What is the Electrophile ?
An electrophile is a chemical species that accepts an electron pair and forms bonds with nucleophiles. Electrophiles are Lewis acids because they accept electrons. Most electrophiles are positively charged, have a partial positive charge on an atom, or have an atom without an octet of electrons.
Addition and substitution reactions are the most common interactions between electrophiles and nucleophiles. Cations like H+ and NO+ polarised neutral molecules like HCl, alkyl halides, acyl halides, and carbonyl compounds, polarizable neutral molecules like Cl2 and Br2, oxidizing agents like organic peracids, chemical species that don't satisfy the octet rule like carbenes and radicals, and some Lewis acids like BH3 and DIBAL are all popular electrophiles in organic syntheses.
They are electron-deficient and are attracted to electrons.
They can either have a positive or a negative charge.
Electrons attack atoms with a lot of electrons, including carbon-carbon double bonds.
The density influences electron movement, which generally occurs from a high-density to a low-density region.
Electrophilic addition and electrophilic replacement reactions should be favored.
Since they accept electrons, they are also known as Lewis acids.
In electrophilic substitution and addition reactions, electrophiles are involved.
This article will study what is electrophile and electrophile examples in detail.
Electrophilic Reagent Examples
1. Addition of Halogens
In halogen addition reactions, these occur between alkenes and electrophiles, most commonly halogens. The use of bromine water to titrate against a sample to determine the number of double bonds present is a common reaction.
C2H4+ Br2 → BrCH2CH2Br
The electrophilic Br-Br molecule forms a -complex with the electron-rich alkene molecule.
Bromine acts as an electrophile and the alkene acts as an electron donor. With the release of Br, the three-membered bromonium ion 2 with two carbon atoms and a bromine atom emerges.
The assault of Br from the backside opens the bromonium ion. The vicinal dibromide has an antiperiplanar configuration as a result of this reaction. When other nucleophiles like water or alcohol are present, they can attack 2 and create alcohol or ether.
2. Addition of Hydrogen Halides
In hydrohalogenation, hydrogen halides such as hydrogen chloride (HCl) are added to alkenes to create alkyl halides. The reaction of HCl with ethylene, for example, produces chloroethane. In comparison to the halogen addition above, the reaction proceeds through a cation intermediate. The following is an example:
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Proton (H+) adds to one of the carbon atoms on the alkene to form cation 1 (by acting as an electrophile).
The adducts 2 and 3 are formed when the chloride ion (Cl) interacts with cation 1.
Chiral Derivatives
Many electrophiles are optically stable and chiral. Optical purity is typically a characteristic of chiral electrophiles. The fructose-derived organocatalyst used in Shi epoxidation is one such reagent. The catalyst will epoxidate trans-disubstituted and trisubstituted alkenes with high enantioselectivity. Until continuing in the catalytic cycle, the Shi catalyst, which is a ketone, is oxidised to the active dioxirane form by stoichiometric oxone.
Electrophile Examples
Given below is the Electrophiles List-
The different types of electrophiles can be classified as:
1. Positively Charged Electrophiles:
H+, SO3H+, NO+, NO2+, X+, R+ , C6H5N2+
2. Neutral Electrophiles: These showcase electron deficiency.
(a) All Lewis acids: BF3, SO3 , FeCl3 , AlCl3 , BeCl2 , SnCl2 , SnCl4 ,ZnCl2.
(b) The neutral atom that accepts electrons from the substrates :
R *COCl, R – * Mg – X, *I – Cl, CH3 – *CN, R*–Cl, R*–O
Which is an Electrophile?
Although the hydrogen ion has a positive charge, it does not qualify as an electrophile because it has absolute empty orbitals in its outer shell.
The ammonium ion, on the other hand, does not have any empty orbitals to draw electrons. As a result, ammonium ions are not considered electrophiles.
Did You Know?
In the presence of superacids, super-electrophiles are characterised as cationic electrophilic reagents with greatly enhanced reactivities. George A. Olah was the first to identify these compounds. By proto-solvation of a cationic electrophile, super-electrophiles form as a doubly electron-deficient super-electrophile. When mixed with hydrofluoric acid, a mixture of acetic acid and boron trifluoride can extract a hydride ion from isobutane through the formation of a superacid from BF3 and HF, as Olah discovered. The [CH3CO2H3]2+ dictation is the responsible reactive intermediate.
Conclusion
This is all about electrophile, a chemical species, and its features. Focus on the examples and explanations given in this article to develop your conceptual foundation.
FAQs on Electrophile
1. What is an electrophile in chemistry, with examples?
An electrophile, meaning "electron-loving," is a chemical species that is deficient in electrons and can accept an electron pair from an electron-rich species (a nucleophile) to form a covalent bond. They are essential reactants in many organic reactions. Electrophiles can be categorised into two main types:
- Positively Charged Electrophiles: These are cations, such as the hydrogen ion (H⁺), nitronium ion (NO₂⁺), and carbocations (R⁺).
- Neutral Electrophiles: These molecules have an atom with an incomplete octet or a partial positive charge. Examples include Lewis acids like Boron trifluoride (BF₃) and Aluminium chloride (AlCl₃), and molecules with polar covalent bonds like Acyl chlorides (RCOCl).
2. How does an electrophile differ from a nucleophile?
The primary difference lies in their relationship with electrons. An electrophile is an "electron-loving" species that is electron-deficient and acts as an electron pair acceptor. Conversely, a nucleophile is a "nucleus-loving" species that is electron-rich and acts as an electron pair donor. Electrophiles are considered Lewis acids, while nucleophiles are considered Lewis bases.
3. What are the key characteristics to identify an electrophile?
To identify if a chemical species is an electrophile, look for the following characteristics:
- Positive Charge: Most cations, like H⁺ and NO₂⁺, are strong electrophiles as they are inherently electron-deficient.
- Partial Positive Charge: In a polar molecule, the atom with the partial positive charge (δ+) can act as an electrophilic centre. For instance, the carbon atom in a carbonyl group (C=O) is electrophilic.
- Incomplete Octet: Neutral atoms that do not have a full octet of valence electrons, such as the boron in BF₃ or aluminium in AlCl₃, are strong electrophiles as they can accept an electron pair to complete their octet.
4. Why are all electrophiles considered Lewis acids?
This is because the fundamental definitions of both terms describe the same chemical behaviour. A Lewis acid is defined as any species that can accept a pair of non-bonding electrons. An electrophile, by its nature, seeks out and accepts an electron pair from a nucleophile to form a new bond. Since both are defined by their ability to accept an electron pair, any species functioning as an electrophile is, by definition, also acting as a Lewis acid.
5. Can an electrophile be a neutral molecule? Provide examples.
Yes, an electrophile can be a neutral molecule. The defining feature of an electrophile is electron deficiency, not necessarily a positive charge. Neutral molecules can be powerful electrophiles if they contain an atom with an incomplete valence shell or are part of a highly polarised bond. Common examples include:
- Boron trifluoride (BF₃) and Aluminium chloride (AlCl₃): The central atoms (B and Al) have only six valence electrons and readily accept an electron pair to complete their octet.
- Sulfur trioxide (SO₃): The sulfur atom is highly electron-deficient due to the pull of three electronegative oxygen atoms.
- Carbonyl compounds (e.g., aldehydes, ketones): The carbon atom in the C=O double bond has a partial positive charge, making it an electrophilic site.
6. Why is a positively charged ion like the ammonium ion (NH₄⁺) not an electrophile?
This is a common point of confusion. Although the ammonium ion (NH₄⁺) has a positive charge, it is not an electrophile. The reason is that the central nitrogen atom already has a complete octet of electrons, and there are no low-energy empty orbitals available to accept an incoming electron pair. For a species to act as an electrophile, it must have a site—an empty or partially filled orbital—where it can accommodate the electrons from a nucleophile. NH₄⁺ lacks such a site.
7. What is the importance of electrophiles in common organic reactions?
Electrophiles are fundamental to many of the most important reactions in organic chemistry. They are the species that are attacked by electron-rich molecules, driving the reaction forward. Their primary roles are seen in:
- Electrophilic Addition Reactions: This is a key reaction for alkenes and alkynes. An electrophile (like H⁺ from HBr or Br⁺ from Br₂) adds across the double or triple bond, breaking the pi bond.
- Electrophilic Aromatic Substitution: This is the characteristic reaction of aromatic compounds like benzene. An electrophile (like NO₂⁺ in nitration) substitutes a hydrogen atom on the aromatic ring.
8. What makes a species a "super-electrophile"?
A super-electrophile is a cationic electrophilic reagent that exhibits significantly enhanced reactivity compared to a conventional electrophile. They are typically generated by reacting a cationic electrophile with a very strong acid (a superacid). This process creates a "doubly electron-deficient" species, meaning its electron-seeking power is greatly intensified. This extreme reactivity allows super-electrophiles to react with even very weak nucleophiles, such as alkanes.
