Alkynes: Structure, Properties, and Reactions

TL;DR

Alkynes are hydrocarbons with at least one carbon-carbon triple bond, making them quite reactive due to exposed pi electrons. Their linear geometry and acidity are key features that influence how they behave in chemical reactions. You'll often see them involved in addition reactions and used to build larger molecules.

1. The Mental Model

Think of alkynes as molecules with a very strong, yet exposed, triple bond between two carbon atoms. This bond is like a high-energy, crowded superhighway that other atoms love to jump onto, making alkynes very reactive, especially compared to their single and double-bonded relatives.

2. The Core Material

Alkynes are unsaturated hydrocarbons, meaning they have carbon-carbon triple bonds. The general formula for non-cyclic alkynes with one triple bond is C$_n$H$_{2n-2}$.

Structure and Bonding

Photo by Artem Podrez on Pexels

The key feature is the carbon-carbon triple bond. Each carbon involved in the triple bond uses sp hybridization. This means one s orbital and one p orbital mix to form two sp hybrid orbitals. The remaining two p orbitals on each carbon remain unhybridized.

• Sigma (σ) bond: Formed by the head-on overlap of two sp hybrid orbitals.
• Pi (π) bonds: Formed by the sideways overlap of the two unhybridized p orbitals. There are two such pi bonds, perpendicular to each other and to the sigma bond.

This sp hybridization leads to a linear geometry around the triple bond, with bond angles of 180°.

graph TD
    A["Carbon 1 (sp)"] --- B["Carbon 2 (sp)"]
    A -- "Sigma (σ) Bond" --> B
    C["Carbon 1 (p)"] -- "Pi (π) Bond 1" --> D["Carbon 2 (p)"]
    E["Carbon 1 (p)"] -- "Pi (π) Bond 2" --> F["Carbon 2 (p)"]
    subgraph Triple Bond Formation
        A --- C
        A --- E
        B --- D
        B --- F
    end
    style A fill:#cef,stroke:#333,stroke-width:2px
    style B fill:#cef,stroke:#333,stroke-width:2px
    style C fill:#fce,stroke:#333,stroke-width:2px
    style D fill:#fce,stroke:#333,stroke-width:2px
    style E fill:#fce,stroke:#333,stroke-width:2px
    style F fill:#fce,stroke:#333,stroke-width:2px

Properties

1. Boiling Points: Generally increase with increasing molecular weight, similar to alkanes and alkenes. They are slightly higher than corresponding alkanes and alkenes due to stronger London dispersion forces from the linear shape allowing closer packing.
2. Solubility: Nonpolar, so they're insoluble in water but soluble in organic solvents (like diethyl ether, acetone).
3. Acidity: This is a crucial property for terminal alkynes (where the triple bond is at the end of the carbon chain, e.g., R-C≡C-H). The hydrogen atom attached to the sp-hybridized carbon is weakly acidic. This is because the sp orbital has more 's' character (50% s, 50% p) compared to sp$^2$ (33% s) or sp$^3$ (25% s). The 's' orbital is closer to the nucleus, so electrons in an sp orbital are held more tightly, making it easier for the hydrogen to be removed as a proton (H$^+$) by a strong base.
   • pKa of terminal alkynes is about 25, much more acidic than alkenes (pKa ~44) or alkanes (pKa ~50), but still less acidic than water (pKa ~16) or alcohols (pKa ~16-18).
   • Strong bases like NaNH$_2$ (sodium amide) or alkyl lithium reagents (R-Li) can deprotonate terminal alkynes to form acetylide anions (R-C≡C$^- $).

Reactions

Alkynes undergo many reactions similar to alkenes, but often twice as much due to having two pi bonds. The most common type of reaction is addition.

1. Hydrogenation (Addition of H$_2$):

   • Complete Hydrogenation: With a platinum (Pt), palladium (Pd), or nickel (Ni) catalyst, alkynes add two molecules of H$_2$ to form alkanes.
     R-C≡C-R' + 2 H$_2$ $\xrightarrow{\text{Pd, Pt, or Ni}}$ R-CH$_2$-CH$_2$-R'
   • Partial Hydrogenation (to Alkenes): You can stop at the alkene stage.
     • Lindlar's Catalyst (Pd/BaSO$_4$ poisoned with quinoline): Gives a cis-alkene.
       R-C≡C-R' + H$_2$ $\xrightarrow{\text{Lindlar's Cat.}}$ cis-R-CH=CH-R'
     • Dissolving Metal Reduction (Na or Li in liquid NH$_3$): Gives a trans-alkene.
       R-C≡C-R' + Na $\xrightarrow{\text{liq. NH}_3}$ trans-R-CH=CH-R'

2. Halogenation (Addition of X$_2$): Adds halogens (Cl$_2$, Br$_2$).

   • R-C≡C-R' + X$_2$ $\longrightarrow$ R-CX=CX-R' (vicinal dihalide alkene)
   • R-CX=CX-R' + X$_2$ $\longrightarrow$ R-CX$_2$-CX$_2$-R' (tetrahalide alkane)

3. Hydrohalogenation (Addition of HX): Adds HBr, HCl, HI. Follows Markovnikov's Rule for unsymmetrical alkynes.

   • R-C≡C-H + H-X $\longrightarrow$ R-C(X)=CH$_2$ (vinyl halide)
   • R-C(X)=CH$_2$ + H-X $\longrightarrow$ R-C(X)$_2$-CH$_3$ (geminal dihalide alkane)
   • Anti-Markovnikov: With HBr and peroxides, you get anti-Markovnikov addition.

4. Hydration (Addition of H$_2$O):

   • Mercury(II) Catalyzed Hydration: Uses H$_2$SO$_4$ and HgSO$_4$. This follows Markovnikov's rule, producing an enol which quickly tautomerizes into a ketone.
     R-C≡C-H + H$_2$O $\xrightarrow{\text{H}_2\text{SO}_4, \text{HgSO}_4}$ [R-C(OH)=CH$_2$] (enol) $\longrightarrow$ R-CO-CH$_3$ (ketone)
   • Hydroboration-Oxidation: Uses BH$_3$ (or R$_2$BH) followed by H$_2$O$_2$/NaOH. This is an anti-Markovnikov addition of water, producing an enol that tautomerizes into an aldehyde for terminal alkynes.
     R-C≡C-H + (sia)$_2$BH $\xrightarrow{\text{1. H}_2\text{O}_2, \text{NaOH}}$ [R-CH=CH-OH] (enol) $\longrightarrow$ R-CH$_2$-CHO (aldehyde)

5. Ozonolysis: Cleavage of the triple bond.

   • R-C≡C-R' $\xrightarrow{\text{1. O}_3 \text{ 2. H}_2\text{O}}$ R-COOH + R'-COOH (carboxylic acids)
   • If a terminal alkyne: R-C≡C-H $\xrightarrow{\text{1. O}_3 \text{ 2. H}_2\text{O}}$ R-COOH + CO$_2$ (carbon dioxide)

6. Alkyne Acidity and Alkylation:

   • Terminal alkynes can be deprotonated by strong bases (like NaNH$_2$) to form acetylide anions (R-C≡C$^- $).
   • These acetylide anions are excellent nucleophiles and can attack primary alkyl halides (R'-CH$_2$-X) in an S$_N$2 reaction to form new carbon-carbon bonds, extending the alkyne chain.
     R-C≡C-H $\xrightarrow{\text{NaNH}_2}$ R-C≡C$^- $Na$^+$ $\xrightarrow{\text{R'-CH}_2\text{X}}$ R-C≡C-CH$_2$-R'

3. Worked Example

Let's consider the reaction of 1-butyne (a terminal alkyne) with different reagents.

Reaction: 1-butyne (CH$_3$-CH$_2$-C≡C-H) undergoes hydration.

Scenario 1: Mercury(II) Catalyzed Hydration
Reagent: H$_2$SO$_4$, HgSO$_4$
Product Prediction: This is a Markovnikov addition, forming an enol that tautomerizes to a ketone. The OH will add to the more substituted carbon (C3), and the H to the less substituted carbon (C4).

CH$_3$-CH$_2$-C≡C-H + H$_2$O $\xrightarrow{\text{H}_2\text{SO}_4, \text{HgSO}_4}$ CH$_3$-CH$_2$-C(OH)=CH$_2$ (enol) $\longrightarrow$ CH$_3$-CH$_2$-CO-CH$_3$ (2-butanone)

Scenario 2: Hydroboration-Oxidation
Reagent: 1. (sia)$_2$BH (or BH$_3$/THF) 2. H$_2$O$_2$, NaOH
Product Prediction: This is an anti-Markovnikov addition, forming an enol that tautomerizes to an aldehyde. The OH will add to the less substituted carbon (C4), and the H to the more substituted carbon (C3).

CH$_3$-CH$_2$-C≡C-H + (sia)$_2$BH $\xrightarrow{\text{1. H}_2\text{O}_2, \text{NaOH}}$ CH$_3$-CH$_2$-CH=CH-OH (enol) $\longrightarrow$ CH$_3$-CH$_2$-CH$_2$-CHO (butanal)

4. Key Takeaways

• Alkynes contain a carbon-carbon triple bond with linear geometry around it, making them highly unsaturated.
• The sp-hybridized carbons of the triple bond lead to weak acidity for terminal alkynes, allowing deprotonation by