Triple Bond in Alkynes

What are Alkynes?

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

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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.

  1. 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).

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  1. It can act as a strong nucleophile by converting into acetylide. 

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  1. Alkynes can react with BH3 and undergo hydroboration reactions to form aldehydes and ketones.

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  1. Alkynes undergo halogenation reactions in the presence of different halogenating agents by different mechanisms and forms haloalkanes.

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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 (Frequently Asked Questions)

Question 1: Give Five Properties of Alkynes.

Answer: Five properties of alkynes are given below:

  1. Alkynes are non-polar, unsaturated hydrocarbons.

  2. Alkynes are highly soluble in organic and non-polar solvents and slightly soluble in polar solvents.

  3. Compared to other hydrocarbons like alkanes and alkenes, alkynes have a high boiling point.

  4. Alkynes in a reaction release a high amount of energy due to the repulsion of electrons.

  5. Alkynes are more acidic than alkanes and alkenes due to SP hybridisation.

Question 2: Explain the Bond Structure of Alkynes.

Answer: In alkynes, a triple bond exists between carbon. The hybridization of triple bonded carbon in alkyne is SP. Bond angle between the two SP hybridised carbon is 180 degrees. The bond length of the triple bond in alkynes is 121 picometer.