Difference Between

Base vs Nucleophile

Acids and bases are two important concepts in chemistry. They have contradictory properties. Nucleophile is a term, which is more prominently used in organic chemistry to describe reaction mechanisms and rates. Structurally, there isn’t a distinguished difference between bases and nucleophiles, but functionally they perform different duties.

Base

Bases are defined in several ways by various scientists. Arrhenius defines a base as a substance that donates OH^- ions to the solution. Bronsted- Lowry defines a base as a substance that can accept a proton. According to Lewis, any electron donor is a base. According to the Arrhenius definition, a compound should have a hydroxide anion and the ability to donate it as a hydroxide ion to be a base. But, according to Lewis and Bronsted- Lowry, there can be molecules, which don’t possess hydroxides, but can act as a base. For example, NH₃ is a Lewis base, because it can donate the electron pair on nitrogen. Na₂CO₃ is a Bronsted- Lowry base without hydroxide groups, but has the ability to accept hydrogens.

Bases have a slippery soap like feeling and a bitter taste. They react easily with acids producing water and salt molecules. Caustic soda, ammonia and baking soda are some of the common bases we come across very often. Bases can be categorized into two, based on their ability to dissociate and produce hydroxide ions. Strong bases like NaOH, KOH are completely ionized in a solution to give ions. Weak bases like NH₃ are partially dissociated and give fewer amounts of hydroxide ions. K_b is the base dissociation constant. It gives an indication of the ability to lose hydroxide ions of a weak base. Acids with a higher pK_a value (more than 13) are weak acids, but their conjugate bases are considered as strong bases. To check whether a substance is a base or not, we can use several indicators like litmus paper or pH paper. Bases show a pH value higher than 7, and it turns red litmus to blue.

Nucleophile

A nucleophile can be any negative ion or any neutral molecule that has at least one unshared electron pair. Nucleophile is a substance that is very electropositive, therefore, like to interact with positive centers.  It can initiate reactions using the lone electron pair. For example, when a nucleophile reacts with an alkyl halide, the lone pair of the nucleophile attacks the carbon atom that bears the halogen. This carbon atom is partially positive charged due to the electronegativity difference between it and the halogen atom. After the nucleophile attaches to the carbon, the halogen leaves. This type of reactions is known as nucleophilic substitution reactions. There is another type of reactions initiated by nucleophiles, called nucleophilic elimination reactions. Nucleophilicity tells about the reaction mechanisms, thus, it is an indication of the reaction rates. For example, if the nucleophilicity is high, then a certain reaction can be fast and if the nucleophilicity is low, reaction rate is slow.  Since nucleophiles donate electrons, according to the Lewis definition, they are bases.