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The Stronger The Acid, The Weaker The Conjugate Base

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Basicity In Organic Chemistry: Some Basicity Trends

Today we’ll look at a simple periodic basicity trend across the periodic table and extrapolate from it some general principles for acid base reactions in organic chemistry.

1. “Basicity” Is Just Another Word For “Stability Of A Lone Pair Of Electrons”

Last time I started writing about acid-base reactions. We looked at this list of stabilities of anions going across the topmost row of the periodic table.

stability-of-ions-in-first-row-of-periodic-table-ch3-is-least-stable-f-is-most-stable

Fluoride ion is the most stable in this series because it’s the most electronegative; carbon is the least stable because it’s the least electronegative.

Because of this, we were able to say that H-F was the most acidic, because it had the most stable conjugate base.

And H-CH3 (methane)was the least acidic, because it had the least stable conjugate base.

comparing-hf-h2o-nh3-ch4-as-acids-depends-on-stability-of-conjugate-bases

Let’s look at the flip side of this reaction. Instead of starting with HF, H2O, H3N, and CH4 and asking how likely they are to donate a proton to a common base (water in our example) , imagine we start with the anions [ F, HO, H2N and H3C ] and have them take a proton away from  a common acid (such as water).

2. Which Of These Acid-Base Reactions Would Be The Most And Least Favorable?

Which reactions would be most favorable? Which would be least favorable?

which-reactions-would-be-most-favored-going-from-f-to-hf-of-ch3-to-ch4-obviously-less-stable-the-anion-more-likely-to-proceed-to-completion

The same principle applies. The less stable the anion, the more likely the reaction will be to proceed to completion. 

So in this case, the reaction of F with H2O would be the least favored, because F is the most stable. And the reaction of H3C with H2O would be the most favored, because H3C is the least stable.

[A clarification: these are equilibrium reactions. So what I mean by favored here is the extent to which the equilibrium would favor the products on the right]

3. The More Stable A Pair Of Electrons Is, The Less Basic It Will Be. The Less Stable A Lone Pair Of Electrons Is, The More Basic It Will Be

Notice the role that each of these anions plays in these reactions: it is accepting a proton from water, so in other words it is acting as a base.

Therefore, our whole discussion of the  “stability” of anions,  for lack of a better term, goes by another name you’re familiar with: basicity. 

In other words:

  • the more stable a lone pair of electrons is, the less basic it will be.
  • the less stable a lone pair of electrons is, the more basic it will be.
the-less-stable-a-pair-of-electrons-is-the-more-basic-it-tends-to-be

4. The Stronger The Acid, The Weaker The Conjugate Base (And Vice Versa)

Let’s tie these two posts together with a common thread:

  •  For any group of acids, H-X (where X can literally be anything), the strongest acid will have the most stable conjugate base. Since stability is inversely correlated with basicity, another way of putting it is:
  • The stronger the acid, the weaker the conjugate base.
  • Today’s post is about how the opposite is also true: The weaker the acid, the stronger the conjugate base.

Next time, we’ll apply this framework to other stability trends we’ve discussed previously.

5. Beware This Misconception: “Weak Acids Are Strong Bases” <– Not True

P.S. One last note: a common misconception students have is that “weak acids are strong bases”. Not true! Methane (CH4) is a weak acid, but it can’t act as a base – it doesn’t have a lone pair.

The proper way to say it is that “weak acids have strong CONJUGATE bases“. So the conjugate base of CH4, CH3(-) is an extremely strong base.

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