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Nucleophile is an ion or molecule with a free electron pair or at least one pi bond containing two electrons. Nucleophile refers to those substances that tend to donate electron pairs to electrophiles to form chemical bonds with them. A nucleophile is an electron-rich molecule that may form covalent bonds by donating electrons to electron-poor sites. It can be negatively charged or neutral with available electrons. Many molecules can act as a nucleophile in a chemical reaction. Water, ammonia, and hydrogen sulfide are all examples of nucleophiles.
Read More: NCERT Solutions For Class 11 Chemistry Organic Chemistry Some Basic Principles and Techniques
Key Terms: Nucleophiles, Electrons, Molecules, Nucleophilic Substitution Reaction, Nucleus, Ambident Nucleophiles, Nucleophilic Attacks
What is a Nucleophile?
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Nucleophile is a reagent that consists of an atom, ion, or molecule that donates electron pair to form a bond.
- A nucleophile is an electron-rich species that can be negatively charged or neutral with available electrons.
- Nucleophiles have the ability to donate electron pairs to form a bond.
- Nucleophiles may have either a negative or a neutral charge.
- The term ‘nucleophile’ is made up of two parts namely Nucleus and Philos. Philos is a Greek term meaning ‘love’.
- Thus, nucleophiles are also thought of as Nucleus Loving Species.
Important Terminologies Related to Nucleophiles
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Here are a few important terms associated with nucleophiles:
| Term | Definition |
|---|---|
| Nucleophilic | Nucleophilic nature of a species expresses the affinity of the species to the positively charged nucleus. |
| Nucleophilicity | A term used to compare the nucleophilic character of different nucleophiles. It is also referred to as the nucleophile strength of a species. |
| Nucleophilic Substitution | A reaction that occurs when an electron-rich nucleophile selectively attacks a positively charged atom in a molecule. It replaces a leaving group by bonding with the positively charged species. |
A type of nucleophilic substitution reaction is Solvolysis wherein the nucleophile in question is a solvent molecule. Water is a good example of a nucleophilic solvent, and solvolysis with water is often referred to as hydrolysis.
Read More: Organic Chemistry: Basic Principles and Techniques
Ambident Nucleophiles
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An ambident nucleophile can be defined as one that can carry out nucleophilic attacks from two or more places in the molecule. Such attacks can often result in the formation of more than one product.
Example of Ambident NucleophileThiocyanate Ion is an example of an ambident nucleophile that has the chemical formula of SCN–. It can execute nucleophilic attacks from either the sulfur atom or the nitrogen atom. The nucleophilic substitution reactions of alkyl halides involving thiocyanate ions often result in the formation of a mixture of the following:
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In other words, an ambident nucleophile is an anionic nucleophile, wherein the negative charge is delocalized by resonance over two like but non-equivalent atoms or over two, unlike atoms. Enolate ions are the most common ambident nucleophiles.
Nucleophile Examples
Types of Nucleophiles
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The various types of Nucleophiles are as follows:
Halogens
- The anionic halogens are great Nucleophiles, while their diatomic forms are the opposite.
- For instance, diatomic iodine (I2) does not act as a nucleophile whereas I– is the strongest nucleophile in a protic solvent.
Oxygen
- No nucleophilic attacks occur during the intermolecular hydrogen bonding that is carried out in compounds containing oxygen and hydrogen.
- Hydroxide ion is an example of a nucleophile where the oxygen atom donates the electron pair.
Nitrogen
- Nitrogen has been known to form several nucleophiles like azides, amines, ammonia, and nitrides.
- Even amides have been known to show nucleophilic qualities.
Sulphur
- It has multiple nucleophilic qualities and has easy polarization due to its large size.
- Hydrogen sulfide (H2S) is an example of a nucleophile containing sulphur.
Mechanisms of Nucleophilic Substitution
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The rate of nucleophilic substitution reaction not only depends on the nucleophilicity of the incoming nucleophiles and the leaving capacity of the leaving group but also on the mechanism through which the reaction takes place. There are two mechanisms of nucleophilic substitution:
SN2 Mechanism
- This mechanism is also known as the substitution nucleophilic bimolecular mechanism.
- While a reaction takes place, the SN2 mechanism follows second-order kinetics and the rate law for the reaction.
- The rate law states that the SN2 reaction mechanism depends on the concentration of the incoming nucleophile along with the substrate.
- Thus, the concentration nucleophilicity of both can increase the rate of reaction.
SN1 Mechanism
- This mechanism is also known as a unimolecular nucleophilic substitution reaction.
- The SN1 reaction mechanism is independent of the nucleophilicity of the incoming nucleophile.
- However, it is dependent on the leaving capacity of the leaving group.
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Things to Remember
- Nucleophiles are substances that donate electron pairs to electrophiles and form chemical bonds.
- Hydroxide and cyanide ions are some prominent examples of Nucleophiles.
- Ambident Nucleophiles carry out nucleophilic attacks from two or more places in the molecules and result in the formation of more than one product.
- Nucleophilic substitution takes place when an electron-rich nucleophile attacks a positively charged atom in a molecule and replaces a leaving group by associating with the positively charged species.
- Solvolysis is an example of a nucleophilic substitution reaction.
- There are two types of Nucleophilic Substitution mechanisms namely SN1 mechanism and SN2 mechanism.
Previous Years’ Questions
- Which of the following is the strongest nucleophile…
- Among the following the strongest nucleophile is…
- Which one is least reactive towards nucleophilic substitution reaction… [AMUEEE 2014]
- Order of reactivity towards nucleophilic substitution reaction… [AMUEEE 2009]
- Which one is most reactive towards nucleophilic addition reaction…
- The correct order of leaving group ability in nucleophilic substitution…
- Pick out the electrophiles from the following species… [AMUEEE 2013]
- Electrophilic addition reaction is easily given by… [UP CPMT 2010]
- Electrophilic reagents are…
- Which of the following can behave as both electrophile and nucleophile...
Sample Questions
Ques. Explain the electronegativity of Nucleophiles. (3 Marks)
Ans. The electronegativity and strength of a nucleophile have an inverse relationship with each other. If electronegativity increases, the nucleophilicity will decrease as it is less likely that the molecule will act as an electron source. Electronegativity increases across the rows and decreases down the columns of the modern periodic table. Whereas the nucleophilicity decreases across the rows and increases down the columns of the periodic table.
Ques. What is nucleophilic substitution reaction? (3 Marks)
Ans. Nucleophilic substitution reaction is an organic reaction in which one nucleophile replaces another. This reaction is very similar to the normal displacement reactions, where, a more reactive element replaces a less reactive element from its salt solution. The group that takes electron pairs and is displaced from the carbon is known as the “leaving group” while the molecule on which substitution takes place is known as the “substrate”. The leaving group leaves as a neutral molecule or anion.
Ques. Give a few examples of strong nucleophiles. (3 Marks)
Ans. Here are a few examples of strong nucleophiles:
- Halides- Br–, I–, Cl–, F–
- Hydroxide Ion- HO–
- Nitrile Ion- CN–
- Sulfide Ion- RS–, HS–
Ques. Name a few weak nucleophiles. (3 Marks)
Ans. Some weak nucleophiles are:
- Water- H2O
- Alcohols- CH3OH
- Carboxylic Acids- RCOOH
- Bulky Bases- tBuOK, DBU
Ques. What is Nucleophilicity? (3 Marks)
Ans. Nucleophilicity refers to the ability of the nucleophiles to donate their lone pairs to a positive center. Nucleophilicity is a kinetic term that relates to the rate at which the nucleophile attacks the substrates (R - LG). The factors that are used to compare the nucleophilicity of different nucleophiles are:
- The Basic Strength of Nucleophiles
- Electronegativity of the Nucleophilic Atom
- Steric Hindrance
- Polar Solvent Effects
- Leaving Capacity of the Leaving Group
- Charged or Uncharged
Ques. How to determine the strength of a nucleophile? (3 Marks)
Ans. The strength of a nucleophile is determined by its nucleophilicity, which is the ability of a nucleophile to donate electrons. A nucleophile with electrons available for bonding is considered to be a good nucleophile. Thus, a negatively charged species has extra electrons that it can donate to obtain stability. Whereas, on the other hand, a nucleophile that does not easily give away its electrons is a weak nucleophile.
Ques. What are Nucleophiles? Explain with examples. (2 Marks)
Ans. Nucleophiles are known to be 'nucleus loving' and attack the low electron density regions in a substrate molecule. They are electron-rich and negative ions. They can form covalent bonds by donating electrons to electron-poor sides. Examples of nucleophiles are OH-, BR-, CL-, NH3, H2O, etc.
Q1. Identify the nucleophile and the electrophile in the following acid-base reaction. (1 Mark)
AlCl3+NH3→AlCl−3−NH+3
Ans. Since, AlCl3 is electron deficient in nature and therefore, it will act as an electrophile. While NH3 remains a lone pair on nitrogen making it rich in electrons. So, it will act as a nucleophile.
Ques. Which of the following is/are Nucleophiles? (1 Mark)
(I) PhS-
(II) Ph3
(III) F-
(IV) All of the Above
Ans. (IV) All of the Above
Ques. Which of the two, SN1 or SN2 reactions are faster? (2 Marks)
Ans. SN1 depends upon the concentration of the leaving group, but SN2 depends upon both the leaving and incoming group's concentration. SN1 reaction will be faster if the reagent is a weak base and polar protic is the solvent used. While SN2 reaction will be faster if the reagent has a stronger base and polar aprotic is the solvent used.
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