Acid-Base IX. Acidity, Basicity, and Solvent – Part 2

Contents

I. Introduction

II. Acids and Bases Have Different Strength in Different Solvents

II.1. Comparison of Acid Strength in Water and Acetic Acid

II.2. Comparison of Base Strength in Water and Ammonia

II.2.1. Base Strength Using Kb and pKb

II.2.2. Base Strength Using KaH and pKaH

III. Conclusion


I. Introduction

Before we see the actual solvent levelling effect, let’s first see the different strength that acids-bases have when they’re dissolved in different solvents. This is the one of the basics you should know before learning the levelling effect.

As I have mentioned before, solvents are chemical and may react with other chemicals, given the right condition. We have seen several times before that solvents may react as an acid or as a base depending on the species dissolved in it. In this post, we will see that the acid/base properties of the solvents can influence the strength of acids and bases.

Note: This post has a lot of calculations and links referring back to the previous posts, especially Acid-Base V and Acid-Base VIII. Be prepared to calculate too. A lot of them will involve you calculating various pK values and comparing your results with what I have written here. Therefore, this is a good place for you to practice.

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II. Acids and Bases Have Different Strength in Different Solvents

In Section II.6. of Acid-Base IV, we have seen how several acids are less acidic in DMSO than in water. Now, I want to give you some basics that you will need to understand the next post, where the real thing is.

Let’s compare the pKa of several acids in different solvents: water and in acetic acid. Later, let’s also compare the pKa of several bases in different solvents: water and in ammonia.

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II.1. Comparison of Acid Strength in Water and Acetic Acid

Remember: acetic acid (pKa = 4.74) is less basic (more acidic) than water (pKa = 15.7)

In this section, we shall talk about the acidity of several acids in water and acetic acid:

  • Perchloric acid, HClO4 (pKa = –10)
  • Hydrobromic acid, HBr (pKa = –9)
  • Nitric acid, HNO3 (pKa = –1.3)
  • Hydrofluoric acid, HF (pKa = 3.2)

Here’s the acid dissociation reactions of the above acids in water. Their equilibrium constants are Ka.

abviii-fig01-acidskinwater

Recall Section III of Acid-Base VII: strong acids are those with Ka > 1. In other words, those with pKa < 0.

Therefore in water, HClO4, HBr, and HNO3 are strong acids. On the other hand, HF is a weak acid.

Now, compare them with the ‘Ka‘ in acetic acid. Note that I put the quotation marks ‘Ka‘ because these are just theoretical Kas calculated through the approximation method we used in predicting the course of a reaction in Acid-Base V (try to calculate them yourself for a practice!). Recall the note I put in The Important Box in that post.

This calculation does not take into account any other factors that may occur in the real world such as intermolecular interaction and solubility to name a few. The real values of Ka in HOAc should be measured experimentally but unfortunately, I could not find the values for these acids. That being said, these calculated values can still show you the interesting possible acidity trend of those acids in acetic acid.

abviii-fig02-acidskinhoac

The first thing you should notice is that all of them have larger pKa values in acetic acid than in water. This means that these acids are less acidic in acetic acid than they are in water. What causes this?

Remember that acetic acid is less basic than water; therefore it accepts proton less readily than water. This means that the acids can’t donate their proton to acetic acid as easily as they can to water. This consequently reduces their acidity.

Secondly, compare the pKa values of HNO3: its value is –1.3 in water, and 4.7 in acetic acid. This means that HNO3 is a strong acid in water, but it is a weak acid in acetic acid!

From here, I think you should be able to connect the dots and conclude that in solvents with pKa smaller than water (pKa < 15.7), acids are less acidic. Consequently, this means that in solvents with pKa larger than water (pKa > 15.7), acids are more acidic.

In other words:

  • In more basic solvents, acids tend to become more acidic
  • In less basic solvents, acids tend to become less acidic

To prove this, try to calculate the ‘pKa‘ of those acids in t-butanol (pKa = 19) as a solvent! Again I should remind you that these are only calculated values, used solely to show the possible trend of acidity in that particular solvent. The actual values of pKas in t-butanol should be measured experimentally.

What about in DMSO?

DMSO is more basic than water, but as we have seen in Acid-Base IV, acids in DMSO are less acidic. Well, I have said earlier in this post that the trend that we see through this method is an approximation. They don’t take into account many factors that may also influence the acidity of certain species. The effect that we saw with DMSO in Acid-Base IV is highly influenced by the solvation capability of DMSO. It is less capable of stabilising the anionic conjugate base compared to water and therefore making the acids dissolved in it less acidic.

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II.2. Comparison of Base Strength in Water and Ammonia

Remember: ammonia (pKa = 38) is less acidic (more basic) than water (pKa = 15.7)

In this section, we shall talk about several bases:

  • Butyllithium, C4H9Li (pKaH of C4H9 = 50)
  • Phenyllithium, C6H5Li (pKaH of C6H5 = 43)
  • Potassium t-butoxide, (CH3)3COK (pKaH of (CH3)3CO = 19)
  • Sodium phenoxide, C6H5ONa (pKaH of C6H5O = 10)

Talking about bases is more complicated than talking about acids. Even though we have talked a lot about seeing a base through its conjugate acid, I believe a lot of people are still used to using Kb and pKb instead of KaH and pKaH.

So this is what I’m going to do: first we talk about the comparison of base strength using Kb and pKb and after that we shall look at the same comparison using KaH and pKaH. Both are equally important.

This where the discussion that we did in Section V. of Acid-Base VIII plays role. I will write it down again here:

For bases:

  • A base dissolved in HSolv is defined as a strong base if:
    • pKb (A(solv)) < 0
    • pKaH (A(solv)) > pKsolv
  • A base dissolved in HSolv is defined as a strong base if:
    • pKb (A(solv)) > 0
    • pKaH (A(solv)) < pKsolv

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II.2.1. Comparison of Base Strength Using Kb and pKb

Remember: ammonia (pKa = 38) is less acidic (more basic) than water (pKa = 15.7)

To obtain Kb and pKb of the aforementioned bases, let’s use the formula we used in Section IV. of Acid-Base VIII. Remember that because the pKaH values are of those dissolved in water, we have to use pKw as pKsolv. Calculate them yourself and see if your answers match those shown below.

Here’s the base dissociation reaction of the aforementioned bases in water along with their calculated Kb and pKb values.

abviii-fig03-baseskbinwater

We have defined strong bases as those with Kb > 1. In other words, those with pKb < 0.

Therefore in water, butyllithium, phenyllithium and potassium butoxide are strong bases. On the other hand, sodium phenoxide is a weak base in water.

Let’s now compare them with the ‘Kb‘ values in ammonia. Again, I should remind you that these values are calculated and not experimentally determined, which is why I put the quotation mark. These values were calculated through the method we used in Acid-Base V, again try and calculate them yourself and compare the results.

abviii-fig04-baseskbinnh3

Because ammonia is less acidic than water, these bases can’t take away ammonia’s proton as easily as they can towards water. This makes them less basic, as indicated by their larger pKb values.

Butyllithium and phenyllithium are still strong bases in ammonia. However, notice that potassium t-butoxide is a weak base (pKb > 0) in ammonia, even though it is a strong base in water.

We can conclude that in solvents with pKa larger than water (pKa > 15.7), bases are less basic. Consequently, this means that in solvents with pKa smaller than water (pKa < 15.7), bases are more basic.

In other words:

  • In more basic solvents, bases tend to become less basic
  • In less basic solvents, bases tend to become more basic

To prove this, try to calculate the ‘pKb‘ of those bases in acetic acid (pKa = 4.74) as a solvent! Again I should remind you that these are only calculated values, used solely to show the possible trend of acidity in that particular solvent.

You might ask ‘Hey, acetic acid is an acid, how can we dissolve a base in an acid? They will react!’. If you do ask so, then you haven’t fully understood Acid-Base VIII and this post. Don’t forget that when you dissolve those bases in water, water also acts as an acid and therefore react with the base (i.e. water donates its proton to the base). Therefore, dissolving those bases in water also counts as dissolving them in an acid, albeit not a very strong acid. Never forget that solvents are also chemicals, they may react with other chemicals in suitable conditions and are not always inert.

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II.2.2. Comparison of Base Strength Using KaH and pKaH

Remember: ammonia (pKa = 38) is less acidic (more basic) than water (pKa = 15.7)

Again, remember here that:

  • A strong base has pKaH (A(solv)) > pKsolv
  • A weak base has pKaH (A(solv)) < pKsolv

The value of pKsolv when the solvent is water is pKw, which is 14. And when the solvent is ammonia, the value of pKsolv is 33. Remember don’t get confused between pKsolv and pKa of ammonia. Refer to Section III. of Acid-Base VIII.

Here’s the base dissociation reaction of the aforementioned bases in water along with their KaH and pKaH values.

abviii-fig05-baseskahinwater

The bases with pKaH > pKw (pKaH > 14) are t-butyllithium, phenyllithium, and potassium t-butoxide. On the other hand, since pKaH of sodium phenoxide is smaller than pKw (pKaH < 14), it is considered a weak base in water.

Therefore, all of these are in line with what we have discussed in Section II.2.1. of this post, these three bases are strong bases in water!

And here are the base dissociation reaction of the aforementioned bases in ammonia along with their calculated ‘KaH‘ and ‘pKaH‘ values. Again, I should remind you that these values are calculated and not experimentally determined, which is why I put the quotation mark.

abviii-fig06-baseskahinnh3

These values were calculated through the method we used in Section IV. of Acid-Base VIII, using the Kb and pKb that we calculatied in Section II.2.1.. Therefore, these values are obtained through the approximation of approximated values, which may represent the trend but not the actual value of KaH and pKaH of those bases in ammonia. Again try and calculate them yourself and compare the results. You can also use the method we used in Acid-Base V and you’ll get the same value.

Because the pKsolv of ammonia is 33, the strong bases in that list are those with pKaH > 33, namely butyllithium and phenyllithium. In ammonia, the pKaH of potassium t-butoxide is smaller than 33 and therefore, along with sodium phenoxide, is considered a weak base.

All of these are in line with what we have discussed in Section II.2.1. of this post.

Try to calculate the ‘pKaH‘ of those bases in acetic acid (pKa = 4.74) as a solvent! Because we don’t know the value of pKsolv for acetic acid, use the method we used in Acid-Base V to calculate them. Again I should remind you that these are only calculated values, used solely to show the possible trend of acidity in that particular solvent.

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III. Conclusion

In this post, we have proved that acids and bases may give different strength in different solvents:

  • In more basic solvents, acids tend to become more acidic
  • In less basic solvents, acids tend to become less acidic
  • In more basic solvents, bases tend to become less basic
  • In less basic solvents, bases tend to become more basic

Again, all of these are approximations. Acidity and basicity strength of certain species in a solvent has to be measured experimentally, although this method can lead you to an educated guess.

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Page last updated: 15ii17
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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