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#1 2008-10-28 01:41:20


Which bond has a larger dipole, carbon-iodine or carbon-chlorine?

Yahoo Answers Chemistry

Although the C-I bond is longer than a C-Cl bond, the C-Cl bond has a larger dipole moment. Explain

For the points on the exam, it is the C-Cl bond. The Pauling electronegativity tables tells you how much ionic content a bond has. Since the electronegativity of chlorine is higher, it has a greater charge, hence a greater dipole.

That is the easy answer. So why did I post? Organic chemists know that iodide is a better leaving group than chloride. Hydrogen iodide is a stronger acid than hydrogen chloride. If chloride has a greater charge in a reaction to form a chloride ion, shouldn't it be reversed?

Let us look at the Pauling electronegativity table and where it came from. Pauling (and others) knew that fluorine bonds with other elements were much stronger than one would predict from the bond contributions of the individual elements alone. That is, if you wanted to predict the bond strength of HF, you could determine the bond strength of F-F and H-H, find their individual contribution, add them up, and you should predict the bond strength of H-F. It turns out that these calculation underestimate the actual bond strengths of fluorine bonds by the greatest amount. Pauling reasoned that if the new H-F bond contained a Coulombic attraction between the new ions, that could account for the increased bond strength of H-F bonds.

If we were to ask Pauling, which bond has the greatest ionic content, carbon-fluorine or carbon-iodine, the answer would be carbon-fluorine. That must be the answer because that was the reason given for why fluorine bonds were stronger than expected. However, we should realize that this reasoning is entirely circular. Fluorine bonds are thought to be ionic to explain why they are stronger than predicted. So, the actual connection is not to ionic content, but to bond strength. Since strong bonds were thought to contain large amounts of ionic character, then ionic bonds are actually simply strong bonds.

Why does this matter? We know that the principle suggested in 1932 by Pauling could not have been correct. Since Pauling knew that opposite charges attract, then an ionic bond can only result in stronger than expected bond strengths and NEVER weaker ones. Weaker bonds would violate Coulombs Law as it would indicate that ions of opposite charge were repellant. It is well known that metal hydride bonds are weaker than predicted, e.g. LiH.

The conclusion that should have been reached is that fluorine bonds are simply stronger than predicted and iodine bonds are not. I do not wish to argue that the homolytic measurements of bond strength prove H-F bonds are stronger than H-I and therefore HF should be a weaker acid. While relative bond strengths are correct, the conclusions are not. We cannot use the bond strength measurements to form radicals to predict the heterolytic bond strengths of HF and HI.

Let me now argue one step further. The purpose of assigning a bond dipole is to indicate the degree of polarization a bond might have. For the heterolytic cleavage of a H-I or C-I bond, we can use acidity data or solvolysis rates. That is, in the ionization of H-I, the oppositely charged ions form from HI. That is the very reaction in question. If one were to ask which bonds will break to form the ions or to attract electrons to add to the carbons, C-Cl or C-I, we should realize C-I bonds will break most easily and the carbons most able to attract electrons to it. Returning to the intent of the original question, in fact, C-I bonds have larger dipoles than C-Cl bonds.

For those of you that are seeking philosophical purity, you could argue that I have used circular reasoning as well. However, I would argue that we are pretty certain that in measuring the pKa of HI, the pH of solutions do truly measure the concentrations of H3O(+) and I(-). Therefore, in arguing that C-I bonds have a large dipole, it would be consistent with the ability of a C-I bond to form C(+) and I(-) ions.


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