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#1 2010-09-25 14:48:33

orgopete
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The effect of atomic charge on electron availability

Atomic Charge

Formal charges are very effective in balancing the electrons in starting materials and products. However, there is an incorrect tendency to think a charge confers a property to the charged atom. In the Ten Electron Table, my objective was to compare structures with the same numbers of protons and electrons in each column. As a result, the properties of each structure was a function of how the electrons differed in each structure. It was easy to note how a nuclear-electron distance variable corresponded with the acidities of each compound. You can make the same observation about adding or subtracting a proton to a structure in the rows of compounds. Just as adding (or subtracting) a proton to a nucleus changed the Coulombic force, so will addition (or subtraction) of an external proton change the nuclear-electron distance and the formal charge.

What is hydrochloric acid? Dissolving hydrogen chloride in water gives hydrochloric acid, a solution of hydronium ions and chloride ions. Despite the opposite charges, this reaction with water shifts to the right and shows the electrons of water can form a stronger bond with a proton/hydrogen atom than a chloride ion can. The negative charge on the chloride ion tells us the electrons surrounding the chloride are in excess. However, because the electrons are in excess, we should not think that they should have a great affinity for cations (or that they are more basic than those of water). We should only expect the electrons of chloride ion are not held as tightly as the electrons of hydrogen chloride.

            HCl + H2O -> H3O(+)  +  Cl(-)

How does a formal charge apply to a structure? I have drawn the Lewis structures for hydronium ion, water, and hydroxide ion below. These structures all contain ten electrons and the oxygen nucleus has eight protons. The formal charges of the oxygen are positive, none, and negative while the true charge of the nucleus is unchanged for all three forms. The formal charges tell us about the electrons of oxygen. A proton (and other nuclei) can change the nuclear-electron distance, but the charge of an electron and proton are unchanged. Therefore, protonation and deprotonation will decrease or increase the nuclear- electron distance, hence reactivity.

          http://www.curvedarrow.com/chem/ZZ5AD5970E.jpg 

What is hydronium ion? I have drawn an alternate formal charge form for hydronium ion. We know that hydrogen bonding is an important property of water. We should expect that if hydrogen bonding prevails in water, it will also be present with hydronium ions. These forms shift the formal charge from an oxygen atom to several atoms. This example shows some of the difficulty in representing molecules in a realistic yet compact form.

         http://www.curvedarrow.com/chem/ZZ3DA57037.jpg

An error commonly associated with the representation of hydronium ion with a positively charged oxygen atom is to create a new bond to it. Despite a formal charge on an oxygen atom, it contains a completed octet and therefore cannot accept more electrons. Do not attempt to form a new bond to the oxygen atom.

This example is excerpted from A Handbook of Organic Chemistry Mechanisms, available at http://www.curvedarrowpress.com/ or Amazon.com

This post is in response to a question about electron availabilty of non-bonded electrons of an oxygen atom. The main discussion which included the Ten Electron Table in present in A Handbook of Organic Chemistry Mechanisms and was presented to give a different model to explain several aspects of chemistry, e.g., acidity, reactivity, and stability by relating them to a simple model and  principle.

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