Ranking acids

[20090900]

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I know that to rank acids you look at the CB and for those of you who have watched Chad's, you use the acronym CARDIO. However I don't understand why when looking at "A" (atom), a negative charge on an oxygen is more stable than a negative charge on a carbon? Carbon is bigger so thus shouldn't it be more stable to oxygen? I looked up the difference in size between the two atoms and it seems to be quite similar. I then looked up the difference in electronegativity and it seems that the difference in EN is greater than the difference in atomic size. Is this why the negative charge on oxygen is more stable than on a carbon?

 

[mrg276]

The trend is that as you go up the periodic table and to the left, basicity increases, therefore a negative charge on oxygen would be more stable (and LESS basic) because the electronegativity of oxygen pulls those lone pairs in stronger (making it more stable) than what a negative charge on carbon would. The negative charge on oxygen would yield a weaker base and the conjugate would be a stronger acid. Hope this helps!