Grade 11

Grade 11Hydrocarbons


alkene


In the world of chemistry, hydrocarbons are particularly fascinating compounds made of hydrogen and carbon. Among the different types of hydrocarbons, alkanes stand out due to their unique chemical properties. The aim of this lesson is to provide a detailed description of alkanes, their structure, properties, and importance in both academic studies and practical applications.

What are alkenes?

Alkanes are types of hydrocarbons that contain at least one carbon-carbon double bond ( -C=C- ). This double bond is the defining characteristic that distinguishes alkanes from other types of hydrocarbons such as alkenes, which contain only single bonds, and alkynes, which contain triple bonds. Because of the double bond, alkanes are also known as unsaturated hydrocarbons.

Chemical structure of alkenes

The general molecular formula of an alkene is C n H 2n . For example, if an alkene has four carbon atoms, following the formula will give C 4 H 8 The simplest alkene is ethene, also known as ethylene, which has the chemical formula C 2 H 4 .

      HH
       ,
        C=C
       ,
      HH
    

Nomenclature of alkenes

The naming of alkenes follows the IUPAC nomenclature system, where the suffix "-ene" is used to indicate the presence of a double bond. The location of the double bond must also be specified in the name. Here is a step-by-step guide to naming alkenes:

  1. Identify the longest carbon chain that contains a double bond. This becomes the parent chain.
  2. Number the carbon atoms in the parent chain starting at the end nearest the double bond.
  3. Use the number of the first carbon atom involved in the double bond as a locator before the radical name.
  4. List the prefixes alphabetically and numbered, if present.

As an example, consider the following compound:

        CH 3
         ,
      CH 3 -CH-CH=CH-CH 2 -CH 3
    

The longest chain with a double bond has five carbon atoms, making it "pentene." Since the double bond starts at the second carbon, the compound is named 2-methyl-2-pentene.

Isomerism in alkynes

Alkenes show two main types of isomerism:

Structural isomerism

This happens when the molecular formula of the alkene is the same but the structural arrangement of the atoms is different. For example, C 4 H 8 can be but-1-ene or but-2-ene.

    But-1-ene: CH 2 =CH-CH 2 -CH 3
    But-2-ene: CH3 -CH=CH- CH3
    

Cis-trans isomerism

This type of isomerism is typical for alkenes because of the restricted rotation around the double bond. In cis isomers, the substituents are on the same side of the double bond. In trans isomers, they are on opposite sides. Let's consider but-2-ene:

    cis-but-2-ene:
          CH 3 H
           ,
        HC=C-CH 3
          
    trans-but-2-ene:
          H CH 3
           ,
        CH 3 -C=CH
    

The double bond prevents free rotation in alkenes, which gives rise to specific configuration and properties in them.

Physical properties of alkenes

The physical properties of alkenes are similar to those of alkanes, but some differences are worth noting:

  • Boiling point: The boiling point of alkenes is generally similar to that of alkenes of comparable size, affected slightly by the presence of the double bond. The increased polarity from the double bond can lead to a slightly higher boiling point.
  • Solubility: Like alkanes, alkenes are nonpolar and insoluble in water, but are soluble in other nonpolar solvents.
  • Density: Alkenes are less dense than water, which means they would float on water if they were liquid at room temperature.

Chemical properties of alkenes

The presence of the double bond makes alkenes more reactive than alkanes. This double bond can undergo different reactions, some of which are as follows:

Hydrogenation

In this reaction, hydrogen gas (H 2) is added to the double bond, converting it into a single bond. This reaction is used industrially to convert unsaturated fats into saturated fats.

    CH 2 =CH 2 + H 2 → CH 3 -CH 3
    

Halogenation

Alkenes react with halogens (such as Cl 2 or Br 2) to form dihaloalkanes.

    CH 2 =CH 2 + Br 2 → CH 2 Br-CH 2 Br
    

Hydration

In the presence of an acidic catalyst, water can be added to an alkene to form an alcohol:

    CH 2 =CH 2 + H 2 O → CH 3 -CH 2 OH
    

Synthesis of alkenes

Alkenes can be synthesized in several ways:

Dehydration of alcohol

Alcohols can be converted to alkane by removing a water molecule. This usually requires an acid catalyst and heat:

    CH 3 -CH 2 OH → CH 2 =CH 2 + H 2 O
    

Cracking of alkenes

Larger alkanes can be broken down into smaller alkanes through cracking, a process used in the petroleum industry.

Applications and importance of alkenes

Alkenes play an important role in both nature and industry:

  • Polymerization: Alkenes are the building blocks of many polymers, such as polyethylene and polypropylene, which are important in the manufacture of plastics.
  • Pheromones: Many insects use alkanes as pheromones for communication.
  • Natural products: Alkanes are found in a variety of essential compounds in plants and animals, such as vitamins and hormones.

Conclusion

Alkenes, with their characteristic carbon-carbon double bonds, are a distinctive and important class of hydrocarbons. Their unique reactivity and physical properties make them important in a wide range of chemical processes and industries. Understanding the structure and behavior of alkenes is important in many areas of chemistry, from organic synthesis to the development of new materials and the study of biological systems.


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