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2013-01-05 16:51:51

MCAT OChem Alkenes
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  1. Alkenes
    Alkenes are compounds containing carbon-carbon double bonds.

    The degree of unsaturation (the number N of double bonds or rings) of a compound of molecular formula CnHm can be determined according to the equation: 

    N = (1/2) (2n + 2 - m) 

    Double bonds are considered functional groups, and alkenes are more reactive than the corresponding alkanes. 
  2. Naming alkenes
    1. When identifying the carbon backbone, select the longest chain that contains the double bond (or the greatest number of double bonds, if more than one is present). 

    2. Number the backbone so that the double bond receives the lowest number possible.
  3. Some Examples of Alkenes
    •  Methylene cyclohexane 
    • Chloroethene 
  4. Cycloalkenes
  5. Physical Properties of Alkenes

    melting point and boiling point
    The melting and boiling points increase with increasing molecular weight. 

    Terminal alkenes usually boil at a lower temperature than internal alkenes and can be separated by fractional distillation. 

    Trans-alkenes generally have a higher melting points than cis-alkenes because their higher symmetry allows better packing in the solid state. They also tend to have lower boiling points than cis-alkenes because they are less polar. 
  6. Physical Properties of Alkenes

    Dipole moment 
    Polarity is a property that results from the asymmetrical distribution of electrons in a particular molecule. In alkenes, this distribution creates dipole moments that are oriented from the electopositive alkyl groups toward the electronegative alkene. 

    In trans-2-buene, the two dipole moments are oriented in opposite directions and cancel each other. On the other hand, cis-2-butene has a net dipole moment, resulting from addition of the two smaller dipoles. The comound is polar, and the additional intermolecular forces tend to raise the boiling point. 

  7. Synthesis of Alkenes 

    E1: Unimolecular Elimnation
    Unimolecular elimination, which is abbreviated E1, is a two-step process proceeding through a carbocation intermediate. 

    The rate of reaction is dependent on the concentration of only one species, namely the substrate. 

    In the first step, the leaving group departs, producing a carbocation. In the second step, a proton is removed by a base. 

    E1 is favored by the same factors that favor SN1 : highly polar solvents, highly branched carbon chains, good leaving groups, and weak nucleophiles in low concentration. These mechanisms are therefore competitive, and directing a reaction toward either E1 or SN1 alone is difficult, although high temperatures tend to favor E1.
  8. Synthesis of Alkenes 

    E2: Bimolecular Elimination
    Bimolecular elmination, E2, occurs in one step.

    Its rate is dependent on the concentration of two species, the substrate and the base. 

    A strong base such as the ethoxide ion (C H O) removes a proton, while a halide ion anti to the proton leaves, resulting in the formation of a double bond. 

  9. Synthesis of Alkenes 

    E2: Bimolecular Elimination

    E2 or SN2 ?
    Controlling E2 versus SN2 is easier than controlling E1 versus SN1. 

    1. Steric hindrance does not greatly affect E2 reactions. Therefore, highly substituted carbon chains, which form the most stable alkenes, undergo E2 most easily and SN2 rarely. 

    2. A strong base favors E2 over SN2. SN2 is favored over E2 by weak Lewis bases (strong nucleophiles).
  10. Key reactions of Alkenes

    1. Electrophilic addition
    In an electrophilic addition, a small ionic or polar covalent molecule is added across a double bond to form a substituted alkane. The general reaction is:

  11. Key reactions of Alkenes

    1. Electrophilic addition
    In an electrophilic addition to an alkene, the double bond is consumed.

    An electrophilic addition to an alkene follows Markownikoff's rules: the electronegative atom attaches to the most substituted carbon of the double bond.
  12. Key reactions of Alkenes

    1. Electrophilic addition

    A. Addition of HX
    Markovnikov's rule holds. 

  13. Key reactions of Alkenes

    1. Electrophilic addition

    B. Addition of X2
    The addition of halogens to a double bond is a rapid process. 

    it is frequently used as a diagnostic tool to test for the presence of double bonds. 

    The double bond acts as a nucleophile and attacks an X2 molecule, displacing X-

    The intermediate carbocation forms a cyclic halonium ion, which is then attacked by X- giving the dihalo compound. 

    Note that this addition is anti, because the X- attacks the cyclic halonium ion in a standard SN2 displacement. 

  14. Key reactions of Alkenes

    1. Electrophilic addition

    C. Addition of H2O
    Water can be added to alkenes under acidic conditions. 

    The double bond is protonated according to Markovnikov's rule, forming the most stable carbocation. 

    The carbocation reacts with water, forming a protonated alcohol, which then loses a proton to yield the alcohol. 

    The reaction is performed at low temperature because the reverse reaction is an acid-catalyzed dehydration that is favored by high temperatures. 

  15. Key reactions of Alkenes

    2. Reduction
    Catalytic hydrogenation is the reductive process of adding molecular hydrogen to a double bond with the aid of a metal catalyst.

    Typical catalysts are platinum, palladium, and nickel.

    *Syn addition.

    Reactions where one stereoisomer is favored are termed stereospecific reactions.
  16. Key reactions of Alkenes

    3. Free Radical Additions
  17. Aromaticity and Aromatic Compounds

    The carbon to which a substituent is attached is termed the ipso carbon.

    One carbon away from the substituent group, on either side, are the ortho carbons.

    Two carbons away, on either side, are the meta carbons.

    And the carbon opposite the substituent group is termed the para carbon.

  18. Aromaticity and Aromatic Compounds

    Electron Donating Groups
    Electron Withdrawing Groups
    When a benzene ring is unsubstituted, it is reasonably resistant to electophilic addition. Certain groups on a benzene ring will tend to either activate or deactivate the ring to electophilic addition reactions.

    Electron-withdrawing groups are ring-deactivating

    Electron-donating groups are ring-activating.

    All electron donating groups are ortho, para directors

    All electron-withdrawing groups, except the halogens, are meta directors

    The halogens are ortho, para directors
  19. Strong Electron Donating Groups




  20. Moderate Electron Donating Groups



  21. Weak Electron Donating Groups

  22. Weak electron withdrawing



  23. Moderate electron withdrawing




  24. Strong Electron withdrawing groups