Preparation of Alkenes
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Questions and Answers

What is the general formula for the alkene family?

  • CnH2n (correct)
  • CnH2n-1
  • CnH2n-2
  • CnH2n+1

    • What is the distinguishing feature of the alkene structure?

  • Carbon-hydrogen bond
  • Carbon-carbon triple bond
  • Carbon-carbon single bond
  • Carbon-carbon double bond (correct)

    • What is the strongest bond in the carbon-carbon double bond?

  • π bond
  • σ bond (correct)
  • Hydrogen-hydrogen bond
  • Carbon-hydrogen bond

    • What is the name of the simplest member of the alkene family?

    <p>Ethylene</p> Signup and view all the answers

    According to the IUPAC system, what is the parent structure of an alkene named?

    <p>The longest continuous chain that contains the carbon-carbon double bond</p> Signup and view all the answers

    What is the ending of the alkane name changed to when naming an alkene?

    <p>-ane to -ene</p> Signup and view all the answers

    How is the position of a double bond designated in an alkene?

    <p>By the number of the first doubly-bonded carbon</p> Signup and view all the answers

    What takes precedence over alkyl groups and halogens in determining the main carbon chain and the direction in which it is numbered?

    <p>The presence of a carbon–carbon double bond</p> Signup and view all the answers

    What is a characteristic of the physical properties of alkenes?

    <p>They are insoluble in water, but soluble in nonpolar solvents</p> Signup and view all the answers

    What is the reason for cis, trans isomerism in alkenes?

    <p>Restricted rotation about a carbon–carbon double bond</p> Signup and view all the answers

    What is the difference between cis-2-butene and trans-2-butene?

    <p>The arrangement of methyl groups and hydrogen atoms around the double bond</p> Signup and view all the answers

    Why are cis-2-butene and trans-2-butene different compounds?

    <p>Because they cannot be converted into one another at room temperature</p> Signup and view all the answers

    What is the process of converting alkyl halides into alkenes?

    <p>Dehydrohalogenation</p> Signup and view all the answers

    What is the primary product when sec-Butyl chloride undergoes dehydrohalogenation?

    <p>A mixture of 1-butene and 2-butene</p> Signup and view all the answers

    What is the role of the hydroxide ion in the dehydrohalogenation mechanism?

    <p>To pull a hydrogen ion away from carbon</p> Signup and view all the answers

    What is the difference between the breaking of C-H and C-X bonds in dehydrohalogenation?

    <p>The bonds break in an unsymmetrical fashion</p> Signup and view all the answers

    What is the general formula for alcohols?

    <p>ROH</p> Signup and view all the answers

    What is the second method of preparing alkenes mentioned in the text?

    <p>Dehydration of alcohols</p> Signup and view all the answers

    What is required for dehydration to occur?

    <p>The presence of an acid and the application of heat</p> Signup and view all the answers

    What is the role of alumina in dehydration?

    <p>It acts as a Lewis acid</p> Signup and view all the answers

    What is a Lewis acid?

    <p>A species that accepts a pair of electrons</p> Signup and view all the answers

    What is the first step in the mechanism of dehydration of alcohols?

    <p>The union of the alcohol with a hydrogen ion</p> Signup and view all the answers

    What is lost in the second step of the dehydration mechanism?

    <p>H2O</p> Signup and view all the answers

    What is the final product of the dehydration mechanism?

    <p>The alkene</p> Signup and view all the answers

    What is the driving force behind the rearrangement of a carbonium ion?

    <p>The formation of a more stable carbonium ion</p> Signup and view all the answers

    What is the result of the dehalogenation of vicinal dihalides?

    <p>The formation of alkenes</p> Signup and view all the answers

    What is the type of migration where a hydrogen atom moves with a pair of electrons?

    <p>Hydride shift</p> Signup and view all the answers

    What is the outcome of the reduction of an alkyne to the double-bond stage?

    <p>The formation of either a cis-alkene or a trans-alkene</p> Signup and view all the answers

    Why does the 3,3-dimethyl-2-butyl cation rearrange to the 2,3-dimethyl-2-butyl cation?

    <p>To form a more stable carbonium ion</p> Signup and view all the answers

    Which type of carbonium ion is the most stable?

    <p>Tertiary carbonium ion</p> Signup and view all the answers

    A carbonium ion can rearrange to form a less stable carbonium ion.

    <p>False</p> Signup and view all the answers

    The dehalogenation of vicinal dihalides is a method for the synthesis of alkanes.

    <p>False</p> Signup and view all the answers

    A hydride shift is a migration of an alkyl group with a pair of electrons.

    <p>False</p> Signup and view all the answers

    Reduction of an alkyne can yield only a cis-alkene.

    <p>False</p> Signup and view all the answers

    The 2-methyl-1-butyl cation rearranges to the 2-methyl-2-butyl cation.

    <p>True</p> Signup and view all the answers

    Alkyl halides can be converted into alkanes through dehydrohalogenation.

    <p>False</p> Signup and view all the answers

    The 3,3-dimethyl-2-butyl cation rearranges to the 2,3-dimethyl-1-butyl cation.

    <p>False</p> Signup and view all the answers

    Isomeric alkenes can be formed in the dehalogenation of vicinal dihalides.

    <p>True</p> Signup and view all the answers

    Tertiary carbonium ions are less stable than primary carbonium ions.

    <p>False</p> Signup and view all the answers

    An alkyl shift is a migration of hydrogen with a pair of electrons.

    <p>False</p> Signup and view all the answers

    Study Notes

    Alkenes

    • Alkenes are unsaturated hydrocarbons with the general formula CnH2n.
    • The simplest alkene is ethylene, C2H4.
    • Alkenes have a carbon-carbon double bond, which is made up of a strong σ bond and a weak π bond.
    • The C─C distance in ethylene is less than in ethane, and the double bond is shorter than the carbon-carbon single bond.

    Names of Alkenes

    • Common names are rarely used, except for three simple alkenes: ethylene, propylene, and isobutylene.
    • Most alkenes are named by the IUPAC system.
    • The IUPAC system rules:
      • Select the longest continuous chain that contains the carbon-carbon double bond as the parent structure.
      • Indicate the position of the double bond in the parent chain by a number.
      • Carbon-carbon double bonds take precedence over alkyl groups and halogens in determining the main carbon chain and direction.

    Physical Properties

    • Alkenes possess physical properties similar to those of alkanes.
    • They are insoluble in water, but soluble in nonpolar solvents like benzene, ether, and chloroform.
    • Boiling point rises with increasing carbon number.

    Cis, Trans Isomerism

    • Alkenes show cis, trans isomerism due to restricted rotation about the carbon-carbon double bond.
    • Example: 2-butene has two stereoisomers, cis-2-butene and trans-2-butene.
    • These compounds cannot be converted into each other at room temperature and have different physical and chemical properties.

    Preparation of Alkenes

      1. Dehydrohalogenation of alkyl halides:
      • Alkyl halides are converted into alkenes by elimination of hydrogen halide.
      • Dehydrohalogenation involves removal of the halogen atom and a hydrogen atom from a carbon adjacent to the one bearing the halogen.
      1. Dehydration of alcohols:
      • Alcohols are compounds of the general formula ROH.
      • Dehydration requires the presence of an acid and heat.
      • Mechanism involves the formation of a protonated alcohol, dissociation into water and a carbonium ion, and the loss of a hydrogen ion to form the alkene.
      1. Dehalogenation of vicinal dihalides:
      • Vicinal dihalides are compounds that have halogens on adjacent carbons.
      • They are prepared by the reaction between a halogen and an alkene.
      1. Reduction of alkynes:
      • Reduction of an alkyne to the double-bond stage can yield either a cis-alkene or a trans-alkene.
      • The choice of reducing agent determines which isomer predominates, except when the triple bond is at the end of a chain.

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    Preparation of Alkenes PDF

    Description

    Test your knowledge of alkenes, a type of unsaturated hydrocarbon with a carbon-carbon double bond. Learn about their general formula, simplest member, and more. This quiz covers the basics of alkenes in organic chemistry.

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