Contents
III. Acid-Base and Their Conjugate Pair
IV. Examples of Acid-Base and Their Conjugate Pair
I. Introduction
In this post, we shall start with the fundamentals: the three theories of acid-base. This is then followed by the concept of acid-base and conjugate pair.
The aims of this post are:
- To refresh your memory about the fundamental theories of acid-base so that you are ready for the more advanced topic in the following posts
- To demonstrate to students how organic compounds are able to act as acids or bases
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II. Acid and Base
There are three acid-base theories that are widely accepted in chemistry:
1. Arrhenius Theory
‘In solution, all acids release H+ ion, whilst all bases release OH– ion. Neutralisation reaction happens because H+ ion and OH– ions react to form water.’
This theory works well with compounds such as HCl, H2SO4, H3PO4, NaOH, KOH, etc. All of those acids release H+ in water, and all of those bases release OH–.
Some organic compounds also exhibit this phenomenon. A famous example is acetic acid (CH3COOH), which is a carboxylic acid. Carboxylic acid, true to its name, is known to be weak acids. Other carboxylic acid examples: formic acid (HCOOH) and benzoic acid (C6H5COOH). All of them dissociate to give H+ in equilibrium.
Unfortunately, this theory fails to explain why nitrogen-containing compounds such as NH3 (ammonia), Et3N (triethylamine) or C5H5N (pyridine), have basic properties even though: 1. they do not release OH– in water, and 2. they are able to react with acids (e.g. HCl) and neutralisation reaction can happen between them.
This theory also fails to explain that some organic compounds have inherent acid/base properties that are not really obvious when you only look at their structures.
2. Brønsted-Lowry Theory
‘An acid is a proton (H+) donor, whilst a base is a proton (H+) acceptor’
Brønsted-Lowry triumphs where Arrhenius theory fails, however it does not go against Arrhenius theory, it merely complements it.
This theory succesfully explains that nitrogen-containing compounds are basic because they are able to accept H+. Examples:
This theory also explains why some organic compounds are able to act as acids or bases. An example is acetophenone:
3. Lewis Theory
‘An acid is an electron pair acceptor, whilst a base is an electron pair donor’
At a glance, you’d probably feel weird. In the Brønsted-Lowry theory, an acid is a donor and a base is an acceptor, but in the Lewis theory everything is the other way around??
Yes, it is indeed! And they are not contradictory, they are in line with each other:
Remember that in the Brønsted-Lowry theory, the acid is a proton donor. This means, the acid gives away a positive charge and therefore the overall charge of the acid goes down and becomes more negative.
You can also get the same thing when you accept an electron. If an acid accepts an electron, it takes in a negative charge and therefore the overall charge of the acid goes down and becomes more negative.
Can you see how they’re basically similar, it’s just they are looking at it from two different perspectives and two different cases.
A classic example of a Lewis acid-base reaction is the reaction of NH3 and BF3 (boron trifluoride):
In the example above, NH3 gives away its free electrons, which makes it a base. On the other hand, BF3 accepts the electrons and makes it an acid.
Let’s have a look at another example that involves an organic compound. Lewis acid TiCl4 is able to accept a pair of electrons from the carbonyl oxygen of propanal. In this reaction, propanal acts as a Lewis base.
This reaction is the initial step in Mukaiyama aldol addition.
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III. Acid-Base and Their Conjugate Pair
When a Brønsted-Lowry acid donates its proton to a Brønsted-Lowry base, the acid becomes a species known as a conjugate base and the base becomes a species known as a conjugate acid:
The picture above shows that the acid HA (as a reactant) loses a proton, giving conjugate base A– as its product.
The proton is accepted by the base B (also a reactant), giving conjugate acid HB+ as its product.
The term acid/base and conjugate acid/base in a species depend on how we look at the reaction. If we look at the above reaction the other way around (the reverse reaction), then:
The picture above shows that the acid is now HB+. It loses a proton, giving conjugate base B as its product.
The base is now A–. It accepts the proton, giving conjugate acid HA as its product.
Let’s summarise both reactions. Let’s call the first reaction the forward reaction and the second reaction the reverse reaction.
| Species | Forward Reaction |
Reverse Reaction |
| HA | acid | conjugate acid |
| B | base | conjugate base |
| A– | conjugate base | base |
| HB+ | conjugate acid | acid |
Now let’s have a look at the table above in two different ways:
1. Column-wise
Let’s have a look at the ‘Forward Reaction’ column.
Here, if you look at the HA/A– pair, you’ll see that HA is an acid, and A– is a conjugate base. Therefore, we can say that: A– is the conjugate base of HA.
In case of the B/HB+ pair, we can say that: HB+ is the conjugate acid of B.
And now let’s have a look at the ‘Reverse Reaction’ column.
Here, if you look at HA/A– pair, you’ll see that HA is an conjugate acid, and A– is a base. Therefore, we can say that: HA is the conjugate acid of A–.
In case of B/HB+ pair, we can say that: B is the conjugate base of HB+.
This is very important to remember, as you will have to be able to know which species has which role as soon as you see a reaction.
2. Row-wise
If you have a look at each of the rows, you’ll realise that they retain their acid/base property. Take HA in the first row for example, it is an acid in the forward reaction, and it is a conjugate acid in the reverse reaction. In both cases it is still a base.
- When you reverse the point of view of a reaction, the role of each species changes.
- No matter from which perspective you choose to see a reaction, an acid will still be an acid, and so does a base.
- A species gets the ‘conjugate’ title, if and only if it is seen as a product (on the right side of the chemical equation).
Therefore, a species that was once an acid (like HA as a reactant in the forward reaction) becomes a conjugate acid when we generate it as a product in the reverse reaction.
This also applies for the rest of the species above. In both examples, H2O is still an acid, NH4+ is also still an acid, and OH– is still a base. Each of them gets the ‘conjugate’ title when they are on the right side of the chemical equation.
To summarise:
- An acid that donates its proton becomes a conjugate base
- A base that accepts proton becomes a conjugate acid
- The ‘conjugate’ title is only given when the species is seen as a product, i.e. on the right side of the chemical equation.
In other words:
- Conjugate acid is the protonated version of a base
- Conjugate base is the unprotonated version of an acid
Note:
This is quite obvious, but I would like to point it out anyway.
Not all bases are neutral like pyridine or triethylamine. Some bases are negatively charged, and such bases, if protonated, will form neutral conjugate acid (HB).
We saw this example in the deprotonation of acetophenone using methoxide ion.
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IV. Examples of Acid-Base and Their Conjugate Pair
Let’s see two real-life examples:
1. HCl and H2O
In this example, HCl loses a proton (proton donor) and becomes Cl– ion. By definition, this means that HCl is an acid and Cl– is its conjugate base.
H2O accepts that proton (proton acceptor) and becomes H3O+ ion. Hence, H2O is a base and H3O+ is its conjugate acid.
Let’s now see the reverse reaction:
In this example, H3O+ ion loses a proton (proton donor) and becomes H2O. By definition, this means that H3O+ is an acid and H2O is its conjugate base.
Cl– ion accepts that proton (proton acceptor) and becomes HCl ion. Hence, Cl– is a base and HCl is its conjugate acid.
Let’s summarise all of that in a table:
| Species | Example 1 Forward |
Example 1 Reverse |
| HCl | acid | conjugate acid |
| H2O | base | conjugate base |
| Cl– | conjugate base | base |
| H3O+ | conjugate acid | acid |
If you look at the table column-wise and row-wise like before, can you see how the arguments from section III hold?
2. NH3 and H2O
In this example, H2O loses a proton (proton donor) and becomes OH– ion. By definition, this means that H2O is an acid and OH– is its conjugate base.
NH3 accepts that proton (proton acceptor) and becomes NH4+ ion. Hence, NH3 is a base and NH4+ is its conjugate acid.
Let’s now see it the other way around:
In this example, NH4+ ion loses a proton (proton donor) and becomes NH3. By definition, this means that NH4+ is an acid and NH3 is its conjugate base.
The OH– ion accepts that proton (proton acceptor) and becomes H2O. Hence, OH– is a base and H2O is its conjugate acid.
Let’s summarise all of that in a table:
| Species | Example 2 Forward | Example 2 Reverse |
| NH3 | base | conjugate base |
| H2O | acid | conjugate acid |
| NH4+ | conjugate acid | acid |
| OH– | conjugate base | base |
If you look at the table column-wise and row-wise like before, can you see how the arguments from section III hold?
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V. Conclusion
So let’s have a look at the overall picture here!
There are three acid/base theories and all of them are useful in organic chemistry. However, the Brønsted-Lowry theory is probably the most common type we will encounter.
Acid/base and conjugate pair relationships in Brønsted-Lowry theory:
- An acid that donates its proton becomes a conjugate base
- A base that accepts proton becomes a conjugate acid
In other words:
- Conjugate acid is the ‘protonated version’ of a base
- Conjugate base is the ‘unprotonated version’ of an acid
These are the only concepts that you need to arm yourself with in order to understand the next post.
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VI. Try It Yourself!
Why not try to do some problems yourself? This time let’s use real organic compounds.
Determine which species donates proton (acid) and which accepts it (base).
Then, determine the ‘unprotonated version’ of the acid (conjugate base), and the ‘protonated version’ of the base (conjugate acid).
1. Acetophenone and NaOMe (sodium methoxide)
Which one is the acid/conjugate base pair or base/conjugate acid pair?
Highlight text to reveal the answer: acetophenone is an acid (donates proton), its conjugate base is acetophenone carbanion. Methoxide ion is a base (accepts proton), its conjugate acid is methanol.
2. Benzophenone and HCl
Let’s have another one:
Which one is the acid/conjugate base pair or base/conjugate acid pair?
Highlight text to reveal the answer: benzophenone is a base (accepts proton), its conjugate acid is protonated benzophenone. HCl is an acid (donates proton), its conjugate base is chloride anion.
3. The reverse reactions
If you need more examples, try doing examples 1 and 2 above by looking at the reverse reaction and determine the conjugate pairs (i.e. the products are now reactants, and the reactants are now products).
Compare the backwards reactions with the forward reactions and make a table like you saw in Section III and Section IV before. You should get a similar table like what we have and you can confirm that you are correct by looking at the table row-wise.
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