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Hydrocarbons

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Hydrocarbons PDF Notes, Important Questions and Synopsis

SYNOPSIS

  1. Alkanes: General formula: CnH2n+2
    • Preparation of alkanes

    • Chemical properties of alkanes


    1. Aromatisation:

      begin mathsize 12px style straight a. space straight R minus straight H     rightwards arrow from heat to Al subscript straight 2 straight O subscript straight 3. Cr subscript straight 2 straight O subscript straight 3 of thin space Cycloalkane thin space rightwards arrow from blank to blank of thin space Benzene   +  its thin space derivative end style

    2. Cracking (pyrolysis):
      begin mathsize 12px style text a. Decane    end text rightwards arrow from heat to Conc. straight H subscript straight 2 SO subscript straight 4 of thin space Heptane  +  Propene  +  Hexane plus Butene plus Pentane plus Pentene plus. .. end style
    Conformations of ethane:

    Conformations →

     

    Eclipsed

    Staggered

    Skew

     

    Conformation in

    which hydrogen atoms attached to two carbon atoms are as close to each other as possible

    Conformation in which hydrogen atoms are as farther away as possible

    Any other intermediate

    conformation in between eclipsed and sawhorse

    Sawhorse projection

     

     

     

    Newman projection

     

    [RKR1] 

     


  2. Geometrical isomers of alkenes

    Trans-isomer is non-polar and has zero dipole moment where a cis-isomer is polar.
    Cis-isomer has higher boiling points compared to trans-isomer because it is polar.
    In case of solids, cis-isomer has a lower melting point than trans-isomer. This is because the trans-isomer is symmetrical and fits well into the crystal lattice.
    . Preparation of alkenes 






     Chemical properties of alkenes 

    Test for Unsaturation: There are two tests to know whether a compound is unsaturated or not

    1. Bromine water test: When an unsaturated compound is reacted with bromine solution in carbon tetrachloride, the reddish-orange colour of bromine solution in carbon tetrachloride is discharged.

    2. Baeyer’s reagent test: When an unsaturated compound is reacted with cold, dilute and aqueous solution of potassium permanganate (Baeyer’s reagent), the pink colour of KMnO4 solution is discharged.

  3. Preparation of alkynes



    Acidic nature of alkynes: Hydrogen atoms of ethyne attached to triple bonded carbon atom are acidic in nature.
    Example:

    begin mathsize 12px style table attributes columnalign left end attributes row cell HC identical to CH text   end text plus text   end text Na text   end text rightwards arrow from blank to blank of text   end text stack HC identical to straight C to the power of minus Na to the power of plus with table attributes columnalign left end attributes row Monosodium row cell text     end text ethynide end cell end table below text   end text plus text   end text 1 half text   end text straight H subscript 2 end cell row cell HC identical to straight C to the power of minus Na to the power of plus text   end text plus text   end text Na text   end text rightwards arrow from blank to blank of text   end text stack Na to the power of plus straight C to the power of minus identical to straight C to the power of minus Na to the power of plus with table attributes columnalign left end attributes row disodium row cell text   end text ethynide end cell end table below text   end text plus text   end text 1 half text   end text straight H subscript 2 end cell end table end style

    Reason:

     

    Alkyne

    Alkene

    Alkane

    Hybridisation of carbon to which H atom is attached

    sp

    sp2

    sp3

    Percentage s character of carbon

    50%

    (maximum)

    66.6%

    25%

    (minimum)

    Electronegativity of carbon atom

    Highest

    Less than alkyne and more than alkane

    Lowest

    Extent of attraction of hydrogen atoms of C-H bonds towards C

    Highest

    Less than alkyne and more than alkane

    Lowest

    Ease of liberation of H atoms as protons

    Highest

    Less than alkyne and more than alkane

    Lowest

    Acidic character

    Highest

    Less than alkyne and more than alkane

    Lowest

      

     Chemical properties of alkynes  

  4. Benzene: 
    Conditions for aromaticity
  1. Planarity: The ring should be planar. In simple words, all the carbons of the ring should be sp2 hybridised.
  2. Complete delocalisation of the p electrons in the ring: The p electrons should be completely delocalized. In simple words, in the ring alternate single and double bonds should exist.
  3. Hückel Rule: Presence of (4n + 2) p electrons in the ring where n is an integer (n = 0, 1, 2, . . .).
Chemical properties of benzene

Directive influence of a functional group in monosubstituted benzene
  1. Ortho–para directing groups: Groups which direct the incoming group to
    ortho and para positions are called ortho–para directing groups.
    Activating groups: Groups which increase the electron density at ortho and para positions of the benzene ring.
    Examples: –NH2, –NHR, –NHCOCH3, –OCH3, –CH3 and –C2H5.Deactivating groups: Groups which decrease the electron
    density at ortho and para positions of the benzene ring.
    Example: –X.
  2. Meta directing groups: The groups which direct the incoming group to meta position are called meta directing groups.
    Examples: –NO2, –CN, –CHO, –COR, –COOH, –COOR and –SO3H.
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