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The Conjugate Base is Always a Stronger Nucleophile

Last updated: August 23rd, 2022 |

Let’s say you get asked this exam question:

All else being equal, what’s a better nucleophile: water or OH(-) ? Acetic acid or OAc(-)? HF or F(-)? [Let’s agree to leave out the counterion for the moment: it isn’t crucial for this discussion]. Butane or Bu(-) ? NH₃ or NH₂(-)? Hydrogen peroxide or HOO(-) ? Given an acid and its conjugate base, which will have the highest affinity for a given electrophile?

Recall that nucleophilicity refers to the “nucleus-seeking” (i.e. positive charge seeking) ability of a species, which is proportional to its electron density. When a proton is stripped from an acid, the electron pair that was previously shared with hydrogen is completely handed over to the conjugate base, which becomes negatively charged. Opposite charges attract, and the strength of the interaction is proportional to the magnitude of the respective positive and negative charges. [Tying it back to physics, this is Coulomb’s law in action]. Therefore, the affinity of this species for a given electrophile will always be higher than that of the acid due to the greater electrostatic attraction between opposite charges.

So given identical reaction conditions (e.g. temperature, solvent, and electrophile) the conjugate base is always a better nucleophile.

And when I say “always” I don’t mean “always*” with fingers crossed behind my back because you may subsequently discover that I only told you part of the truth (a common pedagogical necessity, I am sorry to say). I mean always – as in, I cannot think of an exception to this.

N.B. Don’t forget that when we talk about the “strength” of nucleophiles, we are talking about reaction rate. This is the key difference between nucleophilicity and acidity. The strength of an acid – its pK_a – is measured by the position of the equilibrium between an acid and its conjugate base, which is a function of their relative thermodynamic stability. Not so with nucleophilicty, which is all about speed, or “kinetics” as we like to say.

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

26 Organic Chemistry Tips and Tricks

27 Case Studies of Successful O-Chem Students

Comments

5 thoughts on “The Conjugate Base is Always a Stronger Nucleophile”

Orgboy says:

May 9, 2021 at 3:15 am

There is an excpetion. Nucleophilic strength of OCH3- and OH-. The trends are reversed

Reply
rachel says:

November 2, 2013 at 8:49 pm

I don’t understand why a conjugate base of a strong acid is a stronger nucleophile than say a conjugate base of a weaker acid. What makes the difference in nucleophilocity?

Reply
1. Pedro says:
  
  June 9, 2014 at 2:08 pm
  
  When you have a conjugate base of a weak acid, the base is necessarily a strong base. That is to say it has a greater tendency to capture protons than the conjugate base of a strong acid (ie.: weaker base).
  
  Judging from the above paragraph alone, one might be tempted to think that a stronger base would make for a stronger nucleophile. However, the strength of a conjugate base is measured by its unstability (the more unstable the conjugate base, the stronger the acid). So when you have a weak acid and therefore a strong conjugate base, you have a very unstable species with an enormous tendency for removing protons in order to neutralize its negative charge. Nucleophily though, is a slightly different thing. It is not as much an ability to neutralize the base itself and its negative charge as an urge to neutralize positive charges by donating electrons.
  
  So, when you have a carbocation, for example, you need a species that attacks the positive charge created by the electron deficit generated by the leaving of the exiting group. If you had a very strong base, it would probably be more likely to remove an acidic proton from one of the beta carbons than to bond to the positively charged centre.
  
  Reply
  1. James says:
    
    June 12, 2014 at 6:07 pm
    
    The sentence says, “stronger” nucleophile, not strong nucleophile. The conjugate base of HCl is Cl- which is a better nucleophile than HCl (but not itself a strong base or nucleophile). Thank you for elaborating on the points above.
    
    Reply
  2. Armita says:
    
    April 8, 2016 at 7:57 pm
    
    Pedro, you are mistaken when you say that a strong acid has an unstable conjugate base. It’s actually the opposite, the more stable the conjugate base of an acid, the stronger the acid. Why you may ask? Because if it is so willing to give away a hydrogen, it is because it can withstand the negative charge! A deprotonated acid, is an anion, and this anion is better at deprotonating an acid stron than its conjugate acid. Also, in acid-base reactions, the equilibrium will always favour the weaker acid-base pair! Chateliers principle in play. On the other hand, nucleophilicity involves kinetics and also the willingness of a compound or atom to give its lone pair of electrons to a election deficient atom. Nucleophilicity of an atom follows the opposite trend of electronegativity!
    
    A.I
    
    Reply

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