The primary distinction between internal and terminal alkynes lies in the position of the carbon-carbon triple bond within the molecular structure and the types of atoms or groups attached to the carbons of that triple bond.
Understanding Alkynes: The Basics
Alkynes are a class of unsaturated hydrocarbons characterized by the presence of at least one carbon-carbon triple bond (C≡C). This triple bond consists of one sigma (σ) bond and two pi (π) bonds, making the molecule highly reactive due to the exposed pi electrons. Depending on where this triple bond is located within the carbon chain, alkynes are categorized as either terminal or internal.Terminal Alkynes: At the End of the Chain
Terminal alkynes are compounds where the C≡C unit is positioned at the **end** of the carbon chain. This means that at least one of the sp-hybridized carbons involved in the triple bond is directly attached to a hydrogen atom.- Structure: They are either monosubstituted (R-C≡C-H), where R represents an alkyl or aryl group, or unsubstituted (H-C≡C-H), which is known as ethyne (common name: acetylene). The triple bond is thus "at the end of the structure."
- Key Characteristics:
- Acidic Proton: The most significant feature of terminal alkynes is the presence of an acidic hydrogen atom directly bonded to the sp-hybridized carbon of the triple bond. This acidity arises from the high s-character (50%) of the sp hybrid orbital, which makes the C-H bond more polar and the proton more easily removed by strong bases.
- Reactivity: Due to their acidic proton, terminal alkynes can react with strong bases (e.g., sodium amide, NaNH₂, or Grignard reagents) to form nucleophilic acetylide anions (R-C≡C⁻). These anions are extremely useful in organic synthesis for forming new carbon-carbon bonds.
- Examples:
- Ethyne (Acetylene): H-C≡C-H
- Propyne: CH₃-C≡C-H
- But-1-yne: CH₃CH₂-C≡C-H
Internal Alkynes: Within the Structure
Internal alkynes are compounds where the C≡C unit is located **within** the carbon chain, meaning it is not at an end. Both carbons of the triple bond are bonded to other carbon atoms (alkyl or aryl groups), and no hydrogen atoms are directly attached to the sp-hybridized carbons of the triple bond.- Structure: They are disubstituted alkynes (R-C≡C-R'), where R and R' represent alkyl or aryl groups. The C≡C unit is thus "inside the structure."
- Key Characteristics:
- No Acidic Proton: Unlike terminal alkynes, internal alkynes do not have hydrogen atoms directly bonded to the sp-hybridized carbons of the triple bond. Consequently, they do not exhibit the same acidic properties.
- Stability: Internal alkynes are generally more stable than their terminal isomers. This increased stability is often attributed to the electron-donating effect of the two alkyl or aryl groups through hyperconjugation, which helps to stabilize the triple bond.
- Examples:
- But-2-yne: CH₃-C≡C-CH₃
- Pent-2-yne: CH₃-C≡C-CH₂CH₃
- Hex-3-yne: CH₃CH₂-C≡C-CH₂CH₃
Comparative Summary: Terminal vs. Internal Alkynes
The following table summarizes the key differences between terminal and internal alkynes:
| Feature | Terminal Alkynes | Internal Alkynes |
|---|---|---|
| Structure | R-C≡C-H or H-C≡C-H (triple bond at chain end) | R-C≡C-R' (triple bond within chain) |
| Substituents | Monosubstituted (one R group) or unsubstituted | Disubstituted (two R/R' groups) |
| Acidic Proton | Yes (the H attached to the triple bond carbon) | No (no H directly on triple bond carbons) |
| Reactivity | Can form acetylide anions; more reactive in deprotonation reactions | Does not form acetylide anions; generally more stable |
| Nomenclature | "-1-yne" suffix is characteristic (e.g., But-1-yne) | "-x-yne" where 'x' is > 1 (e.g., But-2-yne) |
| Example | Propyne (CH₃-C≡C-H) | But-2-yne (CH₃-C≡C-CH₃) |
