Summary

• Alkynes undergo addition reactions in an analogous fashion to those of alkenes.
• The high electron density of the p bonds makes them nucleophilic.
• Two factors influence the relative reactivity of alkynes compared to alkenes:

1. increased nucleophilicity of the starting p system (CºC  vs  C=C), and
2. stability of any intermediates (for example carbocations)

Reactions:

• Hydrogenation
• Dissolving Metal reduction
• Hydrohalogenation
• Hydration
• Halogenation
• Ozonolysis

Hydrogenation of Alkynes

Summary

• Alkynes can be partially reduced to cis-alkenes with H₂ in the presence of poisoned catalysts.

(eg. Pd / CaCO₃ / quinoline which is also known as Lindlar's catalyst)
• Alkynes can be reduced to alkanes with H₂ in the presence of catalysts (Pt, Pd, Ni etc.)
• The new C-H s bonds are formed simultaneously from H atoms absorbed onto the metal surface.
• The reaction is stereospecific giving only the syn addition product.
• The "poisoned" catalyst prevents over-reduction, which would give the alkane by reducing the alkene

• Dissolving metal reduction of alkynes
• Hydrogenation of alkenes

CATALYTIC HYDROGENATION
Step 1: Hydrogen gets absorbed onto the metal surface.
Step 2: Alkyne approaches the H atoms absorbed on the metal surface.
Step 3: CºC  reacts with the H atoms on the surface forming the two new C-H s bonds generating the alkene.

 

Dissolving Metal Reduction of Alkynes

Summary

• Alkynes can be reduced to trans-alkenes using Na in NH₃ (liq.)
• This reaction is stereospecific giving only the trans-alkene via an anti addition.
• Note that the stereochemistry of this reaction complements that of catalytic hydrogenation (syn)
• The reaction proceeds via single electron transfer from the Na with H coming from the NH₃
• These reaction conditions do not reduce alkenes, hence the product is the alkene.

• Catalytic hydrogenation of alkynes

MECHANISM FOR THE REDUCTION OF ALKYNES WITH Na / NH3
Step 1: Sodium transfers an electron to the alkyne giving a radical anion.
Step 2: The radical anion removes a proton from the ammonia in an acid / base reaction.
Step 3: A second atom of sodium transfers another electron to the alkyne giving an anion.
Step 4: The anion removes a proton from the ammonia in an acid / base reaction.

 

Reaction of Alkynes with Hydrogen Halides

Summary

• When treated with HX alkynes form vinyl halides.
• Hydrogen halide reactivity order : HI > HBr > HCl > HF (paralleling acidity order).
• Regioselectivity predicted by Markovnikov's rule with the H adding to the C with the most H already present.

• Reaction proceeds via protonation to give the more stable carbocation intermediate.
• Not stereoselective since reaction proceeds via planar carbocation.
• In the presence of excess HX, a second addition can occur to the product alkene giving a geminal dihalide


Can you suggest a reason why this regioselectivity is observed ? 
What would be the product from the reaction of 2-butyne with excess HBr ? 

• For HBr, care must be taken to avoid the formation of radicals as the alternate radical addition occurs with opposite regiochemistry:

Why does this reaction have the opposite regiochemistry ? 

• Addition of HX to alkenes

MECHANISM FOR REACTION OF ALKYNES WITH HBr
Step 1: An acid/base reaction. Protonation of the alkyne to generate the more stable carbocation.  The p electrons act pairs as a Lewis base.
Step 2: Attack of the nucleophilic bromide ion on the electrophilic carbocation creates the alkenyl bromide.
Step 3: In the presence of excess reagent, a second protonation occurs to generate the more stable carbocation.
Step 4: Attack of the nucleophilic bromide ion on the electrophilic carbocation creates the geminal dibromide.

Hydration of Alkynes

Summary

• Alkynes can be hydrated to form enols that immediately tautomerize to ketones
• Reagents:  aq. acid, most commonly H₂SO₄, with a mercury salt
• Regioselectivity predicted by Markovnikov's rule (review)
• Reaction proceeds via protonation to give the more stable carbocation intermediate (review).
• Not stereoselective since reactions proceeds via planar carbocation.

MECHANISM FOR REACTION OF ALKYNES WITH H3O+
Step 1: An acid / base reaction. Protonation of the alkyne to generate the more stable carbocation.  The p electrons act pairs as a Lewis base.
Step 2: Attack of the nucleophilic water molecule on the electrophilic carbocation creates an oxonium ion.
Step 3: An acid / base reaction. Deprotonation by a base generates the alcohol and regenerates the acid catalyst forming an unstable enol.
Step 4: An acid / base reaction. Reprotonation by the acid catalyst occurs on the carbon. The oxygen atom electrons help facilitate this process generating an oxonium ion.
Step 5: Another acid / base reaction. Deprotonation of the oxonium ion creates the ketone. Steps 4 and 5 show the acid catalyzed tautomerization of the enol to the ketone.

Halogenation of Alkynes

Summary

• Overall transformation :  CºC to X-C=C-X (and potentially to X₂C-CX₂)
• Reagent : normally the halogen (e.g. Br₂) in an inert solvent like methylene chloride, CH₂Cl₂.
• Regioselectivity : not relevant since all the new bonds are the same, C-X.
• Reaction proceeds via cyclic halonium ion.
• Stereoselectivity : anti since the two C-X bonds form in separate steps one from X₂ the other X^-.

This CHIME image shows the structure of the bromonium ion formed during the reaction of ethyne with Br2.  Note how the Br  is attached to both the C atoms.  What is the charge on the bromine ?  When the Br- nucleophile attacks, it will attack from the least hindered face, the side opposite to the Br in this intermediate.  Make sure you look at the spacefilling model to show this.

MECHANISM FOR REACTION OF ALKENES WITH HALOGENS
Step 1: The p electrons act as a nucleophile, attacking the bromine, displacing a bromide ion but forming a cyclic bromonium ion as an intermediate.
Step 2: Attack of the nucleophilic bromide from the side away from the bromonium center in an SN2 like fashion opens the cyclic bromonium ion to give overall anti addition.

Ozonolysis of Alkynes

Summary

Overall transformation :  CºC to 2 x CO2H Ozonolysis implies that ozone causes the alkyne to break (-lysis) Reagents : ozone followed by aqueous work-up. Note that each of the CC bonds in the CºC becomes a CO bond:
What would be the products of the ozonolysis reactions of:  (a) ethyne ?  (b) 1-butyne ?	(c) 2-butyne ?  (d) cyclooctyne ?

Related Reactions

• Ozonolysis of Alkenes

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