The key difference between base and nucleophile is that bases are hydrogen acceptors that can perform neutralizing reactions whereas nucleophiles attack electrophiles to initiate some certain organic reactions.
Acids and bases are two important concepts in chemistry. Both of them have contradictory properties. A nucleophile is a term, which we use more prominently in organic chemistry to describe reaction mechanisms and rates. Structurally, there isn’t a distinguished difference between base and nucleophile, but functionally they perform different duties.
CONTENTS
1. Overview and Key Difference
2. What is Base
3. What is Nucleophile
4. Side by Side Comparison – Base vs Nucleophile in Tabular Form
5. Summary
What is Base?
We can define bases in several ways according to the definitions of various scientists. Arrhenius defines a base as a substance that donates OH– ions to the solution. According to Lewis, any electron donor is a base. Bronsted- Lowry defines a base as a substance that can accept a proton. 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.
However, based on Lewis and Bronsted- Lowry theories, there are some 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. Likewise, Na2CO3 is a Bronsted-Lowry base without hydroxide groups but has the ability to accept hydrogens.
Figure 01: Chemical Elements in the Periodic Table that can form Hard, Soft and Intermediate Basic Compounds
Properties of Bases
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. We can categorize bases into two categories, depending on their ability to dissociate and produce hydroxide ions. Strong bases like NaOH, KOH can undergo complete ionization in a solution to give ions. Weak bases like NH3 are partially dissociated and give fewer amounts of hydroxide ions.
Kb is the base dissociation constant. It gives an indication of the ability to lose hydroxide ions of a weak base. Acids with a higher pKa 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.
What is a Nucleophile?
We can name any negative ion or any neutral molecule that has at least one unshared electron pair as a nucleophile. Nucleophile is a substance that is very electropositive, therefore, like to interact with positive centres. 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 has a partial positive charge due to the electronegativity difference between the carbon atom and the halogen atom.
Figure 02: Reactions of Dicarbamoyl Chloride with Nucleophiles
After the nucleophile attaches to the carbon, the halogen leaves. We call this type of reactions as nucleophilic substitution reactions. There is another type of reactions that initiates with the nucleophiles; they are 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 occur fast, and if the nucleophilicity is low, the reaction rate is slow. Since nucleophiles donate electrons, according to the Lewis definition, they are bases.
What is the Difference Between Base and Nucleophile?
The major difference between base and nucleophile lies upon their function. The bases are hydrogen acceptors which can perform neutralizing reactions whereas nucleophiles attack electrophiles to initiate some certain organic reactions. Hence, this is the key difference between base and nucleophile. Moreover, bases act as hydrogen acceptors which can perform neutralizing reactions while nucleophiles attack electrophiles to initiate some certain organic reactions.
As another important difference between base and nucleophile, we can take the type of chemical reactions that they involve in; bases involve in acid neutralizing reactions while nucleophiles involve in nucleophilic reactions. Furthermore, bases have a kinetic chemical nature which means, they react depending on the exposure it depends on. However, nucleophiles have a thermodynamic chemical nature which means they are affected by the other chemical reactions within the environment.
Summary – Base vs Nucleophile
Every nucleophile is a base, but all the bases are not nucleophiles. The key difference between base and nucleophile is that bases are hydrogen acceptors that can perform neutralizing reactions whereas nucleophiles attack electrophiles to initiate some certain organic reactions.
Reference:
1. Britannica, The Editors of Encyclopaedia. “Base.” Encyclopædia Britannica, Encyclopædia Britannica, Inc., 27 Dec. 2017. Available here
2. Libretexts. “Nucleophile.” Chemistry LibreTexts, National Science Foundation, 7 June 2018. Available here
Image Courtesy:
1.”Hardsoftbases”By Tem5psu – Own work, (CC BY-SA 3.0) via Commons Wikimedia
2.”DMCC-Reaktion mit Nukleophilen”By ChemDoc 2010 – Own work, (CC BY-SA 4.0) via Commons Wikimedia