Grade 9
  1. Matter and its nature  1.1. Introduction to matter  1.2. Physical and chemical properties of matter  1.3. States of matter  1.3.1. Solid state  1.3.2. Fluid state  1.3.3. Gaseous state  1.3.4. Plasma and Bose–Einstein condensate  1.4. Changes in the states of matter  1.4.1. Melting and freezing  1.4.2. Evaporation and Condensation  1.4.3. Sublimation and deposition  1.5. Classification of matter  1.6. Elements, Compounds, and Mixtures  1.7. Pure substances and impurities  1.8. Separation Techniques  1.8.1. Filtration  1.8.2. Distillation  1.8.3. Crystallization  1.8.4. Chromatography  1.8.5. Centrifugation  1.8.6. Sublimation  1.8.7. Decantation and sedimentation
  2. Atomic Structure  2.1. Discovery of atoms  2.2. Dalton's atomic theory  2.3. Structure of the atom  2.4. Subatomic particles  2.5. Atomic number and mass number  2.6. Isotopes and Isobars  2.7. Electronic configuration  2.8. Bohr's atomic model  2.9. Modern Atomic Model (Quantum Mechanical Model - Introduction)  2.10. Valency and chemical bonding
  3. C hemical reactions and equations  3.1. Introduction to Chemical Reactions  3.2. Chemical Equation Formulation  3.3. Balancing Chemical Equations  3.4. Types of Chemical Reactions  3.4.1. Combination Reactions  3.4.2. Decomposition reactions  3.4.3. Displacement reactions  3.4.4. Double displacement reactions  3.4.5. Redox reactions  3.4.6. Endothermic and Exothermic Reactions  3.5. Effects of chemical reactions  3.6. Energy Changes in Chemical Reactions  3.7. Catalysts and their role in reactions
  4. Periodic table and periodicity  4.1. History of Periodic Classification  4.2. Mendeleev's periodic table  4.3. Modern Periodic Table  4.4. Periods and groups in the periodic table and periodicity  4.5. Trends in the Periodic Table  4.5.1. Atomic size  4.5.2. Ionization Energy  4.5.3. Electron affinity  4.5.4. Electronegativity  4.5.5. Metallic and Non-Metallic Properties  4.6. Noble gases and their properties
  5. Chemical bond  5.1. Introduction to Chemical Bonding  5.2. Types of chemical bonds  5.2.1. Ionic bond  5.2.2. Covalent bond  5.2.3. Metallic bond  5.2.4. Hydrogen bonding  5.2.5. Van der Waals force  5.3. Properties of Ionic and Covalent Compounds  5.4. Molecular Structure and Geometry (VSEPR Theory - Basics)
  6. Acids, Bases and Salts  6.1. Introduction to Acids and Bases  6.2. Properties of Acids  6.3. Properties of bases  6.4. pH scale and its importance  6.5. Indicators and their uses  6.6. Neutralization reactions  6.7. Strength of Acids and Bases (Strong vs. Weak)  6.8. Preparation of salts  6.9. Uses of acids, bases and salts in daily life
  7. Metals and Nonmetals  7.1. Physical Properties of Metals  7.2. Physical Properties of Nonmetals  7.3. Chemical properties of metals  7.4. Chemical Properties of Nonmetals  7.5. Reactivity Series of Metals  7.6. Extraction of metals  7.7. Corrosion and its prevention  7.8. uses of metals and nonmetals
  8. Carbon and its compounds  8.1. Introduction to Carbon  8.2. Allotropes of carbon (diamond, graphite, fullerene)  8.3. Hydrocarbons  8.3.1. Hydrocarbons  8.3.2. Alkene  8.3.3. Alkynes  8.4. Functional groups in organic chemistry  8.5. Isomerism in organic compounds  8.6. Alcohols and Carboxylic Acids  8.7. Soaps and detergents  8.8. Polymers (Introduction to Natural and Synthetic Polymers)
  9. Solutions and Mixtures  9.1. Types of mixtures  9.2. Homogeneous and Heterogeneous Mixtures  9.3. Concentration of solutions  9.4. Solubility and factors affecting solubility  9.5. Suspensions and Colloids  9.6. Saturated, unsaturated and supersaturated solutions
  10. Air and atmosphere  10.1. Composition of air  10.2. Role of gases in the atmosphere  10.4. Acid rain and its effects  10.5. Greenhouse effect and global warming  10.6. Ozone layer and its depletion  10.7. Air pollution control measures
  11. Water and its importance  11.1. Composition and properties of water  11.2. Hardness of water (temporary and permanent hardness)  11.3. Causes of water pollution  11.4. Effects of Water Pollution  11.5. Water purification methods (filtration, boiling, chlorination)
  12. Industrial Chemistry  12.1. Introduction to Industrial Chemistry  12.2. Important industrial chemicals (sulfuric acid, ammonia, sodium hydroxide)  12.3. Chemical processes in industry  12.4. Uses of Industrial Chemistry in Daily Life  12.5. Environmental impact of industrial chemical processes

Grade 9 → Carbon and its compounds → Hydrocarbons ↓


Alkynes


Introduction to alkynes

In chemistry, alkynes are a fascinating group of hydrocarbons. These are organic compounds composed exclusively of carbon and hydrogen atoms. Alkynes form an important class of hydrocarbons, characterized by the presence of at least one carbon-carbon triple bond. We will explore their structure, properties, and uses in detail in this article.

What are alkynes?

Alkynes are hydrocarbons containing a carbon-carbon triple bond, represented by the general molecular formula:

C n H 2n-2

Here, n is the number of carbon atoms in the molecule. This formula shows that alkynes have two hydrogen atoms less than alkenes, which have carbon-carbon double bonds, while both have fewer hydrogen atoms than alkenes with the same carbon content.

Structure of alkynes

The unique structure of alkynes is essential to their properties. The triple bond is a combination of one sigma (σ) bond and two pi (π) bonds. Sigma bonds form from the end-to-end overlap of sp hybrid orbitals, while pi bonds develop from the side-to-side overlap of unhybridized p orbitals.

This triple bond creates a linear structure for alkynes around the bonded carbon. The bond angles are approximately 180 degrees, which is very different from the tetrahedral structure of alkenes or the planar triangular structure of alkenes.

C≡C H H

Naming of alkynes

Naming of alkynes follows the IUPAC nomenclature system like other hydrocarbons. The steps are as follows:

  1. Identify the longest carbon chain containing a triple bond. This is the parent chain.
  2. Number the chain starting from the end nearest the triple bond.
  3. Use the appropriate prefix for the number of carbon atoms and the suffix "-yne" to indicate the presence of a triple bond.
  4. Indicate the position of the triple bond by writing the number of the first carbon included in the first bond in the name of the compound.

For example, CH≡CH is called ethene. It is also commonly known as acetylene.

Examples of alkynes

Let's look at some examples of alkynes:

  • Propyne: This alkyne, with the formula C 3 H 4, is composed of three carbon atoms, with a triple bond between the first and second carbon atoms. Its structural formula is CH≡C−CH 3.
  • 1-Butyne: This compound has the formula C 4 H 6 and has a triple bond between the first and second carbon atoms. Its structure is CH≡C−CH 2 −CH 3.

Properties of alkynes

1. Physical properties

  • Similar to alkynes and alkenes, alkynes are generally nonpolar due to the presence of only carbon and hydrogen.
  • They are insoluble in water, but soluble in organic solvents such as acetone and benzene.
  • The boiling points of alkynes are generally higher than those of alkenes and alkenes with the same carbon number because of increased London dispersion forces between the molecules due to the linear structure.

2. Chemical properties

Alkynes exhibit some unique chemical reactions due to the presence of triple bonds, which are highly reactive. They undergo addition reactions, which may be complete or partial depending on the reacting agents.

  • Hydration: Alkynes react with water in the presence of sulfuric acid and mercury(II) sulfate catalyst to form aldehydes or ketones.
  • Halogenation: When halogens (such as Cl2 or Br2) are added, the triple bond in alkynes is broken, forming tetrahalo compounds.
  • Hydrogenation: Alkynes can be converted into alkenes through the process of hydrogenation, where hydrogen is added through the triple bond in the presence of a catalyst such as palladium or nickel.

Uses of alkynes

Alkynes have many industrial and commercial applications:

  • Acetylene: This is one of the most important alkynes and is commonly used in welding and cutting due to its ability to produce a high-temperature flame when burned with oxygen.
  • Plastics and synthetic fiber manufacturing: Alkynes serve as a building block in the production of polymers and synthetic fibers.
  • Pharmaceuticals: Many drugs and medicines are synthesized using alkynes as a precursor or functional group.

Reactivity of alkynes

Alkynes are generally more reactive than alkenes due to the presence of the triple bond. The large amount of energy stored in this bond leads them to seek stability through reactions.

An interesting feature of the alkyne group is its acidity which is higher than that of alkene and alkene. The hydrogen atoms bound to the carbon atoms involved in the triple bond can be removed by a strong base, forming the acetylide ion.

Synthesis of alkynes

Alkynes can be synthesized by various methods, the most common of which is the dehydrohalogenation of dihaloalkanes. In this, a molecule of hydrogen halide (HCl, HBr, etc.) is removed from the adjacent carbon atoms of the dihalo to form an alkyne.

CH 2 Br−CH 2 Br → CH≡CH

Conclusion

Alkynes are a fascinating and important group of hydrocarbons that have unique structures and properties. Their appearance, behavior in chemical reactions, and roles in various applications make them an important topic in the study of chemistry. Understanding alkynes enhances our knowledge of organic chemistry and its many uses in everyday life.


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Alkynes

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