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Thread: Inductive Effect (I need further explanation)

  1. #1 Inductive Effect (I need further explanation) 
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    Hi, Can you help me with this?

    What is inductive effect? How does it works?

    These are my explanations:
    ".Inductive Effect."

    Using the example: CF3OH

    The lewis structure would be: (ignore the dots)

    ...F
    ...|
    F-C-O-H
    ...|
    ...F

    The Fluorine is more electronegative than C so it pulls the electron density towards itself. The inductive effect affects all parts in the chain. In this case, Carbon is more affected by the effect. Carbon will be electron deficient (so then it is more electronegative than its original Electronegativity). To compensate with this deficiency, this carbon pulls the electron density of the next part of the chain, which is O. As an effect, Oxygen pulls electron density from Hydrogen. This chain reaction is the inductive effect. The consequence of this effect is that the O-H bond weakens (can also be explained: the effect decreased O's EN). The O pulls the electrons in the O-H bond for itself causing a polarized bond. This is why this molecule is acidic, that it reacts readily. In acidic reaction, the the molecule will be broken and leaves an ion and the H+.

    Oxygen taking the electron pair from the Hydrogen so that Oxygen will have the partial negative charge and Hydrogen will have the partial positive charge since Oxygen is more electronegative than Hydrogen. By calculating the electronegative difference between Oxygen and Hydrogen we would consider their bond as a covalent bond:
    delta EN/(EN of A+ EN of B) = (3.5-2.1)/(3.5+2.1) = 0.25 <--closer to 0 so it's covalent bond.

    On the other hand, if we are to include O-H in the CF3OH, then we have
    to include the inductive effect. The inductive effect includes the weakening of individual parts of the chain as electron density is pulled away from them by the most Electronegative (fluorine, in this case).
    F takes from C which takes from O which takes from H. O initially has an increase in electron density when it takes electrons from H but it decreased when C takes the electron density from it. C was electron deficient and this is the reason why it took the electron density from O.

    Is my explanation correct? Pls. add more information if necessary.

    Some follow-up questions for clarification:
    (1) Did the inductive effect really decreased O's electronegativity? How?
    (2) Did the O-H bond weakened? (so that it becomes acidic?) How?

    Thank you very much for your help! I appreciate it!


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  3. #2  
    Forum Freshman César's Avatar
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    The inductive effect is no more than the transmission of electronegativity differences through covalent bonds as you explain. Mind that I say "electronegativity differences" and not "electronegativity" as an absolute value. Because in the trifluoromethanol the oxygen will be much more electronegative from the point of view of the hydrogen than from the point if view of the carbon atom, which has its own electronegativity increased by the fluorines. Therefore, instead of talking about resulting (apparent) electronegativities I would keep it to electron densities, more visual and more intuitive.

    The hydrogen atom is feeling freer now: he is linked with the oxygen because they are sharing electrons, but since the oxygen goes around with the new gang it is keeping more and more the electrons for itself. The hydrogen is not that linked anymore and has started flirting with OH-'s in the neighbourghood. The molecule is acidic.

    One more thing: avoid the term "chain reaction", it is unproperly used.

    I hope this helps.

    Best regards,

    César


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  4. #3  
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    Hey Cesar, thanks a bunch for your response! It makes more sense now. You are most correct that I should use electron density instead of electronegativity in this case. I really like your explanations especially the second paragraph. It helped a lot. Thanks!
    "Understanding is better than memorizing."
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