Nucleophile

The nucleophile is a substance that has unshared pair of electrons available for bonding. However, this term is introduced by Christopher Kelk Ingold. They are mostly lewis bases. They may be negatively charged or neutral species. For example, hydroxide ion, water, etc. The strength of nucleophiles is greatly influenced by many factors. Such as resonance effect, steric effect, electronegativity, etc.

What is a nucleophile?

“An ion or molecule than can donate electrons is known as a nucleophile.”

Often lewis’s base act as the nucleophiles. The word nucleophile consists of two words, which are the ‘nucleus’ and ‘Philos’. Philos is a greek word which means ‘love’. Therefore, we can say that nucleophiles are nucleus-loving species. The general abbreviation used for nucleophiles is Nu. Some examples of nucleophiles are given below:

In a reaction of the nucleophiles with an alkyl halide. The carbon atom of alkyl halide is a positive center. Hence, it attracts the nucleophile. For example, in the reaction below water act as a nucleophile. However, it attacks the partially positive charge carbon atom of alkyl halide.

CH3Cl + 2H2O → CH3OH + H3O+ + Cl

History

Christopher Kelk Ingold introduced the term nucleophiles in 1933. He replaces the term anionoid presented by the A. J. Lapworthin 1925.

Types of nucleophile

There are the following types of nucleophiles:

Negatively Charged Nucleophiles

When we use negatively charged nucleophiles the resultant products are neutral. This is because the reaction between negative nucleophiles and the substrate neutralizes the charge of the nucleophiles.

HO + RX → ROH + X

neutral nucleophiles

When we use neutral nucleophiles, an initially positively charged product is formed. Later, the proton is removed from the atom that has a positive charge in the initial product. Thus a neutral product is created.

Another example of the use of neutral nucleophiles is the solvolysis reaction.

“The solvolysis is a  reaction in which solvents are present in great excess. Hence, the nucleophiles are the solvent molecule.”

For example, the solvolysis of water converts alkyl halide into alcohol.

CH3Cl + 2H2O → CH3OH + H3O+ + Cl

Halogen nucleophile

The halogens are not nucleophilic in diatomic states. However, their anions are nucleophiles. For example, I2 is not nucleophilic. Whereas I is a good nucleophile.

Carbon nucleophile

Often, carbon act as a nucleophile. Such as in Grignard reagnet (RMgX), acetylides, etc. Enolate ion is also a carbon nucleophile. It is mostly used in a condensation reaction.

Sulfur Nucleophile

Often sulfur act as a nucleophile. Such as:

  • thiols
  • thiol carboxylic acid
  • hydrogen sulfide

Nitrogen Nucleophile

The examples of nitrogen n nucleophiles are the following:

  • amines
  • hydrazine
  • amide
  • azide
  • nitrites

Oxygen nucleophile

Oxygen-containing nucleophiles are the following:

  • hydroxide ion
  • water
  • hydrogen peroxide
  • carboxylate anions

Nucleophilicity

“The ability of nucleophiles to give away the extra electrons and form bonds with the carbon is known as nucleophilicity.”

The strength of nucleophiles is depend upon their structure. For example,

  • A negatively charged nucleophile is stronger as compared to its conjugate acid. Such as H2O  is less nucleophilic than OH.
  • The presence of electron donating groups increases the nucleophilicity of the nucleophiles.

periodic trends in nucleophilicity

As we go down in the periodic table nucleophilicity increases. This is because the atomic number increases. For example, the order of nucleophilicity of halogens is given below:

As we move from left to right in the periodic table nucleophilicity decreases. This is because electronegativity increases. The order of nucleophilicity in a period is given below:

Factors affecting nucleophilicity

There are two main factors that affect the nucleophilicity of nucleophiles.

Resonance effects

The nucleophiles in which the electron pair is delocalized by the resonance are less nucleophilic in nature. For example, the alkoxide ion is more nucleophilic as compared to the carboxylate group. This is because in the carboxylate group electron pair is delocalized. Whereas in the alkoxide ion the charge is localized.

Steric effects

The steric factor greatly influences nucleophilicity. For example, during nucleophilic substitution reaction methoxide ion easily approach the substrate. Whereas the tert-butoxide ion is sterically hindered. Therefore, its nucleophilicity is greatly reduced. The sterically hindered nucleophiles react slowly as compared to other nucleophiles.

Effect of solvent

A polar protic solvent decreases nucleophilicity. This is because they form hydrogen bonds with nucleophiles. Some polar protic solvents are H2O, ROH, RNH2, etc.

However, aprotic solvents lack positively polarized hydrogen. Therefore, they cannot form hydrogen bonds with nucleophiles.

Electronegativity

The more electronegative atom is a poor nucleophile. This is because it cannot donate electrons easily. Therefore, the strength of nucleophiles decreases as electronegativity increases.

Concepts Berg

Why is it called a nucleophile?

Nucleophiles are nucleus-loving species. Therefore, it is derived from two words ‘nucleus’ and ‘Philos’.

What are electrophilic and nucleophile?

The nucleophiles are the species that donate electron pairs. While the electrophile is the species that accept electron pair.

What are the strong nucleophiles?

The conjugate bases are strong nucleophiles. For example, HO has greater nucleophilicity than H2O.

What are the two types of nucleophiles?

There are two types of nucleophiles:

  • Negatively charged nucleophiles
  • Neutral nucleophiles

Is NaCN a strong nucleophile?

NaCN is a strong nucleophile. This is because sodium has greater electropositivity.

Which is a good nucleophile cl^-and f^-?

Cl is a better nucleophile than F. This is because as we go down in the periodic table nucleophilicity decreases.

Is NaOH a nucleophile?

The NaOH is a good nucleophile. This is because the oxygen of hydroxide ion carries greater electron density.

Is aldehyde a nucleophile?

Aldehyde is electrophilic in nature. Therefore, they undergo nucleophilic addition reactions.

Which is better nucleophilic, OH- or NH2-?

-NH2 is more nucleophilic than H2O. This is because the electronegativity of oxygen is more than nitrogen. Therefore, it has less tendency to donate electron pairs.

Reference Books 

  • Organic Chemistry, By Francis A. Carey (University of Virginia)
  • Organic chemistry By T. W. G. Solomons (Department of Chemistry, University of South Florida) & C. B. Fryhle (Chair and Professor of Chemistry at Pacific Lutheran University)
  • A textbook of organic chemistry by M.Younas (University of Punjab)

Reference Links 

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