Acid-Base X. Levelling Effect

Contents

I. Introduction

II. Levelling Effect in Acids

III. Levelling Effect in Bases

IV. Levelling Effect Overview

V. Example of Levelling Effect

VI. Conclusion

VII. Try It Yourself!


I. Introduction

We have seen the relationship between pKa, pKb, and pKsolv in Acid-Base VIII. We have also seen that acids and bases can have different strength in different solvents Acid-Base IX. Both of these posts have prepared you for the levelling effect.

Just like reactants and products, solvents are also chemicals and they can react with other chemicals. Water, for example, can be protonated by an acid (e.g. HCl) or deprotonated by a base (e.g. NH2). What this actually means is that water reacts with both HCl and NH2 to form H3O+ and OH respectively. Therefore, it is implied that water limits the strength of certain acids and bases by reacting with them. This is what the levelling effect is.

There are two types of levelling effect, one that concerns acids and one that concerns bases. The levelling effect states that:

  • Acids stronger than than the conjugate acid of the solvent cannot exist in a considerable quantity in that solvent
  • Bases stronger than than the conjugate base of the solvent cannot exist in a considerable quantity in that solvent

So what does this mean? Let’s break them down slowly. Lots of students get confused by this, so I will try to explain slowly. In this post I will use water as solvent for the examples, but you should know that this applies to any other solvent in general.

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II. Levelling Effect in Acids

‘Acids stronger than the conjugate acid of the solvent cannot exist in a considerable quantity in that solvent’

Look at the general acid dissociation reaction in water.

abvii-fig02-kainwaterchemicalequation

abvii-fig02a-kainwatermathematicalequation

Remember that a strong acid has to have pKa < 0 (Ka > 1). Refer to Acid-Base VIII if you forget about this.

If pKa < 0 (Ka > 1), the equilibrium reaction goes to the right, and the acid HA exists no more in the solution. It has completely protonated water, and the only acid exists in the solution is H3O+. This is what the levelling effect is.

So we can rephrase the definition of levelling effect in water for acid:

Acids stronger than the conjugate acid of water (H3O+) cannot exist in a considerable quantity in water

This is because the acid will fully protonate water to form H3O+ and therefore cease to exist in a considerable quantity in the resulting solution.

HCl, HBr, and HI have small pKa values in water: –7, –9, and –10, respectively. With such small pKa values, the reaction practically goes to completion (towards H3O+ and the conjugate base): all of these acids will fully protonate water to form H3O+.

abix-fig02-hclhbrhidissociation

You will not find any of the acids in the resulting solution. You will only find H3O+ and their counterions: either Cl, Br, or I. The only acidic species in the solution is H3O+.

abix-fig03-hclhbrhisolvation

Therefore, although the three acids have different strength (HI being the strongest, followed by HBr and then HCl), in water they all have the same acidity. Their acidity are levelled down to the acidity of H3O+.

What about weak acids? Let’s take acetic acid (HOAc) for example.

abix-fig04-hoacdissociation

Remember that a weak acid has to have pKa > 0 (Ka < 1). This means in the equilibrium, H3O+, AcO, and HOAc exist, with the quantity of HOAc exceeding the quantity of H3O+ in the resulting solution.

abix-fig05-hoacsolvation

This means that weak acids don’t fully protonate water. HOAc exists predominantly as itself and only a small part of it dissociates to protonate water. This ultimately means that the acidity of a weak acid doesn’t get levelled to the the acidity of water’s conjugate acid (H3O+), because most of it still exists as itself.

To conclude in a less-intimidating form of sentence:

  • Acids stronger than the conjugate acid of the solvent are levelled down to the acidity of the solvent’s conjugate acid, or
  • Acids with pKas smaller than the pKaH of HSolv (a.k.a. pKa of H2Solv+) are levelled down to the acidity of H2Solv+

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III. Levelling Effect in Bases

‘Bases stronger than the conjugate base of the solvent cannot exist in a considerable quantity in that solvent

Look at the general base dissociation reaction in water.

abvii-fig03-kbinwaterchemicalequation

abvii-fig03a-kbinwatermathematicalequation

Remember that a strong base has to have pKb < 0 (Kb > 1); which implies that its pKaH > pKsolv (KaH < Ksolv). In this case pKsolv is pKw. Refer to Acid-Base VIII if you forget about this.

If pKb < 0 (Kb > 1), or in other words if pKaH > pKsolv (KaH < Ksolv), the equilibrium reaction goes to the right, and the base A exists no more in the solution. It has completely deprotonated water, and the only base exists in the solution is OH. This is what the levelling effect is.

So we can rephrase the definition of levelling effect in water for base:

Bases stronger than the conjugate base of water (OH) cannot exist in a considerable quantity in water

This is because the base will fully deprotonate water to form OH and therefore cease to exist in a considerable quantity in the resulting solution.

Bases such as C4H9 (from butyllithium), NH2 (from sodium amide), and (CH3)3CO (from potassium t-butoxide) have large pKaH values in water: 50, 38, and 19, respectively. With such large pKaH values, the reaction practically goes to completion (towards OH and the conjugate acid): all of these bases will fully deprotonate water to form OH.

abix-fig07-bulinanh2tbuokdissociation

You will not find any of the bases in the resulting solution. You will only find OH and the bases’ conjugate acids: either butane (C4H10), ammonia (NH3), or t-butanol ((CH3)3COH). The only basic species in the solution is OH.

abix-fig08-bulinanh2tbuoksolvation

Note: Yes, butane is a gas, so it will leave the solution as soon as it’s formed. What I want to show in this picture is that all C4H9 are converted into butane, so you won’t find any C4H9 left in the resulting solution.

Therefore, although the three bases have different strength (C4H9 being the strongest, followed by NH2 and then (CH3)3CO), in water they all have the same basicity. Their basicity are levelled down to the basicity of OH.

Note: Yes, NH3 in the solution is also basic, but it is less basic than OH because pKaH OH (15.7) is greater than pKaH NH3 (9.2). So although NH3 is present, the base in the solution is still OH.

What about weak bases? Let’s take our favourite ammonia and also sodium phenoxide as examples.

abix-fig09-nh3c6h5odissociation

Remember that a weak base has to have pKb > 0 (Kb < 1); which implies that its pKaH < pKsolv (KaH > Ksolv). This means:

  • In the ammonia dissociation equilibrium, OH, NH4+, and NH3 exist, with the quantity of NH3 exceeding the quantity of OH in the resulting solution.
  • In the phenoxide dissociation equilibrium, OH, C6H5OH, and C6H5O exist, with the quantity of C6H5O exceeding the quantity of OH in the resulting solution

abix-fig10-nh3c6h5osolvation

This means that weak bases don’t fully deprotonate water. If we take ammonia, that means NH3 exists predominantly as itself and only a small part of it dissociates to deprotonate water. Same goes with phenoxide, C6H5O exists predominantly as itself and only a small part of it dissociates to deprotonate water.

This ultimately means that the basicity of a weak base doesn’t get levelled to the the basicity of water’s conjugate base (OH), because most of it still exists as itself.

To conclude in a less-intimidating form of sentence:

  • Bases stronger than the conjugate base of the solvent are levelled down to the basicity of the solvent’s conjugate base, or
  • Bases with pKaHs larger than the pKaH of Solv (a.k.a. pKa of HSolv) are levelled down to the basicity of Solv

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IV. Levelling Effect Overview

A picture paints a thousand words, so let’s put all of those information in one big picture! Since we’re dealing with pKas, let’s combine them with our pKa table.

pkatable_levellingeffect

In the blue region:

These are strong bases with pKaHs greater than the pKaH of water’s conjugate base (a.k.a. pKaH of OH or pKa of H2O). Therefore, they are levelled down to the basicity of water’s conjugate base. They cannot exist in any considerable quantity in that solvent.

In the red region:

These are strong acids with pKas smaller than the pKa of water’s conjugate acid (a.k.a. pKa of H3O+). Therefore, they are levelled down to the basicity of water’s conjugate base. They cannot exist in any considerable quantity in that solvent.

Note: although the arrow points upwards, these acids are much stronger acids compared to H3O+. Therefore, if their acidity is levelled to the acidity of H3O+, we will still say that their acidity is levelled down, not up.

In the green region:

These are weak acids and weak bases. They are not levelled to any acidity or basicity and therefore can exist in that solvent.

In other words, in water:

  • Weak species are those with pKa or pKaH values between the pKaH of OH (15.7) and the pKa of H3O+ (–1.7).
  • Those with pKaHs greater than 15.7 (pKaH of OH) are strong bases.
  • Those with pKas smaller than –1.7 (pKa of H3O+) are strong acids.

What about solvents other than water?

Of course, this works for all solvents in general, not just water.

For example if we use ammonia (NH3):

  • Weak species are those with pKa or pKaH values between the pKaH of NH2 (38) and the pKa of NH4+ (9.2).
  • Those with pKaHs greater than 38 (pKaH of NH2) are strong bases.
  • Those with pKas smaller than 9.2 (pKa of NH4+) are strong acids.

And if we use methanol (CH3OH):

  • Weak species are those with pKa or pKaH values between the pKaH of CH3O (16) and the pKa of CH3OH2+ (–2.2).
  • Those with pKaHs greater than 16 (pKaH of CH3O) are strong bases.
  • Those with pKas smaller than –2.2 (pKa of CH3OH2+) are strong acids.

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V. Example of Levelling Effect

The classic example of levelling effect is the deprotonation of alkyne using sodium amide. This was our main topic in Acid-Base V.

abix-fig11-deprotonationofacetylenegeneral

In that post we have thoroughly talked about this reaction, and we know that the deprotonation of alkyne using sodium amide can take place. But what about the solvent?

Let’s first use water as the solvent.

abix-fig12-deprotonationofacetyleneinwater

If water is used as a solvent, then we can’t use any base with pKaH larger than the pKaH of OH. Since the pKaH of NH2 (38) is larger than the pKaH of OH (15.7), amide anion’s basicity is going to be levelled down to the basicity of OH.

abix-fig13-reactionofamideanionwithwater

Because the strongest base in the solution is OH, the deprotonation reaction can’t take place anymore, because OH is not strong enough to do the job. The Keq of this reaction is about 10–9.3 (calculate it yourself for practice), too small for the reaction to take place.

abix-fig14-deprotonationofacetyleneusingoh

Note: that arrow means reaction does not take place.

So what solvent should we use for this reaction?

You should have guessed that anything with pKaH below the pKaH of NH2 is not going to work. Therefore you won’t be able to use alcohols either.

Knowing that, you should also have guessed that we should use solvents with pKaH above the pKaH of NH2. Judging from the pKa table, we have two options: alkenes (e.g. benzene and toluene) and alkanes (e.g. hexane or heptane).

On paper, you can say that using toluene or hexane will work because their pKaH is above the pKaH of NH2. However in practice (lab settings), you may run into solubility problem. Solvents should be able to separate Na+ from NH2 in order for the amide anion to be able to do its job. Toluene and hexane will not be able to do this as they won’t be able to solvate NaNH2 and therefore won’t be able to separate Na+ from NH2.

So the last option is to use solvents with pKaH that is equal to the pKaH of NH2: ammonia! And indeed, it is the correct choice. Even if the amide anion reacts with ammonia, this reaction will still yield amide anion and ammonia!

abix-fig15-reactionofamideanionwithammonia

That way, the strongest base in the solution is NH2, and thus the reaction is able to take place just like the way we discussed in Acid-Base V.

abix-fig16-deprotonationofacetyleneinammonia

Other examples of levelling effect can be found in Section VII. of this post as a practice.

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VI. Conclusion

We have seen what levelling effect is and how it is important.

Levelling effect in acids:

  • Acids stronger than the conjugate acid of the solvent are levelled down to the acidity of the solvent’s conjugate acid, or
  • Acids with pKas smaller than the pKaH of HSolv (a.k.a. pKa of H2Solv+) are levelled down to the acidity of H2Solv+

Levelling effect in bases:

  • Bases stronger than the conjugate base of the solvent are levelled down to the basicity of the solvent’s conjugate base, or
  • Bases with pKaHs larger than the pKaH of Solv (a.k.a. pKa of HSolv) are levelled down to the basicity of Solv

I have also found two old papers in J. Chem. Educ. that may help you understand solvent levelling effect with real-life analogies. You can download and read them if you want:

  1. Macomber, R.S. J. Chem. Educ. 1984, 61, 128 (DOI: 10.1021/ed061p128.2)
  2. Kramer, A.F. J. Chem. Educ. 1986, 63, 275 (DOI: 10.1021/ed063p275)

If you’re not sure how to get the papers using only those information above, head over to Section 2 of this page to find out how.

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VII. Try It Yourself!

Answer these questions using what we have learned here (highlight texts for answer):

  • Butyllithium (pKaH = 50) is sold commercially as a solution in hexane or heptane. It is a very strong base, they can’t sell it in powder form because it will react with water vapour in the air. So they dissolve it in the only possible solvent: alkanes. I guess you can explain now, why they can’t sell it as a solution in, say, butanol?

Answer: because in butanol, the butyllithium will be levelled down to the basicity of butoxide (C4H9O). Selling butyllithium as a solution in butanol is no difference than selling a solution of lithium butoxide.

  • Sodium methoxide (NaOMe, pKaH = 16) is commercially available as a powder or as a solution. It’s not as reactive as butyllithium, so selling it in powder form is possible, although it is also available in solution form. Guess what solvent is used when sold as solution!

Answer: methanol

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Page last updated: 20ii17
RP


Other Posts in Acid, Base, and pKa Series

Acid-Base
Prologue
Acid-Base I
What are Acids and Bases?
Acid-Base II
What is pKa and Why is It Important?
Acid-Base III
The pKa Table and What Information It Tells You
Acid-Base IV
Factors Affecting the Acidity of Organic Compounds
Acid-Base V
Using pKa to Predict the Course of a Reaction
Part 1
Acid-Base VI
Using pKa to Predict the Course of a Reaction
Part 2
Acid-Base VII
Using pKa to Predict the Course of a Reaction
Part 3
Acid-Base VIII
Acidity, Basicity, and Solvent
Part 1
Acid-Base IX
Acidity, Basicity, and Solvent
Part 2
Acid-Base X
Levelling Effect
Acid-Base XI
Hard and Soft Acids and Bases (HSAB) Theory in Organic Chemistry

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