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#1 2011-12-30 19:41:17

orgopete
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Protonation of ambident anions

The following discussion has been excerpted from A Handbook of Organic Chemistry Mechanisms.

In the original post, the question was why protonation should occur on a nitrogen rather than carbon of a cyanide ion. As I also discussed, formal charges, positive or negative, tell us more about the net charge than any fundamental property of a compound or ion. There is an electron pair on carbon and nitrogen in cyanide. Cyanide has a net charge, but which electrons are more negative? (That is not a good expression as they are equally negative.) Rather the question is which electrons are more reactive?

How does a formal charge apply to a structure? Draw the Lewis structures for hydronium ion, water, and hydroxide ion. 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 alter the oxygen to electron distance, but not the charge of an electron or proton itself. Therefore, protonation and deprotonation will decrease or increase the oxygen-electron distance, hence reactivity. That is, the electrons of an H3O+ ion will be held more tightly than water thus decreasing their attraction to a proton. The electrons of HO- will be held less tightly than water and this will increase their attraction for a proton.
This example is excerpted from A Handbook of Organic Chemistry Mechanisms.

I suggest that reactivity should follow the inverse square law. The further a pair of electrons can extend beyond a nucleus, the more apt it can react with an electrophile (proton for cyanide). I argue that carbon, with fewer protons in its nucleus, does not hold its electrons as closely as nitrogen. Therefore, the electrons of carbon are more reactive. That is, the negative field will extent further from the nucleus. Its field should be greater as the force follows the inverse square law.

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Summary about Atoms

The discussion and models to this point have been about charge and proton (or nuclear)-electron pair distance because their force is determined by the inverse square law. A lesson I learned from how atoms make chemical bonds, 'the more tightly held a pair of electrons becomes, the weaker will its bond to another nucleus become'. A corollary to that is 'a loosely held pair of electrons can form a stronger bond or the easier it becomes to form a bond'. The analogy for an electron pair's ability to attack another nucleus is, 'chemical reactivity is like a boxer. A boxer with a longer reach would have an easier time to hit the nucleus of a another atom'.

A precaution to understanding bond strength above, this is heterolytic bond strength (to form ions). This is different than the homolytic bond strength (to form radicals) you will find in tables. Heterolytic bond strength varies by environment and no universally accepted scale exists to measure it. Breaking HCl into H(+) and Cl(-) is a heterolytic reaction. In water, this is easier for HCl than it is for H2O because chlorine holds its electrons more tightly than oxygen. Chlorine can be thought to increase the proton-electron pair distance in HCl and weaken the proton-electron pair force.

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