A very important element in organic chemistry is acids and bases. It is because of the presence of acids and bases that the synthetic reactions of compounds proceed as electrons are transferred. In other words, without acids and bases, organic synthetic reactions would not occur.
Therefore, we must understand the definitions of acids and bases in advance. We have to understand what compounds are acids and what compounds are bases.
However, whether it is an acid or a base depends on the conditions. What is acid at one time is often a base under other conditions. A substance that is acidic in solution is not necessarily an acid. Even substances that are generally considered to be acids can often be bases.
So let’s understand the definitions of acids and bases. We’ll first understand the Bronsted-Lowry definition and then we’ll check how to think about Lewis acids and Lewis bases.
Table of Contents
- 1 High School Chemistry Is Defined by Arrhenius and Bronsted-Lowry
- 2 A Measure of Acid Strength: pKa Value
- 3 How Are Acidity and Basicity Determined?
- 4 Differences and Way of Thinking About Lewis Acids and Lewis Bases
- 5 There Are Definitions, Types and Characteristics of Acids and Bases
High School Chemistry Is Defined by Arrhenius and Bronsted-Lowry
There are some definitions of acids and bases. One of the easiest and most straightforward is the Arrhenius definition. Arrhenius definition of acids and bases is as follows.
- Acid: a compound that produces H+ in an aqueous solution.
- Base: Compound that produces OH– in aqueous solution.
However, this definition is useless in chemistry. To begin with, chemical reactions don’t only proceed in aqueous solutions.
So the Bronsted-Lowry theory came about. The definition of Bronsted-Lowry defines acids and bases as follows.
- Bronsted acid: a molecule that gives H+
- Bronsted base: molecules that receive H+
In high school chemistry, we were up to the Bronsted-Lowry theory. And even if you learn about Bronsted acids and Bronsted bases, you don’t learn about conjugate acid, conjugate base, indices of acid strength, and other details. Therefore, it is necessary to deeply understand why acids and bases are generated.
Also, to understand Lewis acids and Lewis bases, it is common to first learn about Bronsted acids and Bronsted bases, so let’s learn this content.
In Acids and Bases, There Are Always Conjugate Acid and Conjugate Base
There are many molecules known as acids and bases. For example, ammonia is a base. An ammonia molecule is a base because it receives H+. Water, on the other hand, is an acid because it gives H+.
The molecules that form after the acid gives H+ are called the conjugate base. The fact that the acid has given H+ means that a compound with a negative charge is generated. This compound receives H+ in the reverse reaction, hence the name conjugate base.
On the other hand, the molecule that is formed after the base receives H+ is called a conjugate acid. The name conjugate acid comes from the fact that it gains a positive charge and gives H+ in the reverse reaction.
When ammonia dissolves in water and becomes an ion, NH4+ is a conjugate acid. It is easy to understand if you think of it as a positively charged molecule becoming a conjugate acid after the reaction. On the other hand, when ammonia is dissolved in water, the OH– that is formed is called a conjugate base.
Bronsted Acids and Bronsted Bases Are Relatively Involved
So, are acids and bases absolute? No, acids and bases are relative. Often times they are acids at one time, but under other conditions they are bases.
For example, water is both an acid and a base. If water reacts with hydrochloric acid (HCl), it acts as a Bronsted base. On the other hand, if it reacts with ammonia, water gives H+ and becomes an acid.
Of course, many molecules, not just water, can become acids or bases.
If there is a stronger acid, the compound will act as a base. On the other hand, if you mix a strong basic reagent, the compound will act as an acid. Depending on what acidic or basic reagent is used, the acid and base of the compound will be different.
For example, is an amine an acidic substance or a basic substance? Generally speaking, compounds with amino groups are considered to be basic substances. However, this is because we are dealing with water. Depending on the conditions, amines can be acids.
Once we understand this fact, we can recognize that there are no absolute acidic or basic substances. This is because whether they are acidic or basic depends on what type of reagent you are using.
A Measure of Acid Strength: pKa Value
Then, how does it describe the strength of an acid? The pKa value is a measure of acidity.
For example, let’s say you dissolve hydrochloric acid (HCl) in water. However, not everything is ionized. Only some of it is ionized. At times, it exists as a hydrochloric acid molecule, and at other times, the hydrochloric acid is present as an ion, which indicates acidity.
The equilibrium constant (K) can be produced at equilibrium.
However, in the equilibrium constant (K), water (H2O) is a solvent and is present in excess. And even if it reacts with hydrochloric acid, the amount of water will change only slightly because of the large amount of water present as a solvent. Therefore, we can ignore [H2O] in the above equation.
Therefore, the equilibrium constant can be written as follows, omitting [H2O].
However, it is difficult to understand the equilibrium constant as it is. So, we can apply pKa to the following equation using the log.
- pKa = -log Ka
The lower the pKa value, the more acidic it is. If you think about it the other way around, the higher the pKa value, the higher the basicity of the product. When checking acidity and basicity, be sure to check the pKa value.
For reference, the pKa is as follows for each molecule.
There is no need to remember the pKa values of these molecules. Just understand that there are differences in acidity between these compounds. Also make sure that the lower the pKa value, the more acidic it is, and the higher the pKa value, the more basic it is.
The More Stable the Conjugate Base Is, the More Acidic It Is (the Less Nasicity).
When does it become an acid or a base? Let’s understand that this depends on the nature of the conjugate acid and the conjugate base.
Whether a compound is acidic depends on whether the conjugate base is stable or not. When an acid becomes an ion, it becomes negatively charged by providing H+. In other words, in an ion, a conjugate base is produced.
When the conjugate base state is stable, the compound actively tries to provide H+. It is trying to provide H+ as quickly as possible to become negatively charged.
In the same way, the strength of the base can be thought of in terms of the strength of the conjugate acid. The conjugate acid is what is produced after the base receives H+. The more stable the conjugate acid is, the more the base will try to become stable by receiving H+.
Incidentally, there is always an unshared electron pair (lone pair) in a base. The unshared electron pair receives hydrogen, which stabilizes the conjugate acid.
For this acidity and basicity, you need to check the pKa value. Of course, as mentioned above, there is no point in remembering the pKa value of a compound. However, by checking the stability of the conjugate acid and the conjugate base from the pKa table, you will be able to infer the acid/base reactivity of the compound.
How Are Acidity and Basicity Determined?
So how is acidity (or basicity) determined: the degree of acidity or basicity can be determined by a high or low pKa value. However, remembering the pKa value is useless; we must understand the mechanism by which the acidity or basicity is increased.
In this regard, acidity and basicity are related to the following factors
- Size of an atom
- Resonance structure
- Hybrid orbital
There are many other factors involved in acidity and basicity. However, we can say that these are the main factors.
-The Higher the Electronegativity, the Higher the Acidity
When an atom of high electronegativity is bonded to a hydrogen atom, it causes the molecule to split into positive and negative charges. As a result, the molecule becomes polarized. Due to the high electronegativity, the higher the degree of polarization, the more the atoms attract electrons. This means that hydrogen atoms are more likely to be positively charged, resulting in a higher degree of acidity.
Compare, for example, the second-period elements. In this case, the acidity is ordered by the electronegativity.
- CH4 (pKa: 48) > NH3 (pKa: 33) > H2O (pKa: 15.7) > HF (pKa: 3.2)
The higher the electronegativity, the higher the acidity.
-The Larger the Atom, the Higher the Acidity
However, acidity is not the only factor involved in electronegativity. The size of the atoms is also important. For example, halogens have the following pKa.
- HF (pKa: 3.2) > HCl (pKa: -7) > HBr (pKa: -9) > HI (pKa: -10)
If we think in terms of electronegativity, the fluorine atom has the highest degree of electronegativity. However, the acidity is reversed with respect to the order of the electronegativity. This is related to the size of the atoms.
The larger the atoms, the greater the bond distance between the molecules, the weaker the bond strength. As a result, it is more likely to provide H+ and become an ion. The larger the atom, the easier it is to stabilize the structure of the conjugate base.
-The Structure Is Stabilized by Resonance of the Conjugate Base
Resonance also makes a significant contribution to acidity (or basicity). Resonance of the conjugate base stabilizes the electrons in various locations. This is called the delocalization of electrons. For example, p-nitrophenol can write the following resonance structure.
It is more acidic than phenol because electrons can move to many places. This is because the conjugate base is stabilized by resonance. Similarly, when the conjugate acid is stabilized by resonance, it becomes more basic.
-The More s Character the Hybrid orbital, the More Acidic It Is
Mixed orbitals are related to acidity. There are two types of orbitals: s-orbital and p-orbital, and the higher the percentage of s-orbitals, the more acidic they are.
The s orbital is closer to the nucleus than the p orbitals. Since the nucleus is positively charged, it is easier to stabilize a negative charge. Therefore, even if it is negatively charged as a conjugate base, it is easy to stabilize it. As a result, the acidity is high.
For example, the order of acidity is as follows
- HC ≡ CH (acetylene: pKa25)
- H2C=CH2 (ethylene: pKa44)
- CH3CH3 (ethane: pKa50)
The s-character (percentage of s-orbitals) of ethane is 25%. On the other hand, the s-character of ethylene is 33%. Acetylene’s s-character is 50%. Therefore, the acetylene with high s-character is more acidic.
Differences and Way of Thinking About Lewis Acids and Lewis Bases
This is the end of Bronsted-Lowry detailed discussion of the definitions of acids and bases. It is important in organic chemistry to understand when they become acids and bases.
However, in organic chemistry, even Bronsted-Lowry theory is not sufficient. The reason is simple: there are many synthetic reactions in chemical reactions that do not involve H+. Therefore, we have to understand a different concept for acids and bases.
So, we will use Lewis acids and Lewis bases. Lewis acids and Lewis bases are defined as follows
- Lewis acid: a molecule that receives electrons.
- Lewis bases: molecules that provide electrons.
Lewis acids are Bronsted acids and Lewis bases are also Bronsted bases. For example, they react as follows.
In synthetic organic chemistry, chemical reactions are often described by electron arrows. Electrophilic reagents are used as reagents that receive electrons in these reactions. The electrophilic reagent is a Lewis acid.
On the other hand, there are reagents that attack other compounds. These reagents are called nucleophilic reagents. Nucleophilic reagents are Lewis bases, and it is known that the stronger the basicity, the stronger the nucleophilic property. Lewis acids and Lewis bases allow reactions to be considered by the acid and base, even if the reaction does not involve H+.
How to Identify Electrophilic and Nucleophilic Reagents
So how do we distinguish between a Lewis acid (electrophilic reagent) and a Lewis base (nucleophilic reagent)?
The way to distinguish them is simple: when you draw an arrow in a chemical reaction, the molecule that accepts electrons is a Lewis acid. The more acidic a compound is, the more capable it is of accepting electrons.
When we understand this, we can also consider the following compounds with empty orbitals to be acids in Lewis acids.
- Boron (B)
- Aluminum (Al)
Why do boron and aluminum become Lewis acids? Because they have unoccupied orbital that allows them to accept electrons. That’s why boron (borane: BH3) and aluminum (aluminum chloride: AlCl3) are known to be powerful Lewis acid catalysts.
Metals are often used in Lewis acids. Metal elements have empty orbitals, which often result in Lewis acids that accept electrons.
Compounds with these orbitals do not attack other molecules and make new bonds. Therefore, they can be identified as Lewis acids.
-Distinguish Between Lewis Bases
On the other hand, what about Lewis bases? In nucleophilic reagents, they will always have an unshared electron pair (lone pair). Think of any compound that is strongly nucleophilic (strongly basic) as having an unshared electron pair.
Even if the lone pair is present, it can still be an acid. In fact, although amines are generally basic, in the presence of a stronger base, the amine can become acid.
But without an unshared electron pair, they cannot be nucleophiles. Understand that in order to give an electron, the presence of an electron that is not involved in the bond is essential. This is how you should distinguish between Lewis acids (electrophilic reagents) and Lewis bases (nucleophilic reagents).
There Are Definitions, Types and Characteristics of Acids and Bases
In organic chemistry, most reactions are carried out by acids and bases. All of these compounds have acidity and basicity, and by identifying the conjugate acids and bases, we can determine the difference in acidity (pKa).
Acids and bases by Bronsted-Lowry definition are relative. Depending on what reagents you do with them, they will be acids in some cases and bases in others. With acids and bases, you have to recognize this fact.
However, in chemical reactions, H+ doesn’t necessarily move. Therefore, Bronsted acids and Bronsted bases are not enough. So we have to learn the concept of Lewis acids and Lewis bases. You can distinguish between acids and bases depending on whether they receive or give electrons.
Learning these differences will help you to understand organic chemistry better. Acids and bases are the first steps in organic chemistry, so make sure you understand the concepts.