Alkyne Reactions

By James Ashenhurst

Hydroboration and Oxymercuration of Alkynes

Last updated: September 13th, 2022 |

Hydroboration of Alkynes and Oxymercuration of Alkynes Via Keto-Enol Tautomerism

Alkyne chemistry bears many resemblances to alkene chemistry, but in these first few posts on the subject, the purpose is to illustrate how one seemingly minor change – an extra π bond – can lead to significant differences in chemical behavior.

Previously, we saw that the sp hybridization of alkynes leads to increased acidity, and the second π bond 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 π bond has a very important and surprising consequence.

Table of Contents

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

1. Hydration of Alkenes Gives Alcohols

Several posts ago we talked about the hydration of alkenes. This can be done either with aqueous acid, or with mercury and water (“oxymercuration” – more on that later). Looking at the reaction with alkenes, the pattern is fairly straightforward: break a C-C π bond, and form a C-H and C-OH bond. Also recall that the oxygen ends up on the most substituted carbon [“Markovnikov” selectivity].

hydration of alkenes with mercury oxymercuration and water gives markovnikov alcohols with no rearrangement

2. Hydration of Alkynes With Aqueous Acid Gives… Ketones??… What?!

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.

Well… here’s what we actually observe. We get… a ketone !?

hydration of alkynes with water and acid or with oxymercuration gives a ketone via an enol intermediate how does this happen

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!

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.

Look at the bonds formed and broken. The first set we should understand. Form C-O and form C-H, break C-C π.  It’s that next set of bonds formed/broken that are a big surprise.

3. The First Step In The Hydration of Alkynes Is Formation Of An “Enol”

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 (half spoon half fork) it is part alkene, part alcohol.

first step in hydration of alkynes is formation of enol through attack of water on protonated alkyne break c c pi and form c o

4. The “Enol” Is Converted To A Ketone Through A Process Called “Tautomerization”

Over time, this enol spontaneously converts into the ketone. Note that the two have the same molecular formula – they are constitutional isomers.  And they are in equilibrium with each other.

We call these constitutional isomers which interconvert, “tautomers”. This equilibrium generally favors formation of the ketone due to the strong C-O π bond (compared to C-C π).

step 2 of conversion of alkyne to ketone is enol to ketone through tautomerism equilibrium favors ketone due to stronger c o bonds

Here’s how the whole process works – arrow by arrow.

mechanism for hydration of alkyne with acid and water proceding through vinyl carbocation then enol and then tautomerization to give ketone markovnikov


5. Alkynes Can Also Be “Hydrated” via Oxymercuration

Wait – we’re not done! There’s another way to “hydratealkynes, just like there was with alkenes. We can also perform the same reaction with mercury, water and strong acid [sulfuric acid, H2SO4 is the usual acid of choice]. For interesting reasons we wont get into at the moment, NaBH4 is not generally needed here; it is sufficient to merely have water and acid present.

oxymercuration of alkynes with mercuric sulfate and water and sulfuric acid h2so4 gives markovnikov ketone

6. Hydroboration Of Alkynes Occurs With “Anti-Markovnikov Selectivity”, Giving Aldehydes From Terminal Alkynes

There’s also hydroboration. Remember how it performs “anti-Markovnikov” hydration of alkenes?

Likewise, we can use the same reaction to perform “anti-Markovnikov” hydroboration of alkynes.

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.

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 ketones with H3O+/H2SO4 or via oxymercuration, and aldehydes via hydroboration.

hydroboration of alkenes gives anti markovnikov alkene hydroboration of alkyne gives anti markovnikov enol which tautomerizes to aldehyde

7. Depending On The Alkyne, Mixtures Of Products Can Be Obtained

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.

if both ends of alkyne are equally substituted hydroboration or oxymercuration of alkyne gives mixtures of products

Next Post: Alkyne Reaction Patterns – The Carbocation Pathway


Note 1. Note: while BH3 is sometimes written for this, it’s not strictly correct to do so. Why? Double addition (see  ref)

Instead often use sterically hindered boranes, such as disiamyl borane or 9-BBN that both increase the proportion of addition to the less substituted carbon and also prevent a second hydroboration reaction. 

(Advanced) References and Further Reading

    Brown, H.C.; Zweifel, G.
    J.  Am. Chem. Soc. 1959 81 (6), 1512
    DOI: 10.1021/ja01515a058
    The original paper describing the hydroboration of alkynes, by Nobel Laureate Prof. H. C. Brown (Purdue).
    Miayura, N.; Suzuki, A.
    Org. Synth. 1990, 68, 130
    A procedure by Nobel Laureate Akira Suzuki for the hydroboration of an alkyne with catecholborane. The resulting product can then be subsequently used in a Pd-catalyzed Suzuki coupling reaction.
    A variety of other reagents were developed by H. C. Brown for hydroboration, including catecholborane, 9-BBN, and disiamylborane. The advantage with these reagents is that they will undergo monoadditionto alkynes, whereas borane will add twice. Representative references for the reaction of these reagents with alkynes are below:
  3. Catecholborane (1,3,2-Benzodioxaborole) as a New, General Monohydroboration Reagent for Alkynes. A Convenient Synthesis of Alkeneboronic Esters and Acids from Alkynes via Hydroboration
    Brown, H. C.; Gupta, S. K.
    J. Am. Chem. Soc. 1972 94(12), 4370
    DOI: 10.1021/ja00767a072
  4. 50. Hydroboration of Representative Alkynes with 9-Borabicyclo[3.3.1]nonane-a Simple Synthesis of Versatile Vinyl Bora and gem-Dibora Intermediates
    Brown, H. C.; Scouten, C. G.; Liotta, R.
    J. Am. Chem. Soc. 1979 101 (1), 96
  5. XI. The Hydroboration of Acetylenes-A Convenient Conversion of Internal Acetylenes into cis-Olefins and of Terminal Acetylenes into Aldehydes
    Brown, H. C.; Zweifel, G.
    J. Am. Chem. Soc. 1961, 83(18), 3834
    DOI: 10.1021/ja01479a024
    This paper describes the use of disiamylborane for the selective monohydroboration of alkynes.
    William Whalley Myddleton, Arthur W. Barrett, and John H. Seager
    Journal of the American Chemical Society 1930, 52 (11), 4405-4411
    One of the earliest reports on oxymercuration in the literature.
    Gardner W. Stacy and Richard A. Mikulec
    Org. Synth. 1955, 35, 1
    A pretty standard oxymercuration-hydration reaction of a terminal alkyne in Organic Syntheses, a well-regarded source of independently verified and reproducible organic chemistry laboratory procedures.
  8. Enol acetates, enol ethers, and amines by mercuration of acetylenes
    Paul F. Hudrlik and Anne M. Hudrlik
    The Journal of Organic Chemistry 1973, 38 (25), 4254-4258
    The authors in this publication show that the intermediate mercurinum ion can react with nucleophiles other than water, expanding the scope of this reaction.
  9. An efficient synthesis of .gamma.-methylene-.gamma.-butyrolactone (.alpha.’-angelicalactone). Application to the synthesis of deoxyobtusilactone and deoxyisoobtusilactone
    Richard A. Amos and John A. Katzenellenbogen
    The Journal of Organic Chemistry 1978, 43 (4), 560-564
    Mercurinium ions can also undergo intramolecular cyclizations as well – in this case, the terminal alkyne can cyclize with the carboxylic acid on the other end in the presence of Hg salts to yield lactones.Prof. Bassetti (Italy) published a nice series of papers on the mechanism of mercurinium ion formation from alkynes:
  10. Metalation of alkynes. 1. Effect of alkyne structure on the rate of acetoxymercuration
    Mauro Bassetti and Barbara Floris
    The Journal of Organic Chemistry 1986, 51 (22), 4140-4143
  11. Metalation of alkynes. Part 2. Behaviour of alkynes with mercury(II) acetate in methanol: a systematic reinvestigation
    Mauro Bassetti and Barbara Floris
    J. Chem. Soc., Perkin Trans. 2, 1988, 227-233
  12. Geminal Organometallic Compounds. I. The Synthesis and Structure of 1,1-Diborohexane
    G. Zweifel and H. Arzoumanian
    Journal of the American Chemical Society 1967 89 (2), 291-295
    DOI: 10.1021/ja00978a022
    1-Hexyne undergoes double hydroboration reaction when treated with BH3. This is why bulkier hydroboration reagents such as disiamyl borane or 9-BBN are used.


Comment section

18 thoughts on “Hydroboration and Oxymercuration of Alkynes

  1. What I would really like to know is that WHY THE ANTIMARKONIKOV product is formed in the Hydroboration reactions.Basically I want the simplified mechanism which I can’t find in my books.
    And congratulations of becoming a Dad, Mr.James.

      1. Is the regioselectivity of BH3 any good? Seems like hydroboration via 9-BBN or some other bulky R2BH reagent greatly enhances the anti-Markovnikov product via sterics. While the electronegativity difference is real, you’re hoping for a collision that leads to the π bond attacking boron for the syn transfer of the H to the more substituted side of the alkene (or alkyne)–is there any way something as small as BH3 can reliably select for the anti-M product?

  2. If an alkyne was on the end of a molecule and being reacted, how would it react with H2SO4 and HgSO4. Would it be an aldehyde or a ketone?

    1. Hydration of a terminal alkyne will first form an enol following Markovnikov’s rule. The enol will then undergo Keto-Enol Tautomerization and rearrange to a methyl ketone.

  3. In the very last example (“One final note”), why don’t we use carbocation stability, the reason for Markovnikov’s Rule?
    We could say that the Carbon on the right would form a more stable carbocation, because of hyperconjugation. (As it has 3 alpha-hydrogens while the left carbon has only 2)
    So this would mean that the 2nd product would dominate.
    Or am I missing something?

  4. I know that in the case of alkenes and these reactions (both with Hg and BH3) rearrangements are not observed.
    Is rearrangement possible here? (as to form a more substitued thus more stable C+)

    Thanks in advance.

  5. If water are added via oxymercurium path, don’t a second stage, of using hydroboran to reduce oxymercurium inermidiate, is needed?

  6. What an amazing page ! It cleared all my doubts regarding this topic . My Chemistry teacher couldn’t have taught it better :) Please keep up the good work .

  7. In part 4(mechanism of addition of water),in step2(tautomerization)
    i want to ask about hydrogen that h3O donates.can that H be added by any side so we will have enantiomers or it is stereospecific?

  8. By definition a chiral center must have four different groups attached. Two hydrogens are delivered to the alpha carbon of the ketone. Therefore formation of enantiomers is not possible in this reaction.

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