15.05.02 · orgchem / addition

Electrophilic addition to alkynes: Markovnikov's rule and anti-Markovnikov hydroboration

stub3 tiersLean: nonepending prereqs

Anchor (Master): March's Advanced Organic Chemistry, 7e, Ch. 15

Intuition Beginner

Alkynes have a carbon-carbon triple bond. That triple bond contains two pi bonds — two layers of electron density sitting above and below the molecular axis. These electron clouds attract electrophiles, just as a single pi bond in an alkene does. The key difference is that alkynes can undergo addition twice.

When HBr adds to an alkyne, the first addition follows Markovnikov's rule: the hydrogen bonds to the carbon with more hydrogens, producing a vinyl halide (a halide attached to a carbon that is still part of a double bond). A second equivalent of HBr can then add to the remaining double bond, producing a geminal dihalide — two halogens on the same carbon. Both additions obey Markovnikov's rule.

Anti-Markovnikov addition to alkynes works too. Hydroboration with a bulky borane reagent like disiamylborane adds boron to the less substituted carbon. Subsequent oxidation replaces boron with OH, giving an aldehyde from a terminal alkyne. This is the anti-Markovnikov counterpart to acid-catalysed hydration, which produces a ketone via Markovnikov addition.

Visual Beginner

Consider 1-butyne ( $CH_{3} -CH_{2} -C\equivCH$ ), a terminal alkyne. The triple bond sits between the last two carbons. The terminal carbon (C4) has one hydrogen; the internal carbon (C3) has none.

Path A: Markovnikov hydration. Water adds with Hg $^{2 +}$ catalysis. H $^{+}$ adds to C4 (the carbon with the hydrogen), producing a vinyl carbocation at C3 that is captured by water. The resulting enol ( $CH_{3} -CH_{2} -C(OH)=CH_{2}$ ) tautomerises to the ketone: 2-butanone ( $CH_{3} -CH_{2} -C(=O)-CH_{3}$ ).

Path B: Anti-Markovnikov hydroboration. Disiamylborane adds with boron at C4 (the terminal, less substituted carbon) and hydrogen at C3. Oxidation replaces boron with OH, giving the enol $CH_{3} -CH_{2} -CH_{2} -CH=OH$ , which tautomerises to the aldehyde butanal ( $CH_{3} -CH_{2} -CH_{2} -CHO$ ).

Worked example Beginner

Predict the products of (a) Hg $^{2 +}$ -catalysed hydration and (b) hydroboration-oxidation of phenylacetylene ( $C_{6} H_{5} -C\equivCH$ ).

Phenylacetylene is a terminal alkyne with a phenyl group on C1 and a hydrogen on C2.

(a) Mercuric-ion hydration. H $^{+}$ adds to C2 (Markovnikov — the carbon with the hydrogen), producing a vinyl carbocation at C1 stabilised by the phenyl group. Water attacks C1, giving an enol that tautomerises to acetophenone ( $C_{6} H_{5} -C(=O)-CH_{3}$ ), a methyl ketone.

(b) Hydroboration-oxidation with disiamylborane. Boron adds to C2 (the terminal carbon, anti-Markovnikov), and hydrogen adds to C1. After oxidation, the product is phenylacetaldehyde ( $C_{6} H_{5} -CH_{2} -CHO$ ), an aldehyde. The bulky borane ensures boron adds to the less hindered terminal carbon.

These two reactions convert the same starting material into two different carbonyl compounds — one a ketone and one an aldehyde. The choice between them is a choice of mechanism: ionic hydration (Markovnikov) or concerted hydroboration (anti-Markovnikov).

Check your understanding Beginner

Formal definition Intermediate+

Electrophilic addition to a triple bond proceeds through either a vinyl carbocation (Markovnikov pathway) or a concerted adatomium-type intermediate. For terminal alkynes $RC \equiv CH$ , the regioselectivity is governed by the stability of the intermediate: protonation at C1 gives the more stable secondary vinyl carbocation (Markovnikov), while hydroboration delivers boron to the less substituted carbon (anti-Markovnikov) via a concerted four-centre transition state.

The overall stoichiometry for hydration of a terminal alkyne yields a methyl ketone (Markovnikov product via $Hg^{2 +}$ catalysis) or an aldehyde (anti-Markovnikov via hydroboration-oxidation with disiamylborane or 9-BBN).

Key mechanism Intermediate+

Electrophilic addition to alkynes proceeds through the same carbocation-formation logic as alkene addition 15.05.01, but with important differences arising from the sp-hybridised carbon geometry and the possibility of double addition.

Vinyl carbocations. The first electrophilic attack on an alkyne generates a vinyl carbocation — a carbocation at an sp $^{2}$ carbon that is part of the remaining double bond. Vinyl carbocations are less stable than their alkyl counterparts because the sp $^{2}$ orbital holding the positive charge has more s-character (33% vs 25% for an sp $^{3}$ -derived carbocation), concentrating the charge closer to the nucleus and raising its energy.

The instability of vinyl carbocations means that some alkyne additions proceed through alternative pathways. Mercuric-ion hydration (HgSO $_{4}$ , H $_{2}$ SO $_{4}$ , H $_{2}$ O) avoids the free vinyl carbocation by forming a cyclic mercurinium ion intermediate, analogous to the bromonium ion in alkene chemistry. The mercury bridges both carbons, and water attacks the more substituted carbon in a Markovnikov fashion. Subsequent protonolysis of the C-Hg bond and tautomerisation gives the ketone.

Tautomerism of enol intermediates. Both hydration pathways (Markovnikov and anti-Markovnikov) produce an enol as the initial product — a compound with an OH group on a vinyl carbon. Enols are thermodynamically unstable relative to their carbonyl tautomers. The keto-enol tautomerism proceeds through a 1,3-proton shift: the vinyl hydrogen migrates to the oxygen, and the C=C double bond becomes C=O. Under acidic or basic conditions, this equilibrium overwhelmingly favours the carbonyl form (typically $> 99.9%$ keto at room temperature for simple enols).

For terminal alkynes:

Markovnikov hydration places the OH on the internal carbon; tautomerisation gives a ketone.
Anti-Markovnikov hydroboration places the OH on the terminal carbon; tautomerisation gives an aldehyde.

This divergence makes alkyne hydration a versatile route to both ketones and aldehydes from the same starting material, controlled entirely by reagent choice.

Hydroboration reagents for alkynes. BH $_{3}$ itself is unsuitable for terminal alkynes because it adds twice — the first addition gives a vinylborane whose remaining double bond undergoes a second hydroboration. Bulky borane reagents solve this problem:

Disiamylborane (Sia $_{2}$ BH) is sufficiently bulky that only one addition occurs. It adds to terminal alkynes with boron at the terminal carbon (anti-Markovnikov), and the steric bulk prevents a second addition. Oxidation gives the aldehyde.
9-BBN (9-borabicyclo[3.3.1]nonane) serves a similar role for internal alkynes, where steric differentiation between the two carbons of the triple bond is required for regioselectivity.

Lindlar catalyst for cis-alkenes. Partial hydrogenation of alkynes to cis-alkenes is achieved using Lindlar catalyst (Pd/CaCO $_{3}$ poisoned with Pb(OAc) $_{2}$ and quinoline). The catalyst delivers both hydrogen atoms from the metal surface to the same face of the alkyne, giving syn addition and the cis-alkene selectively. This is the standard synthetic route to (Z)-alkenes. The catalyst poison reduces palladium activity so that the alkene product is not further reduced to the alkane.

Counterexamples to common slips

"Alkyne addition gives the same products as alkene addition." The first addition to an alkyne gives a vinyl halide or enol, different from the alkyl halide or alcohol produced by alkene addition. The enol tautomerises to a carbonyl — a functional group not produced by direct alkene hydration.
"BH $_{3}$ is the best hydroboration reagent for alkynes." BH $_{3}$ adds twice to terminal alkynes. Disiamylborane or 9-BBN should be used to ensure mono-addition and clean regioselectivity.
"Alkynes always undergo two additions." With the right reagents (one equivalent of halide, Lindlar catalyst for partial hydrogenation, bulky boranes), stopping at the mono-addition stage is straightforward.
"Hydration of any alkyne gives a ketone." Only Markovnikov hydration of a terminal alkyne gives a ketone. Hydroboration-oxidation gives an aldehyde. For symmetric internal alkynes, both carbons are equivalent and hydration gives a single ketone product.

Exercises Intermediate+

Exercise 4 (medium, short-answer).

Explain why BH $_{3}$ is not a suitable hydroboration reagent for terminal alkynes but is suitable for most alkenes. What property of the borane reagent solves this problem for alkynes?

Hint

BH $_{3}$ adds across a triple bond and the product still has a double bond. What does this mean for a second addition?

Answer

When BH $_{3}$ adds to a terminal alkyne, it produces a vinylborane. The vinylborane still contains a C=C double bond that is more reactive than an alkene double bond (it is electron-rich from the adjacent boron). BH $_{3}$ adds a second time to this double bond, giving a diboryl product. For alkenes, BH $_{3}$ adds once and the product (an alkylborane) has no remaining unsaturation. Bulky boranes like disiamylborane (Sia $_{2}$ BH) solve the problem for alkynes: after one addition, the steric bulk around the vinylborane double bond prevents a second approach, ensuring mono-addition.

Exercise 6 (hard, short-answer).

A chemist treats 1-pentyne with Na in liquid NH $_{3}$ and obtains a different alkene than when using Lindlar catalyst. Identify both products and explain the stereochemical difference.

Hint

Na/NH $_{3}$ is a dissolving-metal reduction that proceeds through a radical anion intermediate. Lindlar catalyst gives syn addition.

Answer

Lindlar catalyst gives cis-2-pentene ((Z)-2-pentene) via syn addition of hydrogen. Na/NH $_{3}$ gives trans-2-pentene ((E)-2-pentene) via anti addition. The dissolving-metal reduction proceeds through electron transfer to form a vinyl radical anion, which is protonated by NH $_{3}$ to give a vinyl radical. A second electron transfer gives a vinyl anion, which adopts the more stable trans configuration (the two alkyl groups are trans to minimise steric repulsion), and a final protonation gives the trans-alkene. The stereochemical divergence makes these two reagents complementary: Lindlar for cis, Na/NH $_{3}$ for trans.

Exercise 7 (hard, short-answer).

When 1-hexyne is treated with BH $_{3}$ (one equivalent) followed by H $_{2}$ O $_{2}$ /NaOH, the product is not hexanal but a mixture. Explain why, and identify the correct reagent for obtaining hexanal.

Hint

BH $_{3}$ has three B-H bonds. Each BH $_{3}$ can add to three separate alkyne molecules, but with terminal alkynes the problem is double addition to a single alkyne.

Answer

BH $_{3}$ adds twice to a terminal alkyne. The first addition gives a vinylborane (anti-Markovnikov), but the vinylborane double bond is more reactive than an alkene double bond and undergoes a second hydroboration, producing a 1,1-diborylalkane. Oxidation of the diboryl product gives the primary alcohol (1-hexanol) rather than the aldehyde. To obtain hexanal, use disiamylborane (Sia $_{2}$ BH), which is bulky enough to prevent the second addition. After oxidation, the mono-boryl intermediate gives hexanal.

Regioselectivity of dihalide addition to alkynes Master

Alkynes add two equivalents of Cl $_{2}$ or Br $_{2}$ to give tetrahaloalkanes. The first addition produces a trans-dihaloalkene via anti addition through a halonium ion intermediate, and the second addition produces the tetrahalide. The stereochemistry of the first addition is reliably trans because the halonium ion intermediate constrains the halide nucleophile to attack from the opposite face — the same bromonium-ion logic that governs Br $_{2}$ addition to alkenes 15.05.01, extended to the sp-hybridised substrate.

For unsymmetrical internal alkynes, the regiochemistry of the first addition is governed by electronic effects. Electron-withdrawing groups on one carbon of the triple bond make that carbon less nucleophilic, directing the electrophilic halogen to the more electron-rich carbon. This electronic bias can override the modest steric differences between the two carbons. The selectivity is generally lower than for alkene halogenation because the linear geometry of the alkyne reduces the steric differentiation available to the approaching electrophile.

The geminal dihalide motif produced by double Markovnikov addition of HX to terminal alkynes is a versatile synthetic handle. Treatment with strong base (KOH in ethanol) effects double dehydrohalogenation, regenerating the alkyne — the reverse of the double addition. This protection-deprotection strategy allows chemists to mask an alkyne as a geminal dihalide, carry out transformations on other functional groups, and then regenerate the triple bond. The strategy is nontrivial to execute on polyfunctional substrates because the strongly basic conditions required for dehydrohalogenation can affect other base-sensitive groups.

Oxidative cleavage and ozonolysis of alkynes Master

Ozonolysis of alkynes is less commonly used than ozonolysis of alkenes, partly because the products (carboxylic acids rather than aldehydes or ketones) are less synthetically versatile and partly because the reaction is harder to control. Ozone attacks the triple bond to form a cyclic molozonide that fragments to give two carboxylic acid groups:

R-C \equiv C-R’ + O_{3} + H_{2} O ⟶ R-COOH + R’-COOH

For terminal alkynes, one product is carbon dioxide (from the terminal carbon, which has no alkyl group to become part of a carboxylic acid):

R-C \equiv CH + O_{3} + H_{2} O ⟶ R-COOH + CO_{2}

This destructive cleavage can be useful analytically — it identifies the position of the triple bond by the identity of the acid products — but is rarely used in synthesis because the carbon count decreases and the triple bond is consumed without generating a new carbon-carbon bond.

Potassium permanganate under basic conditions (KMnO $_{4}$ , OH $^{-}$ , heat) similarly cleaves alkynes to carboxylic acids. The mechanism proceeds through initial addition of permanganate to the triple bond, forming a cyclic manganate ester that fragments. The reaction is cleaner for alkynes than for alkenes because the intermediate diol (from syn addition of two OH groups) is not isolable — it is immediately oxidised further to the dicarboxylic acid.

Metal-catalysed alkyne hydration Master

The classical mercuric-ion hydration (HgSO $_{4}$ /H $_{2}$ SO $_{4}$ ) gives Markovnikov addition of water to alkynes, but mercury toxicity and environmental concerns have driven the development of transition-metal alternatives. The most significant advances use ruthenium and gold catalysts.

Ruthenium-catalysed hydration. RuCl $_{3}$ catalyses the Markovnikov hydration of terminal and internal alkynes to ketones under milder conditions than the mercury system. The mechanism involves coordination of the alkyne to Ru(II), nucleophilic attack by water on the coordinated alkyne (anti addition), and tautomerisation of the resulting enol to the ketone. The regiochemistry is Markovnikov: for terminal alkynes, the carbonyl ends up on the internal carbon, giving a methyl ketone. The ruthenium catalyst is less toxic than mercury and operates at ambient temperature in aqueous solvent.

Gold-catalysed hydration. Au(I) and Au(III) complexes catalyse alkyne hydration with high efficiency and functional-group tolerance. The mechanism exploits the strong pi-philicity of gold: Au $^{+}$ coordinates to the alkyne triple bond, activating it toward nucleophilic attack by water. The gold acts as a soft Lewis acid, polarising the triple bond without forming a classical carbocation. For terminal alkynes, the regiochemistry is Markovnikov, producing methyl ketones. The mild conditions (room temperature, neutral pH, low catalyst loading) make gold catalysis suitable for complex substrates where the harsh acidic conditions of mercury catalysis would cause side reactions.

Gold-catalysed hydration has been extended to intramolecular variants where a pendant hydroxyl group adds to an internal alkyne, forming cyclic enol ethers with complete regio- and stereocontrol. These cyclisations are key steps in the synthesis of oxygen heterocycles found in natural products.

Alkyne metathesis. Alkyne metathesis, catalysed by high-valent molybdenum or tungsten alkylidyne complexes (Schrock-type catalysts), exchanges the alkylidene fragments between two alkynes. The mechanism proceeds through a [2+2] cycloaddition of the alkyne with the metal alkylidyne to form a metallacyclobutadiene intermediate, which then fragments to give the new alkyne and the new metal alkylidyne. This is directly analogous to alkene metathesis (which proceeds through a metallacyclobutane) but operates at the higher oxidation state required for triple-bond activation.

Alkyne metathesis is thermodynamically driven by removal of a volatile alkyne byproduct (typically 2-butyne from terminal alkyne substrates), and the reaction can be driven to completion under reduced pressure or with a continuous flow of inert gas to strip the byproduct. The synthetic utility lies in the formation of carbon-carbon bonds under mild, neutral conditions — the reaction tolerates a wide range of functional groups because it does not involve acidic or basic intermediates. Recent developments have produced air-stable molybdenum catalysts that operate at room temperature, bringing alkyne metathesis into the mainstream synthetic toolbox.

Alkyne metathesis connects to the Diels-Alder cycloaddition 15.05.03 pending because the internal alkyne products of metathesis are excellent dienophiles in [4+2] cycloadditions, and the combination of metathesis followed by Diels-Alder cyclisation is a powerful two-step route to polycyclic frameworks.

Connections Master

Electrophilic addition to alkenes 15.05.01. Alkyne addition is the direct extension of alkene addition to a substrate with two pi bonds. The same carbocation stability logic governs regiochemistry, and the same cyclic ion intermediates (halonium, mercurinium) control stereochemistry. The key difference is the possibility of double addition and the intermediacy of enols that tautomerise to carbonyls.
Keto-enol tautomerism 15.03.02 pending. Every alkyne hydration reaction produces an enol intermediate. The position of the keto-enol equilibrium and the rate of tautomerisation are treated in the acid-base organic chemistry unit. Understanding tautomerism is a prerequisite for predicting whether an alkyne hydration gives a ketone or aldehyde.
Diels-Alder cycloaddition 15.05.03 pending. Alkynes serve as dienophiles in [4+2] cycloadditions. The same electron-rich triple bond that attracts electrophiles in addition reactions donates electron density to the diene in cycloadditions. Partial reduction of alkynes to cis-alkenes (Lindlar) or trans-alkenes (Na/NH $_{3}$ ) provides stereochemically defined dienophile precursors for Diels-Alder reactions.
Carbonyl nucleophilic addition 15.07.01. The ketones and aldehydes produced by alkyne hydration are substrates for carbonyl addition reactions. The synthetic connection is direct: alkyne hydration followed by nucleophilic addition to the resulting carbonyl is a two-step route to functionalised alcohols.
Retrosynthetic analysis 15.10.01. Alkyne hydration and hydroboration-oxidation are standard functional-group interconversions in retrosynthetic planning. The ability to choose between ketone and aldehyde products from the same alkyne gives the synthetic chemist two distinct disconnection options.
Organometallic synthesis 15.09.01. Metal-catalysed alkyne hydration (Ru, Au) and alkyne metathesis (Mo, W) are examples of transition-metal catalysis in organic synthesis. The organometallic unit treats the electronic structure of metal-alkyne complexes that underpins these catalytic cycles.

Historical notes Master

The hydration of alkynes using mercuric sulfate was developed in the late nineteenth century by Mikhail Kucherov (1881), who demonstrated that acetylene could be converted to acetaldehyde using HgSO $_{4}$ in dilute sulfuric acid. The Kucherov reaction became the industrial basis for acetaldehyde production (the starting material for acetic acid) before being superseded by the Wacker process in the 1960s. The mercury-catalysed process is now largely of historical and pedagogical interest due to mercury toxicity.

Herbert Brown's hydroboration work (1957 onward) was extended to alkynes by Brown and Zweifel in the 1960s, who demonstrated that disiamylborane gives clean mono-addition to terminal alkynes with anti-Markovnikov regiochemistry. The resulting vinylboranes were shown to be versatile intermediates: oxidation gives aldehydes, and protonolysis or halogenation gives stereodefined alkenes. Brown received the Nobel Prize in Chemistry in 1979 for his body of work on organoborane chemistry.

The Lindlar catalyst for partial hydrogenation of alkynes to cis-alkenes was reported by Herbert Lindlar in 1952. The deactivation of palladium with lead acetate and quinoline was an empirical discovery — the mechanism of catalyst poisoning was not fully understood until surface-science techniques became available decades later. Lindlar catalyst remains the standard reagent for cis-alkene synthesis from alkynes.

Gold-catalysed alkyne hydration emerged in the late 1990s and 2000s, driven by the recognition that gold's strong pi-philicity (a consequence of relativistic effects on the 6s orbital) makes it exceptionally effective at activating alkynes toward nucleophilic attack without generating harsh acidic conditions. Teles et al. (1998) reported gold-catalysed hydration, and the field has expanded rapidly since. Alkyne metathesis was developed in the 1990s-2000s, with contributions from Schrock (molybdenum alkylidyne catalysts) and Furstner (air-stable silica-supported molybdenum catalysts).

Bibliography Master

@book{McMurry2019,
  author = {McMurry, John},
  title = {Organic Chemistry},
  edition = {10th},
  publisher = {Cengage},
  year = {2019}
}

@book{Clayden2012,
  author = {Clayden, Jonathan and Greeves, Nick and Warren, Stuart},
  title = {Organic Chemistry},
  edition = {2nd},
  publisher = {Oxford University Press},
  year = {2012}
}

@book{March2013,
  author = {Smith, Michael B.},
  title = {March's Advanced Organic Chemistry},
  edition = {7th},
  publisher = {Wiley},
  year = {2013}
}

@article{Kucherov1881,
  author = {Kucherov, Mikhail},
  title = {Uber eine neue Methode der synthese von Aldehyden},
  journal = {Ber. dtsch. chem. Ges.},
  volume = {14},
  year = {1881},
  pages = {1540--1542}
}

@article{Lindlar1952,
  author = {Lindlar, Herbert},
  title = {Ein neuer Katalysator fur selektive Hydrierungen},
  journal = {Helv. Chim. Acta},
  volume = {35},
  year = {1952},
  pages = {446--450}
}

@article{BrownZweifel1961,
  author = {Brown, Herbert C. and Zweifel, George},
  title = {Hydroboration of Alkynes. A Convenient Synthesis of Aldehydes via Organoboranes},
  journal = {J. Am. Chem. Soc.},
  volume = {83},
  year = {1961},
  pages = {3834--3840}
}

@article{Teles1998,
  author = {Teles, Jorg Henning and Brode, Stephan and Chabanus, Mathieu},
  title = {Cationic Gold(I) Complexes: Highly Efficient Catalysts for the Addition of Alcohols to Alkynes},
  journal = {Angew. Chem. Int. Ed.},
  volume = {37},
  year = {1998},
  pages = {1415--1418}
}