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Acid Base Reactions

By James Ashenhurst

The Stronger The Acid, The Weaker The Conjugate Base

Last updated: December 29th, 2022 |

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”
  2. Which Of These Acid-Base Reactions Would Be The Most And Least Favorable?
  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
  4. The Stronger The Acid, The Weaker The Conjugate Base (And Vice Versa)
  5. Beware This Misconception: “Weak Acids Are Strong Bases” <– Not True
  6. Notes

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

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.

Next Post: Walkthrough of Acid-Base Reactions (3) – Acidity Trends

Notes

Related Articles

00 General Chemistry Review
01 Bonding, Structure, and Resonance
02 Acid Base Reactions
03 Alkanes and Nomenclature
04 Conformations and Cycloalkanes
05 A Primer On Organic Reactions
06 Free Radical Reactions
07 Stereochemistry and Chirality
08 Substitution Reactions
09 Elimination Reactions
10 Rearrangements
11 SN1/SN2/E1/E2 Decision
12 Alkene Reactions
13 Alkyne Reactions
14 Alcohols, Epoxides and Ethers
15 Organometallics
16 Spectroscopy
17 Dienes and MO Theory
18 Aromaticity
19 Reactions of Aromatic Molecules
20 Aldehydes and Ketones
21 Carboxylic Acid Derivatives
22 Enols and Enolates
23 Amines
24 Carbohydrates
25 Fun and Miscellaneous

Comments

Comment section

29 thoughts on “The Stronger The Acid, The Weaker The Conjugate Base”

  3. I am not understanding F- reaction with base. As you have written that F- with H2O doesn’t proceed but the product in this reaction is HF which is the strongest acid and nature always want more stability so why doesn’t it proceed?

    Reply

    1. Because the other product is HO(-) which is a considerably stronger base than F(-) (by about a factor of 11 orders of magnitude, compare the pKa of H2O (14) to that of HF (3). ). That’s why it doesn’t proceed.

      Reply

  4. What would happen if I were to compare HF and HI? I am lost beacuse Flouride ion ,as you pointed out, has the highest electronegativity so a stable lone pair thus rendering HF the stonger acid and HI the weaker acid but that’s not the case.
    So what is meant by stability of lone pairs? Is it related to eletronegativity or size of the ion?

    Reply

  6. I study for JEE ADVANCED, an engineering entrance exam in India. Honestly there are lots of books for one to study organic chemistry from. If you guys are to publish an Organic chemistry book for the entrance I bet it’ll sell like flash sale Android phones all over. Also thank you for the wonderful and TO THE POINT explanation.

    Reply

  8. Which of these factors has the greatest effect on basicity? Is there a hierarchy we should go through when asked, for instance, to rank the basicity of five or six conjugate bases?

    Reply

  9. This site is really great, I want to ask with reference to your last note that if a compound A is more basic than B then can we infer that B is more acidic then A. Please help urgently, I am a beginner with organic chemistry
    Thank you

    Reply

  11. Sir
    Does bascity order also change with changing solvents
    Like in nucliophilcity
    F- >Cl- in polar aprotic
    But
    Cl-<F- in polar protic

    Reply

    1. The order of basicity F(-), Cl(-), Br(-), and I(-) is the same in both polar protic and polar aprotic solvents.

      However the *magnitude* of basicity for F(-) is much higher in polar aprotic solvents than in polar protic solvents. Hydrogen bonding greatly attenuates the basicity of F(-). In cases where water can be (mostly) removed, anhydrous F(-) is an extremely strong base.

      Reply

  13. Please help me classify the following in order of increasing basicity..
    Aniline, 2-Toluene, dinitribenzene and 4-chloroaniline

    Reply

  14. Thank you so much for the explanation! It has helped me a lot :-)
    However I still cannot figure out which one is a stronger base between -OH and -OC(CH3)3..

    Reply

    1. They’re quite similar, but t-butoxide is a bit more basic, mostly because the oxygen of the t-butoxide won’t be able to accept as many hydrogen bond donors as hydroxide (and hence be more “unstable”)

      Reply

  16. This is an incredible site! I really appreciate your hard work and all the effort you put into this. Studying from your site has drastically increased my marks in organic chemistry at university and I totally owe it all to you! Thank you once again! :D

    Reply

      1. The main difference is that nucleophilicity is much more sensitive to steric hindrance.

        Basicity is a measure of the equilibrium between a Lewis base and a proton (H+). These reactions are often reversible. So the position of the equilibrium tells you the relative energy differences between the starting material and reactants.

        Nucleophilicity is a measure of the reaction between a Lewis base and any other electrophile besides H+ (most commonly carbon in our cases). These reactions are generally not reversible. So to measure nucleophilicity we generally have to measure the reaction *rates*.

        Reactions between Lewis bases and carbon are much more sensitive to steric hindrance than are reactions of Lewis bases with H+ (because attacking carbon requires orbital overlap with a sigma* orbital that may be difficult to access on a tetrahedral carbon atom via backside attack).

        Reply

  18. THIS SITE IS GREAT!!! I couldn’t find videos on Khan Academy but this had a ton of info and it was explained in a way I could understand! Thanks a ton!

    Reply

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