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Electrophilic reagents are chemical entities that gain electrons or a share of electrons from other molecules or ions during chemical processes. A reagent is a compound or mixture added to a system to start or test a chemical reaction.
- Also, electrophiles are known as lewis acid as they accept electrons in a chemical reaction.
- Electrophiles are positively charged and have a partial positive charge on atoms.
- Electrophiles are atoms without octets of electrons.
- Addition and substitution are the reactions where electrophile and nucleophile interaction is seen.
- An example of Electrophilic Reagents is the function of Bromine.
- The electrophilic Br-Br molecule is known to form a complex with the electron-rich alkene molecule.
- Bromine, in this case, acts as an electrophile, while the alkene serves as an electron donor.
Chemical species such as carbenes, radicals, lewis acids like BH3 and DIBAL are the popular electrophile in the organic reaction.
Read More: Ambident Nucleophile
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Key Terms: Electrophilic, Substitution Reaction, Electron, Aromatic Substitution, Chlorine, Lewis Acid, Alkene, Alkyne, Aromatic Compounds
Electrophilic Reagents
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Electrophilic Reagents are those reagents which acquire electrons, or share electrons which previously belonged to foreign molecules.
- Electrophilic Reagents are also sometimes called Electrophiles.
- Electrophilic reagents include positively charged ions, for example, H+ and NO2+; neutral molecules with an electron deficiency, for example, SO3; and highly polarized molecules, for example CH3CO2–Br+.
- Simply, a reagent which takes away an electron pair is known as an electrophile (E+).
Read Also: Limiting Reagent
Characteristics of Electrophiles
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The following are the important characteristics of Electrophile:
- Electrophiles are the electron-deficient species that are attracted to electrons.
- Electrons attack atoms with lots of electrons such as carbon double bonds and triple bonds.
- Electrophilic addition and electrophilic replacement reactions are seen in a chemical reaction.
- The other name of electrophile is lewis acids.
Read More:
| Topic-Relevant Concepts | ||
|---|---|---|
| Huckel Rule | Saturated Hydrocarbons | Conformational Isomers |
| Unsaturated Hydrocarbons | Conformation | Benzene |
Electrophilic Reagent Examples
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Below mentioned are some examples of electrophilic reagents:
Addition of Halogen
In halogen addition reaction, there is a reaction between alkene /alkenes and electrophiles. This kind of reaction mainly involved halogens such as bromine, chlorine, and iodine.
The use of halogens such as bromine /chlorine /iodine water helps us to determine the number of double bonds present in any reaction.
For example: C2 H4 + Br2 → Br-CH2-CH2 -Br
Bromine electrophile forms a complex between electron-rich alkene or alkyne molecules. In the above reaction, bromine is acting as an electrophile, and alkene or the alkyne acts as an electron donor.
| C2H4 + Cl2 → Cl-CH2-CH2-Cl |
In this reaction, chlorine electrophiles form a complex between electron-rich alkene or alkyne molecules. Here chlorine will act as an electrophile whereas alkene is acting as an electron donor.
Addition of Hydrogen Halides
In hydrohalogenation reaction, hydrogen halides such as hydrogen chloride/hydrogen bromide/hydrogen iodide to alkene or alkyne to create alkyl halides.
For example: The reaction with hydrogen chloride with ethylene produces chloroethane.
Some reactions take place when there are different halides such as bromine or iodine.
It will produce iodoethane or bromoethane as a product.
Chiral Derivatives
Most of the electrophiles are optically stable and chiral. Optical purity is an important characteristic of chiral electrophiles.
Electrophiles Examples
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Electrophiles can be classified as positive and neutral electrophiles.
Examples of positively charged electrophiles:
| H+, NO+, NO2+, R+ |
Examples of negatively charged electrophiles:
| BF3, SO3, ZnCl2, SnCl4 |
Electrophile Example Diagram
Electrophilic Substitution Reaction
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An electrophilic substitution reaction is a reaction that involves a functional group attached to a compound that is replaced by an electrophile.
Types of Electrophilic Substitution Reaction
Below are the two types of electrophilic substitution reaction example:
1) Electrophilic Aromatic Substitution Reaction:
- In an electrophilic aromatic reaction, an atom that is attached to an aromatic ring is replaced with an electrophile.
- Examples of electrophilic aromatic substitution reactions are nitration, aromatic sulfonation, and Friedel-Crafts reaction.
2) Electrophilic Aliphatic Substitution Reaction:
In an electrophilic aliphatic substitution reaction, an electrophile always replaces the functional group from an aliphatic compound in a given reaction.
Check-Out: Functional Group, Types of Amides, Structure & Examples
Mechanism of Electrophilic Substitution Reaction
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The Electrophilic substitution reaction generally takes place in the following three steps:
1) Generation of electrophile
2) Formation of Carbocation
3) Removal of a proton from carbonation (intermediate)
Step 1: Generation of Electrophile
Anhydrous aluminum chloride (AlCl3 ) is a lewis acid that is used for the generation of electrophile from chlorination, alkylation, and acylation of the aromatic ring. The resulting electrophile are Cl+, R+ are as shown below:
Step 2: Formation of Carbocation
In this step, the electrophile attacks the aromatic ring and forms an arenium ion. The carbon present in the arenium ion is SP3 hybridized.
Step 3: Removal of Proton
To restore the aromatic character of any compound, Arenium ion releases a proton from SP3 hybridized carbon when it is attacked by the AlCl4, thus electrophile replaces a hydrogen atom in the benzene ring.
Things to Remember
- Electrophilic substitution reactions result in the inversion of the configuration of electrophilic attack which occurs at an angle of 180.
- There are two types of electrophiles, positive (eg: NO2+, R+) and negative (eg: SO3, ZnCl2,)
- Halogens such as bromine, chlorine, and iodine are used in addition to halogen.
- Important characteristics of chiral electrophiles include Optical purity.
- The following is the general order of reactivity toward electrophilic substitution reactions: Aniline > Phenol > Anisole > Acetanilide > Toluene > Chlorobenzene > Fluorobenzene > Benzoic acid > Benzaldehyde > Nitrobenzene.
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Previous Year Questions
- Which of the following alkane cannot be made in good yield by Wurtz reaction ?...[NEET 2020]
- Which of the following compounds has the lowest boiling point?...[NEET 1998]
- Which of the following reagents will be able to distinguish between 1-butyne and 2-butyne?...[NEET 1980]
- Dihedral angle in staggered form of ethane is….[NEET 2000]
- With respect to the conformers of ethane, which of the following statements is true ?..[NEET 2017]
- A compound is treated with NaNH2 to give sodium salt. Identify the compound...[NEET 1993]
- Acetylenic hydrogens are acidic because...[NEET 1989]
- In Friedal-Craft's alkylation besides AlCl3 the other reactants are...[NEET 1999]
- Which one is the correct order of acidity ?..[NEET 2017]
- Liquid hydrocarbons can be converted to a mixture of gaseous hydrocarbons by….[NEET 2010]
Sample Questions
Ques: What is Electrophile? Give an example. (2 marks)
Ans: An electrophile is defined as a chemical species which mostly accepts the electron pair and makes bonds with nucleophile substances.
Examples of electrophiles are Alcl3 ,AlBr3 etc.
Ques: What are the two types of electrophilic reactions? (2 marks)
Ans: Below are the two types of electrophilic substitution reactions-
- Electrophilic Aliphatic substitution reaction
- Electrophilic Aromatic substitution reaction
Ques: What Catalysts are used in chlorination and bromination of aromatic rings? (1 mark)
Ans: For chlorination of aromatic rings ,catalysts such as ALCl3, FeCl3 lewis acids are used whereas for bromination catalysts such as AlBr3 or FeBr3 can be used.
Ques: Why are lewis acids generally used as a catalyst in electrophilic reactions? (1 mark)
Ans: Lewis acids generally form a complex with chlorine & other halides, liberating high amounts of electrophilic chlorine species. Lewis acids are generally used as a catalyst because they increase the rate of the reactions.
Ques: What is nucleophile ? Give an example (1 mark)
Ans: Nucleophiles are the electron rich species that donate electron pairs to electron deficient compounds .Examples of nucleophiles are water, ammonia, cyanide etc.
Ques: How will you identify electrophile in a chemical reaction? (1 mark)
Ans: Electrophiles are species which have empty electron orbitals that are drawn to electron rich centers and have positive charges.
Ques: What are the three types of nucleophile? (1 mark)
Ans: There are the three types of nucleophiles such as lone pair, Pi bond & Sigma bonds.
Ques: Which functional groups are electrophiles? (1 mark)
Ans: Alkyl halides or the alkane which are connected to halogen atoms ( Fluorine, Chlorine or Bromine are the good electrophiles, they are the ones who can participate in nucleophilic substitution reaction as well as in the elimination reaction.
Ques: What is the electrophilic reagent in Friedel-Crafts reaction? (2 marks)
Ans: An acyl group is added to an aromatic ring as part of the Friedel-Crafts acylation procedure.
- The acyl halide's halogen participates in the formation of a complex with the Lewis acid, which results in the generation of a highly electrophilic acylium ion.
- This acylium ion has the general formula RCO+ and is stabilized by resonance.
Ques: Write the difference between electrophile and nucleophile. (4 marks)
Ans: Following are the differences between electrophile and nucleophile,
| Electrophile | Nucleophile |
|---|---|
| An electrophile lacks electrons. | A nucleophile have extra electrons. |
| It is drawn to negative charges (electron seeking). | It is drawn to a positive charge (nucleus seeking). |
| It attacks the substrate's nucleophilic core, causing an electrophilic reaction. | It attacks the substrate's electrophilic core, causing a nucleophilic reaction. |
| It accepts electron pairs. (Acid Lewis) | It is a donor of electron pairs. (Lewis base) |
| E.g: H+, NO2+, BF3, etc. | E.g: OH-, Cl-, CN-, etc. |
Ques: Write the electrophilic substitution reaction mechanism for benzene with an example. (5 marks)
Ans: Electrophilic substitution reactions take place in three phases, which are as follows: Generation of an electrophile (a), formation of a carbocation (an intermediate), and removal of a proton from the intermediate (b).
The atom linked to the ring is replaced by an electrophile in electrophilic aromatic substitution processes. Aromatic nitration, sulphonation, and Friedel-Crafts reactions are examples of these reactions. The aromaticity of benzene is not altered during the process. As a result, the spontaneity of these reactions is quite great.
Mechanism of Electrophilic Substitution on Benzene(nitration)
Nitrobenzene is produced when benzene is treated with concentrated nitric acid in the presence of concentrated sulfuric acid at 323-333K. This electrophilic substitution process is known as benzene nitration.
STEP 1: Generation of Electrophile
The first step is to combine HNO3 and H2SO4 to produce a powerful electrophile, the nitronium ion. After accepting a proton from H2SO4, HNO3 dissociates to generate the nitronium ion NO+2.
STEP 2: Formation of Arenium ion
The produced electrophile NO+2 attacks the benzene ring, forming a positively charged cyclohexadienyl cation, commonly known as arenium ion, with onesp3 hybridised carbon. Through resonance, the positive charge is distributed over all three carbon atoms, making the ring somewhat stable.
STEP-3: Removal of the proton
In the penultimate step, the arenium ion loses its proton from the sp3 carbon, resulting in the synthesis of nitrobenzene.
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