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Alkanes are saturated hydrocarbons which can be represented by the general formula CnH2n + 2.

Preparation of alkanes

  • Reduction of alkyl halides, RX, where X = F, Cl, Br, or I: (substitution of halogen by hydrogen)
    1. Reduction by dissolving metals: Reduction by dissolving metals, e.g., zinc and acetic or hydrochloric acid, zinc and sodium hydroxide, zinc–copper couple and ethanol.
      Zn Description: 45091.png Zn2+ + 2e
      RX + eDescription: 45098.png X + R. Description: 45106.png R:
      R: + C2H5OH Description: 45113.png R – H + OC2H5
    2. Reduction by reducing agents such as LiAlH4 and NaBH4.
      Description: 45121.png
    3. Using organometallic compounds such as Grignard reagent: Alkyl halides react with either Mg or Li in dry ether to give organometallic compounds having a basic carbanionic site.
    4. Hydrogenation of alkenes in the presence of Pd, Pt, or Ni: This addition is an example of heterogeneous catalysis involving syn-addition.
    5. Reduction of alcohols, carbonyl compounds, acids and acid derivatives:
      Description: 45165.png
      Description: 45173.png
      Description: 45180.png
    6. Wurtz reaction: An ethereal solution of an alkyl halide (preferably the bromide or iodide) is treated with sodium, when alkane is obtained.
      For example,
      Description: 45189.png
    7. Corey–House synthesis: A superior method for coupling is the Corey–House synthesis which could be employed for obtaining alkanes containing odd number of carbon atoms (unsymmetrical alkanes).
    8. Kolbe’s electrolytic method: A concentrated solution of the sodium or potassium salt of a carboxylic acid or a mixture of carboxylic acids is electrolyzed.
      For example,
      R1CO2K + R2CO2K + 2H2O R1 – R2 + 2CO2 + H2 + 2KOH
    9. Decarboxylation of carboxylate salts: By heating a mixture of the sodium salt of a carboxylic acid and soda-lime, alkanes can be obtained.
      Description: 45199.png
      Description: 45207.png

Chemical properties

Direct fluorination is usually explosive. So special conditions are necessary for the preparation of the fluorine derivatives of alkanes.
Description: 45234.png
Reactivity of X2: F2 > Cl2 > Br2 > I2
The mechanism of chlorination of methane is as follows:
Chain initiation step:
Description: 45249.png ΔH = + 243 kJ/mol
The required enthalpy comes from ultraviolet (UV) light or heat supplied.
Chain propagation step:
  1. Description: 45258.png ΔH = –4 kJ/mol
  2. Description: 45267.png ΔH = –96 kJ/mol
The sum of the two chain propagation steps in the overall reaction is
CH4 + Cl2 CH3Cl + HCl; ΔH = –100 kJ/mol
In propagation steps, the same free radical intermediates, Cl* and H3C*, are being formed and consumed.
Chain termination step: Chains terminate on those rare occasions when any two free-radical intermediates collide to form a covalent bond.
Description: 45277.png
H3C* + Cl* Description: 45287.png CH3 – Cl
Description: 45294.png
Radical inhibitors stop chain propagation by reacting with free radical intermediates. For example,
Description: 45302.png
In more complex alkanes, the abstraction of each different kind of hydrogen atom gives a different isomeric product. Three factors determine the relative yields of isomeric product:
  1. Probability factor: This factor is based on the number of each kind of hydrogen atoms in the alkane molecule. For example, in CH3CH2CH2CH3, there are six equivalent 1° H and four equivalent 2° H. The probability of abstracting a 1° H to 2° H is 6 to 4, or 3 to 2.
  2. Reactivity of H*: The order of reactivity of hydrogen atoms is 3° > 2° > 1°.
  3. Reactivity of X*: The more reactive Cl* is less selective and more influenced by the probability factor. The less reactive Br* is more selective and less influenced by the probability factor, as summarized by the reactivity–selectivity principle. If the attacking species is more reactive, it will be less selective and the yields will be determined by the probability factor as well as reactivity of hydrogen atoms. If the species attacking is less reactive and more selective, the yield of the product is governed exclusively by reactivity of hydrogen atoms.
    Description: 45314.png
    Description: 45323.png
    The rate of abstraction of hydrogen atoms is always found to follow the sequence 3° > 2° > 1°. At room temperature (25°C), the relative rates in chlorination are 5.0 : 3.8 : 1.0, respectively, for 3°, 2°, and 1° hydrogen atoms.

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