Addition Reactions - Organic Chemistry

Addition Reactions - Organic Chemistry

A reaction in which the substrate and the reagent add up to form a product...


Addition Reactions
A reaction in which the substrate and the reagent add up to form a product is called addition reaction.

Compounds having multiple bonds such as,etc. undergo addition reactions.

  1. Electrophilic Addition Reaction: This is a characteristic reaction of unsaturated hydrocarbons,



    On bromination trans-2- butene gives meso-dibromide and cis-2- butene gives recemic (dl) modification. Thus, addition of Br2to the -bond is the trans (or anti) addition. The reaction is, therefore, stereoselective.

    The relative rates of addition of some substituted alkenes are as;



    Electron withdrawing groups will have reverse effect.

    Markownikov Rule: In case of addition of unsymmetrical reagents (i.e., HX) to unsymmetrical alkenes, the negative part of the addendum is added to that doubly bonded carbon which is less hydrogenated.

    In the presence of organic peroxide or under conditions of radical formation, anti-Markonikov addition results. The peroxide converts into a free radical which generates a bromine-free radical from HBr.

    Free radical addition of HF, Hl or HCl is energetically not favorable.

    Addition of Br2 to Conjugated dienes



    The 1,2 product predominates at lower temperature and 1,4-product predominates at higher temperature.

    This may be due to stabilization of 1, 4-product by hyper conjugation.
     
  2. Addition to Alkynes:
    Alkenes are less reactive than alkenes because the -electrons are more tightly held by the carbons. Electrophilic addition to alkynes resembles those of alkenes.

      

    In the hydration of alkynes with sulphuric acid, the catalyst (such as mercuric salt) is necessary.



    In case of aromatic compounds (e.g., benzene) addition reaction is energetically not favorable due to loss of resonance energy.
     
  3. Nucleophilic Addition Reaction:
    Electron-releasing groups conjugated to carbon-carbon multiple bonds favor electrophilic addition while electron withdrawing groups conjugated to carbon-carbon multiple bonds favor nucleophilic addition.



    Polar functional groups, e.g., etc., also undergo nucleophilic addition.


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