Alkynes | Organic Chemistry 2

Alkynes are studied in this chapter: name and properties of alkynes, preparation of alkynes, reduction reactions, electrophilic addition reactions, anti-Markovnikov additions

Nomenclature of Alkynes

Name: the -ane ending of the corresponding alkane is replaced by -yne

The rules for naming alkenes also apply to alkynes
Substituents bearing a triple bond are alkynyl groups
Alkynes take precedence over alkenes but not over alcohols


Properties of Alkynes

Physical properties:

  • Boiling and melting points: low melting points and boiling points, similar to corresponding alkanes and alkenes, increase with increasing carbon number.
  • Solubility: soluble in organic solvents, insoluble in water.
  • Acidity: terminal alkynes are remarkably acidic (pKa ~ 25).

Absorption spectroscopy:

NMR: 1H δ ~ 2-3 ppm (coupling constant J between 2-4 Hz); 13C δ ~ 65-95 ppm
IR: C ≡ C bond stretching frequency ν ~ 2100 cm-1

Geometry and orbitals:

The carbons of the C ≡ C bond of an alkyne are sp-hybridized ⇒ linear geometry. The triple bond of an alkyne is composed of:

  • 1 σ bond ⇒ formed by end-on overlap of 2 sp-hybrid orbitals of carbon atoms.
  • 2 π bonds ⇒ each formed by side-by-side overlap of 2 2p orbitals of carbon atoms.


  • As with alkenes, both π bonds of the triple bond of an alkyne are weaker than the σ bond, making them more easily broken ⇒ alkynes undergo addition reactions.
  • Alkynes are more polarizable than alkenes (the electrons in their π bonds are more loosely held) ⇒ alkynes are more reactive than alkenes.

Preparation of Alkynes

Double Elimination from 1,2-dihaloakanes:


Double elimination reaction from a vicinal dihaloalkane ⇒ 2 equivalents of strong base are necessary (ex: NaNH2, liquid NH3). The starting vicinal dihaloalkane can be formed by electrophilic halogenation of alkenes.


Alkylation of alkynyl anions:


  1. Deprotonation of an alkyne with a strong base (BuLi, RMgBr or LiNH2, liquid NH3)
  2. Alkylation by electrophilic addition of haloalkane, carbonyl or epoxide

Electrophilic Addition Reactions

Addition of hydrogen halides:


  1. Protonation of the alkyne forming a vinyl cation intermediate
  2. Nucleophilic attack of the halide ion (X-) forming an alkyl chloride

The first addition of HX is often followed by a second addition of HX in the same two-step manner ⇒ geminal dihaloalkane formation.
Markovnikov's rule is followed: H+ adds to the less substituted carbon to form the more stable carbocation.


Halogenation - Addition of halogen:


  1. Addition of electrophile (X+) to a π bond, forming a cyclic bromonium or chloronium intermediate

  2. Nucleophilic attack of the halide ion (X⁻) from the back side of the cyclic intermediate to form a trans dihalide

The first addition of X2 is often followed by a second addition to form a tetrahalide.


Hydration - Addition of water:


  1. Addition of the electrophile (H+) to a π bond
  2. Nucleophilic attack of H2O and loss of a proton
  3. Tautomerization: an unstable enol is formed, which rearranges to a carbonyl group

Markovnikov's rule is followed: H+ adds to the less substituted carbon to form the more stable carbocation.

Anti-Markovnikov Additions

Addition of HBr with radicals:

Radical addition of HBr
A cis-trans mixture is obtained


Hydroboration - Oxidation:


  1. Hydroboration: addition of borane to form an organoborane
  2. Oxidation with basic H2O2 to form an enol
  3. Tautomerization of the enol to form a carbonyl compound

Anti-Markovnikov addition: hydroboration of an alkyne adds BR2 to the less substituted carbon. 

Ozonolysis of Alkynes

Ozonolysis of alkynes:

Alkynes undergo oxidative cleavage to form carboxylic acids.


Ozonolysis of terminal alkynes:

Terminal alkynes undergo oxidative cleavage and the terminal side is converted into carbon dioxide.

Reduction of Alkynes

Reduction of an alkyne to an alkane:


Reduction of an alkyne to a cis alkene:

The Lindlar catalyst, composed of palladium deposited on calcium carbonate, is deactivated after one equivalent of H2 addition, allowing the selective hydrogenation of alkynes to cis alkenes without further reduction to saturated alkanes.


Reduction of an alkyne to a trans alkene (Birch reduction):


Sequential addition of electrons from Na and protons from NH2 to the triple bond.

Check your knowledge about this Chapter

The general formula for alkynes is CnH2n-2, where n is the number of carbon atoms. Alkynes are hydrocarbons with at least one triple bond between two carbon atoms, leading to two fewer hydrogen atoms than an alkene, which has the formula CnH2n. Alkanes have single bonds only and follow the formula CnH2n+2.


Alkynes are named by replacing the -ane suffix in the corresponding alkane with -yne. The key principles of their nomenclature are:

  • Find the longest continuous carbon chain containing the triple bond.

  • Assign numbers to the carbon atoms in the triple bond, starting from the end that gives the triple bond the lowest number.

  • Use the appropriate numerical prefix to indicate the position of the triple bond in the chain.

  • Identify and name any substituent groups attached to the carbon chain, and specify their locations using locants.

  • Combine the base name of the alkyne, the locants for the triple bond and substituents, and the names of any substituents to give the final systematic name.


The carbons of the triple bond in an alkyne are sp-hybridized, resulting in a linear geometry. The triple bond of an alkyne consists of: 

  • one σ bond, formed by the end-on overlap of two sp-hybrid orbitals of carbon atoms
  • two π bonds, with each π bond formed by the side-by-side overlap of two 2p orbitals of carbon atoms.

Alkynes are significantly more acidic than other hydrocarbons, such as alkanes and alkenes, due to the sp hybridization of their carbon atoms.  The carbon-hydrogen bond in terminal alkynes is composed of an sp-hybridized carbon and an s-orbital hydrogen, which results in a higher percentage of s-character in the bond. When a hydrogen is removed from an alkyne, the resulting acetylide anion is stabilized by the sp hybridized carbon, which can effectively distribute the negative charge over the p-orbitals of the carbon.


Alkynes exhibit increased reactivity compared to alkenes due to the weaker nature of both π bonds within the triple bond, making them more susceptible to breaking and facilitating addition reactions. Additionally, alkynes are more polarizable than alkenes, signifying that the electrons in their π bonds are loosely held, which contributes to their increased reactivity in various chemical reactions.

Dehydrohalogenation is a chemical reaction that involves the elimination of hydrogen halides (HX) from organic compounds. For example, when we subject a vicinal dihalide to dehydrohalogenation with a strong base, we can form an alkyne. The reaction typically proceeds through the formation of an alkene intermediate, which then undergoes a second dehydrohalogenation to form the alkyne. This is a common method to synthesize alkynes from dihaloalkanes when treated with a strong base such as sodium amide (NaNH2) in liquid ammonia.

Electrophilic addition reactions to alkynes follow a two-step mechanism where the electron-rich triple bond attacks an electrophile to form a vinylic carbocation intermediate. This reactive intermediate is then attacked by a nucleophile, leading to the addition product. These reactions are regioselective, often following Markovnikov's rule; however, various conditions and catalysts can alter the outcome of the addition.

Alkynes can undergo several types of electrophilic addition reactions such as:

  • Halogens: Alkynes react with halogens such as Br2 or Clto form dihalides or tetrahalides depending on the molar ratio used.
  • Hydrogen halides: Reaction with HX (X = Cl, Br, I) can yield vinyl halides or geminal dihalides, with the latter as the major product in excess of HX.
  • Water: In the presence of a strong acid catalyst, alkynes hydrate to form enols which quickly tautomerize to ketones.
  • Hydrogen: Alkynes can be hydrogenated to alkenes using a poisoned catalyst (Lindlar's catalyst) or fully to alkanes with a regular catalyst like Pd/C.

Regioselectivity in electrophilic addition to alkynes is influenced by the stability of the intermediate carbocations formed during the reaction. More substituted carbocations are more stable due to greater alkyl group stabilization, known as hyperconjugation, and the inductive effect. For asymmetrical alkynes, electrophilic addition tends to occur in a way that forms the more stable carbocation, leading to the more substituted alkenyl product, following Markovnikov's rule. Additionally, the nature of the electrophile and solvent effects can also play roles in the outcome of the reaction.

The hydrohalogenation of alkynes involves the addition of hydrogen halides (HX) to the alkyne to form first a vinyl halide, and then an dihaloalkane if excess HX is present. Unlike alkenes, where the addition is typically done in one step, alkynes undergo a two-step addition due to the presence of two π bonds. The addition follows Markovnikov's rule, where the hydrogen atom attaches to the carbon with more hydrogen substituents and the halogen to the carbon with more alkyl substituents.

Alkynes react with halogens through a halogen addition reaction where the π bonds of the alkyne are broken to form dihaloalkanes. Initially, the alkyne reacts with one equivalent of the halogen to form a dihalogenated alkene (vinyl halide), and upon further reaction with a second equivalent of halogen, a tetrahaloalkane is formed. This reaction proceeds through a cyclic halonium ion intermediate, and the addition of the halogens is anti, meaning they add on opposite sides of the alkyne plane.

Anti-Markovnikov addition refers to the addition of a protic acid (H-X) to an alkyne in such a way that the hydrogen (H) attaches to the carbon with fewer hydrogens and the halide (X) or other group attaches to the less substituted carbon. This is the opposite of what we observe in Markovnikov's rule, where the hydrogen attaches to the less substituted carbon.

Anti-Markovnikov addition in alkynes is favored when the reacting alkyne is treated with reagents that form radicals. For example, this type of addition is observed in the presence of peroxides in the reaction with hydrogen bromide (HBr), which leads to the formation of radicals. It can also occur in hydroboration-oxidation reactions, where borane (BH3) adds first in an anti-Markovnikov manner, followed by oxidation which replaces the boron with a hydroxyl group (-OH), giving an enol that tautomerizes to a ketone or aldehyde.

In the hydrogenation of alkynes to alkenes, a poisoned catalyst, such as the Lindlar catalyst (palladium on calcium carbonate partially deactivated), is often used. This selective catalyst allows for the addition of hydrogen to the triple bond to form a cis-alkene without further reduction to an alkane.

For full hydrogenation of alkynes to alkanes, a more active catalyst like palladium, platinum, or nickel is used, which facilitates the addition of hydrogen across the triple bond to generate the corresponding alkane.

The products of alkyne reduction can vary depending on the hydrogenation catalyst used. Using a traditional catalyst like palladium on carbon (Pd/C) will typically reduce an alkyne completely to an alkane. In contrast, using a poisoned catalyst, such as the Lindlar catalyst, allows for the selective reduction of an alkyne to a cis-alkene, stopping at the alkene stage without further reduction to an alkane. Alternatively, sodium in liquid ammonia (the Birch reduction) reduces an alkyne to a trans-alkene by transferring electrons and protons in a stepwise manner.