Alkynes are highly reactive hydrocarbons that consist of at least one triple bond. The triple bond consists of one sigma (σ) and two pi (π) bonds. The carbon-carbon pi bond is formed by side-by-side overlapping of p orbitals, each from a different atom. The pi bonds are easily broken, and the valence electrons are set free. They make the molecule electron-rich, attract electrophiles, and form new bonds with other atoms. Therefore, alkynes produce various compounds ^[1-4].

Types of Alkyne Reactions ^[1-5]

1. Addition

The most common reaction of alkynes is the addition reaction. This reaction involves breaking the pi bond and adding two atoms to the molecule.

General Formula for Addition Reaction of Alkyne

R₁ – C ≡ C – R₂ + A – B     →     R₁ – AC = CB – R₂

Types of Addition Reaction of Alkyne

1.1 Hydrogenation

Hydrogenation is a process in which a hydrogen molecule is added to the pi bond. Alkynes undergo hydrogenation in the presence of a catalyst, like platinum or palladium, to form alkenes. Alkenes further undergo hydrogenation to form alkanes. The mechanism of the alkyne hydrogenation is similar to that of alkene. The catalyst absorbs the two hydrogen atoms and adds to the compound in a syn addition.

Examples

• 2-Butyne (C₄H₆) reacts with hydrogen (H₂) in the presence of a Pd/C catalyst to form 2-butene (C₄H₈), which is then reduced to butane (C₄H₁₀).

CH₃ – C ≡ C – CH₃ + H₂ → CH₃ – HC = CH – CH₃

CH₃ – HC = CH – CH₃ + H₂ → CH₃ – H₂C – CH₂ – CH₃

• Phenylacetylene (Ph – C ≡ CH) undergoes hydrogenation in the presence of a Pd/C catalyst to form polystyrene (Ph – HC = CH₂), which is further reduced to ethylbenzene (Ph – H₂C – CH₃).

Ph – C ≡ CH + H₂ → Ph – HC = CH₂

Ph – HC = CH₂ + H₂ → Ph – H₂C – CH₃

1.2 Halogenation

Halogenation is the process of adding a halogen molecule to the pi bond. When halogens are added to alkynes, the result is dihaloalkenes. The alkenes undergo further reaction to give tetrahaloalkane. The mechanism is generally very complex and proceeds through [2+1] cycloaddition.

Examples

• But-2-yne (C₄H₆) reacts with excess bromine (Br₂) to give an intermediate 2,3-dibromobut-2-ene (C₄H₆Br₂), which further reacts with another Br₂ molecule to give 2,2,3,3-tetrabromobutane (C₄H₆Br₄).

CH₃ – C ≡ C – CH₃ + Br₂ → CH₃ – BrC = CBr – CH₃

CH₃ – BrC = CBr – CH₃ + Br₂ → CH₃ – Br₂C – CBr₂ – CH₃

• 1-phenyl-1-propyne (Ph – C ≡ C – CH₃) reacts with chlorine (Cl₂) in presence of CCl₄ to give 1,2-dichloro-1-phenyl-1-propene (Ph – ClC = ClC – CH₃) which undergoes further chlorination to give 1,1,2,2–tetrachloro-1-phenylpropane (Ph – Cl₂C – Cl₂C – CH₃).

Ph – C ≡ C – CH₃ + Cl₂ → Ph – ClC = CCl – CH₃

Ph – ClC = CCl – CH₃ + Cl₂ → Ph – Cl₂C – CCl₂ – CH₃

1.3 Hydrohalogenation

Hydrohalogenation refers to adding a hydrogen halide to the pi bond. Adding a hydrogen halide molecule to pi bonds is known as hydrohalogenation. The result is haloalkene, followed by dihaloalkane per Markovnikov’s rule.

Examples

• When hydrogen bromide (HBr) is added to pent-1-yne (C₃H₇ – C ≡ CH + HBr), the result is 2-bromopent-1-ene (C₃H₇ – BrC = CH₂). This compound reacts with a second molecule of HBr to form 2,2-dibromopentane (C₃H₇ – Br₂C – CH₃).

C₃H₇ – C ≡ CH + HBr → C₃H₇ – BrC = CH₂

C₃H₇ – BrC = CH₂ + HBr → C₃H₇ – Br₂C – CH₃

• When hydrogen chloride (HCl) is added to hex-3-yne (C₂H₅ – C ≡ C – C₂H₅), the result is 3-chloro-hex-3-ene (C₂H₅ – ClC = CH – C₂H₅), which reacts with another molecule of HCl to form 3,3-dichlorohexane (C₂H₅ – Cl₂C – CH₂ – C₂H₅).

C₂H₅ – C ≡ C – C₂H₅ + HCl → C₂H₅ – ClC = CH – C₂H₅

C₂H₅ – ClC = CH – C₂H₅ + HCl → C₂H₅ – Cl₂C – CH₂ – C₂H₅

1.4 Hydration

The addition of a water molecule to the pi bond is known as hydration. This process is carried out in the presence of mercuric sulfate. Unlike alkene, alcohol is not formed because of enol-ketone tautomerization. When water is added, the initial product is an enol, which then rearranges to a more stable keto tautomer.

Examples

• Pent-1-yne (C₃H₇ – C ≡ CH) is hydrated to give an enol (C₃H₇ – C(OH) = CH₂), which then tautomerizes to 2-pentanone (C₃H₇ – CO – CH₃).

C₃H₇ – C ≡ CH + H₂O → C₃H₇ – C(OH) = CH₂ → C₃H₇ – CO – CH₃

• Acetylene (HC ≡ CH) is hydrated to give acetaldehyde (H₃C – CHO).

HC ≡ CH + H₂O → H₃C – CHO

• Propyne (CH₃ – C ≡ CH) is hydrated to give acetone (CH₃ – CO – CH₃)

CH₃ – C ≡ CH → CH₃ – CO – CH₃

2. Oxidation

Alkynes react with oxidizing agents like potassium permanganate resulting in products depending on the reaction conditions – gentle or strong. For example, alkynes form vicinal dicarbonyl compounds with neutral potassium permanganate (gentle) and carboxylic acid with basic potassium permanganate (strong). The latter is a result of oxidative cleavage.

Example

• The reaction between but-2-yne (CH₃ – C ≡ C – CH₃) and neutral potassium permanganate (KMnO₄) yields 2,3-butadione.

CH₃ – C ≡ C – CH₃ + KMnO₄ → CH₃ – OC – CO – CH₃

Under more vigorous conditions, this reaction yields acetic acid.

CH₃ – C ≡ C – CH₃ + KMnO₄ → 2 CH₃ – COOH

3. Ozonolysis

Ozonolysis is the process by which an ozone molecule is added to an unsaturated compound to cleave its double or triple bond. Ozonolysis of alkyne results in a mixture of carboxylic acids.

Example

• Pent-2-yne reacts with ozone to produce a mixture of acetic and propanoic acids.

C₂H₅ – C ≡ C – CH₃ + O₃ → CH₃ – COOH   +   C₂H₅ – COOH

4. Other Reactions of Alkyne

4.1 Hydroboration Oxidation

In this reaction, an alkyne is treated with borane followed by oxidation with alkaline hydrogen peroxide to form an enol. The unstable enol quickly transforms into a more stable aldehyde or ketone.

Example

• Propyne is transformed into propanal

CH₃ – C ≡ CH → CH₃ – HC = C(OH) – H → CH₃ – CH₂ – CHO

• But-2-yne is transformed into 2-butanone

CH₃ – C ≡ C – CH₃ → CH₃ – (OH)C = CH – CH₃ → CH₃ – CO – CH₂ – CH₃

4.2 Hydroxyhalogenation

Hydroxyhalogenation is halogenation carried out in the presence of excess water. In this case, the nucleophilic hydroxide and the halogen attach to the carbon atoms to give a halohydrin intermediate. The halohydrin is an enol and undergoes keto-enol tautomerization to give halogenated ketone.

Example

• Phenylacetylene (Ph – C ≡ CH) reacts with chlorine (Cl₂) in the presence of excess water to give 2-chloroacetophenone (Ph – OC – CH₂Cl).

Ph – C ≡ CH + Cl₂/H₂O → Ph – (OH)C = CHCl → Ph – OC – CH₂Cl

4.3 Alkoxyhalogenation

Alkoxyhalogenation is also a halogenation process that is carried out in the presence of excess alcohol. The double addition of the alkoxy and the halogen group gives an acetal.

Example

• Phenylacetylene (Ph – C ≡ CH) reacts with chlorine (Cl₂) in the presence of excess ethanol (EtOH) to give (1,1-dichloro-2,2-diethoxy ethyl)benzene.

Ph – C ≡ CH + Cl₂/EtOH → Ph – EtC = CHCl → Ph – (EtO)₂C – CHCl₂

4.4 Deprotonation

The terminal alkynes are highly acidic and can be deprotonated by a strong base like sodium hydride (NaH) or sodium amide (NaNH₂). The reaction is nucleophilic substitution.

Example

• Deprotonation of propyne (CH₃ – C ≡ CH) gives sodium propynylide (CH₃ – C ≡ C^– Na⁺)

CH₃ – C ≡ CH + NaNH₂ → CH₃ – C ≡ C^– Na⁺ + NH₃

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