Acidic and Basic Strength

The acidic and basic strength in organic chemistry is defined as how strong or weak the acid or the base is. In simple words, the terms strong and weak show the ability of acidic and basic solutions to conduct electricity. If electricity is conducted strongly by the solution, it is called a strong acid or base. If the electricity is conducted weakly by the solution, it is called a weak acid or base. The acid and basic strength can be defined by several theories, as explained below.

Arrhenius Theory

As per Arrhenius Theory, acidic and basic strength in organic chemistry is defined as:

• When dissolved in water, acids release H+ ions. The greater the number of H+ ions produced in the solution, the stronger the acidic strength.
• Bases release OH– ions when dissolved in water. The greater the number of OH– ions produced in the solution, the stronger the basic strength.

Bronsted Lowry Theory

J.N. Bronsted and T.M. Lowry published the theory of proton donors and acceptors in acid-base reactions. Bronsted-Lowry theory’s main feature is identifying the proton (H+) transfer happening in the acid-base reaction. This is shown in the following equation:

HA + B ⇌ A– + HB+

E.g. HCl+ + H2O ⇌ CL– + H3O+

and HOH+ + NH3 ⇌ NH4+ + OH–

In this case, HOH is a base in the first reaction and acid in the second reaction.

To check if a substance is an acid or a base, count the number of hydrogen on each element before and after the reaction. If the number of hydrogens decreases after the reaction, the element is the acid since it loses hydrogen ions. If the number of hydrogens increases after the reaction, the element is the base since it gains hydrogen ions. These definitions are applicable to the reactants on the left. If the reaction is observed in reverse, we can identify a new acid and base. The elements on the right side of the equation are known as the conjugate acid and conjugate base of those on the left side of the equation. It should be noted that the original acid turns into the conjugate base after the reaction.

Bases are Proton Acceptors, and Acids are Proton Donors

For an equilibrium reaction, a transfer of electrons needs to occur. The acid will lose an electron away, and the base will gain the electron. Acids and Bases that work in this way are called conjugate pairs consisting of conjugate acids and conjugate bases. As per the reaction illustration given earlier: HA + B ⇌ A– + HB+

A Stands for an Acidic compound, and B stands for a Basic compound.

• A donates H to form HB+.
• B accepts H from A, which forms HB+.
• A– becomes the conjugate base of HA, and in the reverse reaction, it accepts an H from HB to recreate HA in order to remain in equilibrium.
• HB+ becomes a conjugate acid of B, and in the reverse reaction, it donates an H to A– recreating B in order to remain in equilibrium.

Lewis Theory

As per Lewis Theory, the acidic and basic strength in organic chemistry can be defined as below:

Lewis Acids:

• Acids have empty orbitals and can accept electron pairs from lewis bases.
• It is used to describe substances with trigonal planar structure and an empty p-orbital.
• Lewis acids can be considered electrophiles.
• Some examples of Lewis acids are: 
• H+ ions
• Cations of d-block elements that can have high oxidation states and can accept electron pairs such as Fe3+
• When forming coordination compounds with water, cations of metals such as Mg2+ and Li+ can accept electron pairs and act as lewis acids.
• Pentahalides of group elements such as Antimony, Arsenic and Phosphorus
• Any electron-deficient π system can behave as an electron pair acceptor.

Lewis Bases

• Bases have Highest Occupied Molecular Orbital.
• They can donate electron pairs to a Lewis acid.
• These are anionic in nature, and their basic strength depends on the ionisation constant of the corresponding acid.
• Lewis bases can be considered nucleophiles.
• Some examples of Lewis bases are:
• Pyridine and its derivatives can act as electron-pair donors, hence classified as lewis bases.
• Oxygen, Sulphur, Tellurium, Selenium compounds with -2 oxidation states
• Simple anions having electron pairs also act as lewis bases by donating the electrons such as H- and F-
• Any electron-rich π system such as Benzene, ethyne and ethene etc. can behave as an electron pair donors

The acidic strength of an acid is weak, and the basic strength of the conjugate Lewis base is strong. Water can be classified as both Lewis acid and base since it can donate or accept electron pairs based on the reaction.

Conclusion

The acidic and basic strength in organic chemistry tells us how strong or weak the acid or the base is. The terms strong and weak show the ability of acidic and basic solutions to conduct electricity. If electricity is conducted strongly by the solution, it is called a strong acid or base. Theories such as Arrhenius Theory, Bronsted-Lowry Theory and Lewis Theory explain the acidic and basic strength of substances. If the acid is strong, the conjugate base is weak. If the acid is weak, the conjugate base is strong. Water can be classified as both Lewis acid and base since it can donate or accept electron pairs based on the reaction.