electrophile

Examples of electrophile in the following topics:

• Acid-Base Catalysis

  • Three examples are shown in equations 1 through 3; electrophiles are colored red, and nucleophiles are colored blue.
  • In the former addition reaction, bromine (an electrophile) attacks the nucleophilic double bond of 1-butene to give an electrophilic cyclic-bromonium intermediate (enclosed in square brackets) accompanied by a nucleophilic bromide ion.
  • However, this electrophilic character may be enhanced or diminished by substituents.
  • This generates the bromonium cation, Br(+), a powerful electrophile.
  • We can increase the electrophile reactivity by converting the ester to its conjugate acid, CH3C(OH)OC2H5(+).

• Electrophilic Substitution of the Phenol Aromatic Ring

  • The facility with which the aromatic ring of phenols and phenol ethers undergoes electrophilic substitution has been noted.
  • Carbon dioxide is a weak electrophile and normally does not react with aromatic compounds; however, the negative charge concentration on the phenolate ring enables the carboxylation reaction shown in the second step.

• Substitution of the Hydroxyl Hydrogen

  • This reaction class could be termed electrophilic substitution at oxygen, and is defined as follows (E is an electrophile).
  • This reaction is believed to proceed by the rapid bonding of a strong electrophile to a carboxylate anion.
  • Alkynes may also serve as electrophiles in substitution reactions of this kind, as illustrated by the synthesis of vinyl acetate from acetylene.
  • Electrophilic species such as acids or halogens are necessary initiators of lactonizations.
  • Even the weak electrophile iodine initiates iodolactonization of γ,δ- and δ,ε-unsaturated acids.

• Reactive Intermediates

  • Electrophile: An electron deficient atom, ion or molecule that has an affinity for an electron pair, and will bond to a base or nucleophile.
  • Nucleophile: An atom, ion or molecule that has an electron pair that may be donated in bonding to an electrophile (or Lewis acid).
  • In this sense they are electrophiles, but the non-bonding electron pair also gives carbenes nucleophilic character.
  • As a rule, the electrophilic character dominates carbene reactivity.
  • The importance of electrophile / nucleophile terminology comes from the fact that many organic reactions involve at some stage the bonding of a nucleophile to an electrophile, a process that generally leads to a stable intermediate or product.

• Substitution Reactions of Benzene and Other Aromatic Compounds

  • Since the reagents and conditions employed in these reactions are electrophilic, these reactions are commonly referred to as Electrophilic Aromatic Substitution.
  • The catalysts and co-reagents serve to generate the strong electrophilic species needed to effect the initial step of the substitution.
  • The specific electrophile believed to function in each type of reaction is listed in the right hand column.

• A Mechanism for Electrophilic Substitution Reactions of Benzene

  • A two-step mechanism has been proposed for these electrophilic substitution reactions.
  • In the first, slow or rate-determining, step the electrophile forms a sigma-bond to the benzene ring, generating a positively charged benzenonium intermediate.
  • This mechanism for electrophilic aromatic substitution should be considered in context with other mechanisms involving carbocation intermediates.
  • The carbocation intermediate in electrophilic aromatic substitution (the benzenonium ion) is stabilized by charge delocalization (resonance) so it is not subject to rearrangement.

• Substitution of the Hydroxyl Hydrogen

  • The oxygen atom of an alcohol is nucleophilic and is therefore prone to attack by electrophiles.
  • This powerful nucleophile then attacks the weak electrophile.
  • In the following equation the electrophile may be regarded as Cl(+).
  • This reaction provides examples of both strong electrophilic substitution (first equation below), and weak electrophilic substitution (second equation).
  • The electrophilic atom in the acid chlorides and anhydrides is colored red.

• Addition by Electrophilic Reagents

  • When the addition reactions of electrophilic reagents, such as strong Brønsted acids and halogens, to alkynes are studied we find a curious paradox.
  • As a rule, electrophilic addition reactions to alkenes and alkynes proceed by initial formation of a pi-complex, in which the electrophile accepts electrons from and becomes weakly bonded to the multiple bond.
  • Why are the reactions of alkynes with electrophilic reagents more sluggish than the corresponding reactions of alkenes?
  • Since the initial interaction between an electrophile and an alkene or alkyne is the formation of a pi-complex, in which the electrophile accepts electrons from and becomes weakly bonded to the multiple bond, the relatively slower reactions of alkynes becomes understandable.
  • Despite these differences, electrophilic additions to alkynes have emerged as exceptionally useful synthetic transforms.

• Addition of Lewis Acids (Electrophilic Reagents)

  • The proton is not the only electrophilic species that initiates addition reactions to the double bond.
  • The electrophilic character of the halogens is well known.
  • The electrophilic moiety of these reagents is the halogen.
  • To apply this mechanism we need to determine the electrophilic moiety in each of the reagents.
  • By using electronegativity differences we can dissect common addition reagents into electrophilic and nucleophilic moieties, as shown below.

• Directed ortho-Metalation

  • The organometal species produced in this way could then react with a variety of common electrophilic reagents (e.g. alkyl halides, carbonyl compounds and halogens).
  • The following equation illustrates this Directed ortho Metalation (DoM) reaction, where DMG refers to a directing metalation group and E+ is an electrophile.
  • Electrophilic substitution of aromatic rings generally gives a mixture of ortho and para substitution products when an existing substituent activates the ring or meta products when the substituent is deactivating.
  • Electrophilic iodination by the action of molecular iodine in the presence of sodium nitrate and acetic acid (a source of iodinium cation) gives a high yield of para-iodoanisole.
  • Direct electrophilic substitution would normally occur at the meta position, so the action of the amide DMG is particularly noteworthy.