What is difference between Base and Nucleophile?

A base and a nucleophile are organic compounds which has opposite properties. Here we will discuss about acid base reaction, elimination reaction and nucleophilic substitution reaction which are favored by bases and nucleophiles.

Hi Friends, in this article we will learn about base and nucleophiles in organic chemistry.

Let’s start with base first.

What is a base?

There are three definitions are given for the concept of base. First is Arrhenius base; which states that a base is a chemical compound which dissociates in aqueous solution to form hydroxide ion (OH).

A simple example of Arrhenius base is sodium hydroxide, which dissociate in water to produce hydroxide ions.

Second definition is Bronsted base which states that a base is a chemical compound which accept hydrogen ion (also known as proton) from acid compound.

An example of Bronsted base is ammonia, it accepts a proton from hydrochloric acid to produce ammonium chloride.

                                

Third definition of base is Lewis base; it states that a base is chemical compound which is electron pair donar and it share a pair of electrons with an electron deficient molecule.

For example, bromine is reacts with iron(III) bromide to produce iron(IV) bromide. Here bromine donates a pair of electrons to Fe atom, hence it is acts as Lewis base.

What is nucleophile?

A nucleophile is a chemical species which is electron rich and it can donate a pair of electrons to an electron deficient species (also known as electrophile) to form new bond. A nucleophile can be negatively charged of neutral.

For example; sodium hydroxide reacts with butyl bromide to produce butyl alcohol. Here hydroxide ion act as nucleophile and it substitute bromine atom. Here the nucleophile (-OH) negatively charged. This is an example of nucleophilic substitution reaction.

Consider another example, water reacts with carbon dioxide to produce carbonic acid. Here water molecule acts as nucleophile. It is neutral molecule. This is an example of addition reaction.

Stability of bases

Reaction of acid and base is a reversible reaction. Hence stability of base is depended upon the position of equilibrium.

Similarities between a base and a nucleophile

Base and nucleophile are electron rich.

Both reacts with electron deficient species.

Difference between base and nucleophile

Base	Nucleophile
Reacts with acid proton	React with electron deficient species other than proton
Reaction with acid is reversible	Reaction with electrophile is irreversible.
Basicity is measured by position of equilibrium	Nucleophilicity is measured by rate of reaction
Temperature affects the stability of base	Along with temperature, steric hindrance and solvent also affects the stability of nucleophile

Types of Bases and nucleophiles

Based on reactivity of the bases; they can be differentiated as strong base and weak base. A strong base easily abstracts a proton to form stable molecule. Whereas a weak base reacts slowly with acid compounds.

In case of nucleophiles, there are two types of nucleophiles can be categorized as strong and weak nucleophiles. A strong nucleophile reacts with electron deficient species with higher rate of reaction than a weak nucleophile.

According to above classification the bases and nucleophiles are of four different types. They are described following table;

• Strong base – strong nucleophile

• Strong base- weak nucleophile

• Weak base- strong nucleophile

• Weak base- weak nucleophile

What is basicity and nucleophilicity?

The basicity of a chemical compound is a measure of its ability to accept a proton (H+) to form a stable product. This reaction is known as neutralization reaction.

Nucleophilicity of compound is the ability to displace a leaving group in a substitution reaction (SN1 / SN2)

Important note

If a molecule form bond with hydrogen atom, then it is called as Base.

If a molecule forms bond with atom other that hydrogen, then it is called as Nucleophile.

Let’s understand this with few examples;

Consider a reaction of sodium ethoxide with methyl chloride, it produces ethyl methyl ether. Here ethoxide ion substitutes Cl atom to form bond with carbon. Therefore, in this reaction ethoxide ion is playing a role as Nucleophile. This is an example of nucleophilic substitution reaction.

Now see the reaction of sodium ethoxide with diethyl malonate. Here ethoxide ion abstract most acidic proton to form neutral molecule and enolate ion. In this example ethoxide ion forms with hydrogen atom, therefore it is playing a role of Base. This is an example of elimination reaction.

I hope that it is been clear to you that what is a difference between a base and a nucleophile.

There are another kind of bases which are known as non-nucleophilic base. They are in general sterically hindered bases which does not act as a nucleophile. For example; 

Always remember one thing “All nucleophiles are bases but all bases are not nucleophiles”

Now Let’s recap what we learned here;

A strong base is a molecule which has ability to form bond with a proton from very weak acid. A strong base dissociates 100 % when placed in water.

A strong nucleophile is a molecule which has ability to react faster to form bond with an atom other than proton. In other words, the faster the reaction the better or stronger the nucleophile.

That’s all for this topic. If you feel this information helpful, then please do write in the comment section below.

See you in the next blog ..!!