Understanding the Structure, Bonding, and Reactivity of 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.
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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It can act as a strong nucleophile by converting into acetylide.
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Alkynes can react with BH3 and undergo hydroboration reactions to form aldehydes and ketones.
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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 on Triple Bond in Alkynes: Complete Guide for Students
1. What exactly is a triple bond in an alkyne?
A triple bond is a type of chemical bond where two carbon atoms share three pairs of electrons. In alkynes, this bond consists of one strong sigma (σ) bond and two weaker pi (π) bonds. This makes the bond very strong and electron-rich, defining the properties of alkynes.
2. How can I identify and name an alkyne based on its structure?
You can identify an alkyne by looking for a carbon-carbon triple bond (C≡C) in its structure. To name it according to IUPAC rules, you find the longest carbon chain containing the triple bond and change the suffix of the parent alkane from '-ane' to '-yne'. A number is used to indicate the starting position of the triple bond.
3. What is the general formula used for alkynes?
The general formula for acyclic alkynes (hydrocarbons with one triple bond and no rings) is CnH2n-2. In this formula, 'n' represents the number of carbon atoms, which must be two or more.
4. Can you give some common examples of alkynes?
Of course! The simplest and most well-known alkyne is ethyne (C₂H₂), which is commonly called acetylene. Other basic examples include:
- Propyne (C₃H₄)
- But-1-yne (C₄H₆)
- But-2-yne (C₄H₆)
5. How many sigma and pi bonds make up a carbon-carbon triple bond?
A carbon-carbon triple bond is always made of one sigma (σ) bond and two pi (π) bonds. The sigma bond lies directly between the two carbon atoms, while the two pi bonds are located above, below, and to the sides of the sigma bond.
6. How does the triple bond influence the shape of an alkyne molecule?
The triple bond forces the two carbon atoms and the atoms directly attached to them to lie in a straight line. This is because the carbon atoms in the triple bond are sp hybridised, which results in a linear geometry with a bond angle of 180°.
7. Why are alkynes considered more acidic than alkanes and alkenes?
The acidity of a terminal alkyne (where the triple bond is at the end of the chain) is due to the sp hybridisation of its carbon atom. This hybrid orbital has 50% 's-character,' making the carbon atom very electronegative. This allows it to stabilize the negative charge better when it loses a hydrogen ion (H⁺), making it more acidic compared to sp² or sp³ hybridised carbons in alkenes and alkanes.
8. What makes the triple bond in an alkyne a reactive part of the molecule?
The triple bond is an area of high electron density because of the two relatively weak pi (π) bonds. These pi electrons are more exposed and less tightly held than the sigma electrons. This makes them easily available to react with other atoms or molecules, particularly in addition reactions.
