What is the triple bond in alkynes its structure hybridization and reactions
Alkynes are the organic molecule that contains triple bonds between the carbon atoms. Its general formula is CnH2n-2. They are also known as acetylenes. In this article, we will deal with the structure of alkynes. Alkynes are the most common term studied by the students in general organic chemistry.
Structure of Alkynes
(Image will be uploaded soon)
Triple bond in alkynes shows the following features:
The hybridization of triple bonded carbon in alkyne is SP.
The bond angle between the two SP hybridised carbon is 180 degrees.
The bond length of the triple bond in alkynes is 121 picometer.
Cyclic alkynes exist rarely in nature.
The bond strength of alkyne is highest among the saturated (alkanes) and unsaturated hydrocarbons (alkenes and alkynes).
Triple bond of alkynes is made up of one sigma and two pi bonds.
Properties of Alkynes
Physical Properties
Alkynes are non-polar, unsaturated hydrocarbons.
Alkynes are highly soluble in organic and non-polar solvents and slightly soluble in polar solvents.
Compared to other hydrocarbons like alkanes and alkenes, alkynes have a high boiling point.
Alkynes in a reaction release a high amount of energy due to the repulsion of electrons.
Alkynes are more acidic than alkanes and alkenes due to SP hybridisation.
Chemical Properties
The triple bond in alkynes makes it an unstable molecule. Due to its instability, it becomes reactive and undergoes several reactions.
Hydrogenation - Alkynes undergoes two types of hydrogenation reactions. Complete hydrogenation (in presence of Pd-C/ H2) and partial hydrogenation (in presence of Linder’s catalyst/H2).
(Image will be uploaded soon)
It can act as a strong nucleophile by converting into acetylide.
(Image will be uploaded soon)
Alkynes can react with BH3 and undergo hydroboration reactions to form aldehydes and ketones.
(Image will be uploaded soon)
Alkynes undergo halogenation reactions in the presence of different halogenating agents by different mechanisms and forms haloalkanes.
(Image will be uploaded soon)
Did You Know?
Alkynes are commonly used as fruits ripener.
Most of the alkynes are used for making organic solvent.
Alkynes are used as rocket fuel.
PVC is a polymer made up of an alkyne.
FAQs on Triple Bond in Alkynes Structure and Bonding Explained
1. What is a triple bond in alkynes?
A triple bond in alkynes is a covalent bond between two carbon atoms consisting of one sigma (σ) bond and two pi (π) bonds. This type of bond is represented as C≡C.
- Found only in alkynes, which are unsaturated hydrocarbons.
- The presence of a C≡C bond makes alkynes more reactive than alkanes.
- Example: ethyne (C2H2) has the structure H–C≡C–H.
2. How is a triple bond formed in alkynes?
A triple bond in alkynes is formed by the overlap of sp hybrid orbitals for the sigma bond and side-by-side overlap of unhybridized p orbitals for two pi bonds.
- Each carbon in the triple bond undergoes sp hybridization.
- One sp–sp overlap forms the σ bond.
- Two p–p sideways overlaps form the two π bonds.
3. What is the bond length of a triple bond in alkynes?
The bond length of a C≡C triple bond in alkynes is approximately 1.20 Å (angstroms).
- It is shorter than a double bond (C=C ≈ 1.34 Å).
- It is much shorter than a single bond (C–C ≈ 1.54 Å).
- Shorter bond length indicates stronger bond strength.
4. Why are alkynes more reactive than alkanes?
Alkynes are more reactive than alkanes because the two pi (π) bonds in the triple bond are weaker and more exposed than sigma bonds.
- Pi bonds are easily broken in chemical reactions.
- Alkynes undergo addition reactions such as hydrogenation and halogenation.
- Example (hydrogenation): C2H2(g) + 2H2(g) → C2H6(g) (in presence of catalyst).
5. What is the general formula of alkynes with a triple bond?
The general formula of alkynes containing one triple bond is CnH2n−2 (for open-chain alkynes).
- n ≥ 2
- Example: For n = 2 → C2H2 (ethyne).
- Example: For n = 3 → C3H4 (propyne).
6. What is the bond angle in alkynes with a triple bond?
The bond angle in alkynes is 180° because the carbon atoms involved in the triple bond are sp hybridized.
- sp hybridization results in a linear geometry.
- Example: In ethyne (C2H2), the H–C≡C–H structure is linear.
- The 180° angle minimizes electron pair repulsion.
7. What is the difference between a double bond and a triple bond?
The main difference is that a double bond (C=C) has one sigma and one pi bond, while a triple bond (C≡C) has one sigma and two pi bonds.
- Double bond: sp2 hybridization, bond angle ≈ 120°.
- Triple bond: sp hybridization, bond angle = 180°.
- Triple bonds are shorter and stronger than double bonds.
8. What are terminal and internal alkynes?
A terminal alkyne has the triple bond at the end of the carbon chain, while an internal alkyne has the triple bond between two carbon atoms inside the chain.
- Terminal alkyne example: propyne (CH3–C≡CH).
- Internal alkyne example: 2-butyne (CH3–C≡C–CH3).
- Terminal alkynes are slightly acidic due to the acidic hydrogen attached to sp carbon.
9. How do alkynes undergo addition reactions at the triple bond?
Alkynes undergo addition reactions by breaking the two pi bonds of the triple bond and adding atoms across the C≡C bond.
- Hydrogenation: C2H2(g) + H2(g) → C2H4(g) (partial, with Lindlar catalyst).
- Further hydrogenation: C2H4(g) + H2(g) → C2H6(g).
- Halogenation: C2H2(g) + Br2(l) → C2H2Br2.
10. Why are terminal alkynes acidic?
Terminal alkynes are acidic because the hydrogen attached to an sp-hybridized carbon is more easily removed due to high s-character (50%).
- Greater s-character increases electronegativity of carbon.
- The conjugate base (acetylide ion) is stabilized.
- Example reaction: 2HC≡CH + 2Na → 2HC≡CNa + H2(g).
