxygen ends up on the most substituted carbon [“Markovnikov” selectivity].<\/em><\/span><\/p>\n

    \"hydration<\/p>\n

    2. Hydration of Alkynes With Aqueous Acid Gives… Ketones??… What?!<\/h2>\n

    So what happens when we try this reaction on alkynes? We might expect to observe the same pattern, right? After all, it’s just a simple addition reaction.<\/p>\n

    Well… here’s what we actually observe. We get… a ketone<\/strong> !?<\/p>\n

    \"hydration<\/p>\n

    Now what’s going on here? This seems like the type of thing that drives new organic chemistry students around the bend. Just when you think you understand your surroundings, you pick up the most innocuous looking rock, and underneath it find a poisonous snake!<\/p>\n

    Don’t panic! It’s a new concept in organic chemistry we’ll be exploring here called tautomerism – one that gets much more discussion in Org 2, but it’s not as weird as you initially might think. (See post: Keto-Enol Tautomerism<\/a><\/em>)<\/p>\n

    Look at the bonds formed and broken. The first set we should understand. Form C-O and form C-H, break C-C \u03c0.\u00a0\u00a0It’s that\u00a0next<\/em> set of bonds formed\/broken that are a big surprise.<\/p>\n

    3. The First Step In The Hydration of Alkynes Is Formation Of An “Enol”<\/h2>\n

    If you monitor this reaction closely – one way to do it is in an NMR tube – it’s actually possible to observe the first product of this reaction, which is the one shown below. We call this an “enol”, by the way – kind of like a spork<\/a> (half spoon half fork) it is part alkene, part alcohol. (See post: Reactions of Enols<\/a>)\u00a0<\/em><\/p>\n

    \"first<\/p>\n

    4. The “Enol” Is Converted To A Ketone Through A Process Called “Tautomerization”<\/h2>\n

    Over time, this enol spontaneously converts into the ketone. Note that the two have the same molecular formula – they are\u00a0constitutional isomers.\u00a0<\/em> And they are in equilibrium with each other.<\/p>\n

    We call these constitutional isomers which interconvert, “tautomers”. This equilibrium generally favors formation of the ketone due to the strong C-O\u00a0\u03c0 bond (compared to C-C\u00a0\u03c0).<\/p>\n

    \"step<\/p>\n

    Here’s how the whole process works – arrow by arrow.<\/p>\n

    \"mechanism<\/p>\n

     <\/p>\n

    5. Alkynes Can Also Be “Hydrated” via Oxymercuration<\/h2>\n

    Wait – we’re not done! There’s another way to “hydrate” alkynes, just like there was with alkenes. We can also perform the same reaction with mercury, water and strong acid [sulfuric acid, H2<\/sub>SO4<\/sub> is the usual acid of choice]<\/em><\/span>. For interesting reasons we wont get into at the moment, sodium borohydride (NaBH4<\/sub>)is not generally used for removal of the mercury with alkynes; it is sufficient to merely have water and acid present.<\/p>\n

    \"oxymercuration<\/p>\n

    6. Hydroboration Of Alkynes Occurs With “Anti-Markovnikov Selectivity”, Giving Aldehydes From Terminal Alkynes<\/h2>\n

    There’s also hydroboration. Remember how hydroboration-oxidation of alkenes with BH3<\/sub> and H2<\/sub>O2<\/sub> gives us “anti-Markovnikov” hydration of alkenes? (See post: Hydroboration of Alkenes<\/a><\/em>)<\/p>\n

    Likewise, we can use the same reaction to perform “anti-Markovnikov” hydroboration of alkynes<\/strong>.<\/p>\n

    Just as in the cases above, we initially obtain an enol. However, under the reaction conditions, keto-enol tautomerism results in formation of the aldehyde. (For more, see article: <\/em><\/span>Hydroboration of Alkynes With R2BH<\/em><\/span><\/a>)<\/p>\n

    \"hydroboration<\/p>\n

    Bottom line here: if we start with a “terminal” alkyne, that is an alkyne where one of the carbons is attached directly to H – then we will obtain\u00a0ketones<\/strong> with H3<\/sub>O+\/H2<\/sub>SO4<\/sub> or via oxymercuration, and\u00a0aldehydes<\/strong>\u00a0via hydroboration.<\/p>\n

    7. Depending On The Alkyne, Mixtures Of Products Can Be Obtained<\/h2>\n

    One final note: if we use an alkyne where both ends are directly attached to carbon, we will obtain a mixture of products. That’s just “Markovnikov’s rule” – remember that if each carbon in the multiple bond is attached to an identical number of hydrogens, then we can’t determine which is the “most substituted” for our purposes. Like in this example.<\/p>\n

    \"if<\/p>\n

    Next Post: Alkyne Reaction Patterns – The Carbocation Pathway<\/a><\/strong><\/p>\n

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    <\/a>Notes<\/h2>\n

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