Jacky asked in 科學及數學化學 · 1 decade ago

mesomeric effect

my text book state that since the pi electron cloud is more polarizable

than sigma electron cloud,so mesomeric effect is usually stronger than

inductive effect.

I cannot understand the reason why M effect is stronger than I effect

Please give me the detailed explanation

1 Answer

Rating
  • 冷風
    Lv 5
    1 decade ago
    Favorite Answer

    As you know, resonance effect is much stronger than inductive effect.

    Mesomeric effect is a form of resonance, so it is stronger than inductive effect.

    Inductive effect:shifting electrons through a sigma bond.

    Mesomeric effect:moving a pair of electron.

    At the moment, I want to introduce more information of mesomeric effect to you.

    For example, there are two organic acids, benzoic acid and 4-hydroxybenzoic acid.

    圖片參考:http://us.f6.yahoofs.com/hkblog/d1WWTL6FGRtxq8qXRS...

    4-hydroxybenzoic acid is a weaker acid because of the involvement of the electron pairs in the O atom of the phenolic group(positive mesomeric effect, +M), destabilizing the carboxylate anion formed.

    圖片參考:http://us.f6.yahoofs.com/hkblog/d1WWTL6FGRtxq8qXRS...

    That is to say, in 4-hydroxybenzoate ion, the negative charge in the O atom of COOH group is less delocalized.

    There are some groups which have mesomeric effect when compounds attach them.

    +M:─OR, ─OOR, ─OH, ─NH2

    -M:─NO2

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