The Inductive Effect and Electronegativity

  1. According to my A level Chemistry text book, the presence of the O atom in a C=O bond in any carboxylic acid (CH3COOH, for instance) pulls the electrons towards it causing the O-H bond to "weaken", thereby making the molecule more susceptible to dissociation, or in other words, accounts for the "acidity" of carboxylic acids.
    If i am not wrong, the act of pulling electrons away from the O-H bond would give rise to an even more polar O-H bond. What eludes me is how this accounts for a decrease in bond energy. As a matter of fact, when I apply the somewhat simple (considering the number of variables involved in such a situation) case of Coulomb's law for two point charges, it appears to me this separation of charges would make the bond all the stronger.
    Can someone please point out to me where I'm going wrong with this? Perhaps the O-H bond does become stronger as a result of the negative inductive effect mentioned earlier in the post, but becomes more prone to the tendency of H2O to separate cations and anions? So the bond doesn't become weaker per se, and just becomes more "prone to dissociation"? I am lost.
  2. jcsd
  3. Carboxylic acid acidity is better explained by resonance structures of the anion produced by deprotonation. The negative charge can resonate between both oxygens thus lowering the charge density and stabilizing the anion. I'm not sure if that is what the book explanation is trying to get at but in general resonance effects are much stronger than inductive effects. Perhaps posting the relevant text verbatim can help us help you interpret the book explanation better.
  4. I'm glad you brought this up. The book does make a reference to this concept as well (and it makes perfect sense to me), but as a separate point. The other point deals with the acid in and of itself wheras this point deals with the conjugate base, namely CH3COO-. The book lists the two ideas separately and independently.

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