Content Curator
Physical properties of alkynes (unsaturated hydrocarbons) are due to the triple bond between carbon atoms. Alkynes are divided into two categories: terminal and internal. Terminal alkynes are triple-bonded compounds with a carbon atom sharing the triple bond with the carbon at the chain's termination. Internal alkynes are compounds that have a triple bond between two carbon atoms that are not terminal. Alkynes have the typical molecular formula CnH2n-2. Alkynes have a lot in common with alkenes in terms of physical qualities.
Also Read: Alkanes
| Table of Contents |
Key Terms: Alkynes, Hydrocarbons, sp Hybridisation, Boiling, Melting, Triple Bond, Isomerism, IUPAC Nomenclature
Physical Properties of Alkynes
[Click Here for Sample Questions]
Listed below are some of the important physical properties of alkynes:
- Alkynes have properties that are similar to those of alkanes and alkenes.
- Unsaturated carbon with a triple bond at the carbon site is known as alkynes.
- The presence of sp hybridization in alkenes is responsible for its unique structure.
- With the exception of ethylene, which has a strong odor, all alkynes are odorless and colorless.
- The first three alkynes are gases, followed by the next eight alkynes that are liquids. All alkynes with a number greater than eleven are solids.
- Alkynes have a slight polarity. This makes them more acidic in comparison to alkanes and alkenes.
- Alkynes are highly insoluble in water and highly soluble in polar solvents.
- The boiling and melting points of alkynes increases as their molecular structure becomes more complex.
- Due to the one extra bond at the carbon site, the boiling temperatures of alkynes are slightly higher than those of their corresponding alkenes.
- The force of repulsion between the electrons in alkenes is very high causing the compounds to be highly unstable in nature. These compounds also produce a high amount of energy.
Structure of Alkynes
Also Read:
| Huckel Rule of Aromaticity | Hydrocarbons Important Questions | Boiling Point |
| Melting Point | Intermolecular Forces | Change of State |
Chemical Properties of Alkynes
[Click Here for Sample Questions]
The chemical properties of alkynes are listed as follows:
- Alkynes are slightly acidic in nature.
- Alkynes are slightly electronegative in their natural state.
- Alkynes have triple bonded carbon atoms that are sp hybridized, whereas alkanes have single bond atoms that are sp3 hybridized. This results in an electronegativity difference making it easier for alkynes to attract the shared electron pair of the C-H bond.
- When a strong base like NaNH2 reacts with ethyne, sodium acetylide and hydrogen (H2) gas are produced. Alkanes and alkenes, on the other hand, do not undergo such reactions.
- The hydrogen atoms connected to the carbon-carbon triple bond in alkynes are slightly acidic in nature.
Also Read: Solvent Chemistry
Nomenclature of Alkynes
[Click Here for Sample Questions]
- Common System: Different members of alkynes are named in the form of acetylene derivatives.
- IUPAC System: The nomenclature of alkynes is quite similar to the nomenclature of alkanes, according to the IUPAC standard. As a result, the name is nearly identical, with the exception that the suffix "yne" replaces the suffix "ane" of the comparable alkanes. Furthermore, the counting of positions begins with the first triple-bonded carbon.
Isomerism in Alkynes
[Click Here for Sample Questions]
Members of the alkyne group, ethyne and propyne, have a single structure. The upper members of this group, on the other hand, have more than one structure. The compound butyne, for example, has two distinct structures.
Isomerism in Alkynes
Pentyne also has three distinct isomers. As a result, the compound pentyne has both chain and position isomerism. From the figure above we see that Pentyne has three isomers:
- Pent-1-yne
- Pent-2-yne
- 3-Methylbut-1-yne
Position isomerism is seen in pent-1-yne and pent-2-yne, chain isomerism in pent-2-yne and 3-Methylbut-1-yne.
- Functional Isomerism: Alkynes show functional isomerism in the form of dienes like but-1-yne and but-1,2-diene. Functional isomerism can also be found in but-1-yne and but-1,3-diene.
- Ring-Chain Isomers: Alkynes display ring-chain isomerism when compared to cycloalkanes. Propyne and cyclopropene are a few examples.
Also Read: Cis- Trans-Isomers
Things to Remember
- The triple bond property aids in explaining the characteristics and bonding in alkynes.
- The uniqueness of the alkyne structure is enabled by hybridization due to triple bonds.
- Alkynes have a nonpolar bonding strength, are linear, and are acidic owing to the triple bond.
- Organic solvents dissolve alkynes.
- Alkynes have a higher boiling point than alkanes and alkenes and possess a slight electronegative nature.
- The IUPAC system is followed in naming alkynes.
- Alkynes show isomerism: functional, ring chain or positional isomerism
Also Read:
| Carbon & its Compounds | Unsaturated Hydrocarbons | Saturated Hydrocarbons |
| Sigma & Pi Bonds | Hydrogen Bonding | Nomenclature of Functional Groups |
Sample Questions
Ques. What is Isomerism? What are its types? (3 marks)
Ans. The phenomenon of isomerism occurs when two or more compounds have the same molecular formula but different physical and chemical characteristics. Isomerism can be classified into two kinds.
- Structural Isomerism
- Stereoisomerism
Ques. Do alkynes undergo a halogenation reaction? (3 marks)
Ans. Alkynes can easily undergo hydration reactions under the right conditions (temperature and pressure). When alkynes react with halogens, hydrogen, and other similar elements, they produce a saturated molecule. Two atoms of H2 or halides or halogens can be added to their structure since they have a triple bond.
Ques. Explain Polymerization in Alkynes. (3 marks)
Ans. Under the right conditions, alkynes can polymerize linearly and cyclically. Polymerization occurs, resulting in molecules with a larger molecular weight than the initial alkyne. Ethyne, for example, will polymerize to produce polyacetylene or polyethylene (with a larger molecular weight).
High temperatures and the availability of a catalyst are required for cyclic polymerization. As in putting ethyne through a red-hot iron tube at a temperature of at least 877K to produce benzene.
Ques. What is the importance of acetylene? (3 marks)
Ans. Following are the important uses of acetylene:
- Soldering metals and arc welding
- Glass industry
- Synthetic rubber manufacturing
- Food preservative
- Manufacturing of acetic acid
- Metal precipitation
Ques. What is stereoisomerism? (3 marks)
Ans. In stereoisomerism, the compounds have the same chemical formula but differ in their spatial arrangement of atoms/groups of atoms. It is divided into two distinct subclasses:
- Configurational isomerism
- Conformational isomerism
Geometrical and optical isomerism are two other types of configurational isomerism.
Ques. How is alkyne prepared from Vicinal Dihalide? (3 marks)
Ans. The dehydrohalogenation reaction occurs when vicinal dihalides are treated with potassium hydroxide (alcoholic). In this process, a hydrogen halide molecule is removed, resulting in the formation of alkenyl halide. Alkenyl halide is also converted to alkyne when it is treated with sodamide.
Ques. How is alkyne prepared from Calcium carbide? (3 marks)
Ans. Ethyne is made in an industrial setting. Calcium carbide is treated with water to produce ethyne in this process. Calcium carbide can be made by heating quicklime and coke together. Quicklime can be made by heating limestone. Following are the reactions involved in its manufacturing:
Also Read:
| Chapter Related Articles | ||
|---|---|---|
| Chemical Reactions & Equations | Oxidizing Agent | Aliphatic Hydrocarbons |
| Reducing Agent | Preparation of Alkynes | Addition Reaction of Alkynes |
Also Read:
Comments