Ace Your Next Organic Chemistry
Exam.

With these Downloadable PDF Study Guides

Our Study Guides

Alkyne Reactions

By James Ashenhurst

Hydroboration and Oxymercuration of Alkynes

Last updated: June 28th, 2019 |

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

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].

oxymerc-alkenes

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 !?

oxymerc-alkynes

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 (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.

2-step 1 alkyne oxymerc

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).

3-step 2 alkyne oxymerc

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

oxymerc-fixed

 

5. Alkynes Can Also Be “Hydrated” via Oxymercuration

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, 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.

5-alkyne oxymerc

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. [Note: while BH3 is sometimes written for this, it’s not strictly correct to do so. [Why? Double addition.] 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. ]

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.

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.

7-mixture

Next Post: Alkyne Reaction Patterns – The Carbocation Pathway

 

Related Posts:

Comments

Comment section

16 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 .

Leave a Reply

Your email address will not be published. Required fields are marked *

This site uses Akismet to reduce spam. Learn how your comment data is processed.