electrophilic addition

Examples of electrophilic addition in the following topics:

• Addition Reactions

  • There are two main types of polar addition reactions: electrophilic addition and nucleophilic addition.
  • Most addition reactions to alkenes follow the mechanism of electrophilic addition.
  • An example is the Prins reaction, where the electrophile is a carbonyl group.
  • In nucleophilic addition reactions, the nucleophile donates an electron pair to the electrophile (one of the atoms in the double bond).
  • Top to bottom: electrophilic addition to alkene, nucleophilic addition of nucleophile to carbonyl, and free radical addition of halide to alkene.

• 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.
  • Although these electrophilic additions to alkynes are sluggish, they do take place and generally display Markovnikov Rule regioselectivity and anti-stereoselectivity.
  • Because of their high electronegativity, halogen substituents on a double bond act to reduce its nucleophilicity, and thereby decrease the rate of electrophilic addition reactions.
  • 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.
  • Despite these differences, electrophilic additions to alkynes have emerged as exceptionally useful synthetic transforms.

• Preparation of 1º-Amines

  • Nucleophile addition to aldehydes and ketones is often catalyzed by acids.
  • The reaction of ammonia with aldehydes or ketones occurs by a reversible addition-elimination pathway to give imines (compounds having a C=N function).
  • Acid chlorides react with ammonia to give amides, also by an addition-elimination path, and these are reduced to amines by LiAlH4.
  • Here a strongly electrophilic nitrogen species (NO2(+)) bonds to a nucleophilic carbon compound.
  • The Hofmann rearrangement of 1º-amides provides an additional synthesis of 1º-amines.

• 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 addition of chlorine and bromine to alkenes, as shown in the following general equation, proceeds by an initial electrophilic attack on the pi-electrons of the double bond.
  • The electrophilic moiety of these reagents is the halogen.
  • By using electronegativity differences we can dissect common addition reagents into electrophilic and nucleophilic moieties, as shown below.

• 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.
  • If we take saturated aldehydes (RCH=O) as a reference, the additional alkyl substituent present in ketones slightly reduces this electrophilicity, but the general reactivity pattern of these classes is similar.
  • This generates the bromonium cation, Br(+), a powerful electrophile.

• 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.
  • Intramolecular carboxyl group additions to alkenes generate cyclic esters known as lactones.
  • 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.

• Polar Functions and Umpolung

  • The resulting carbocation is a very strong electrophile capable of bonding to weak nucleophiles, as in Friedel-Crafts alkylation.
  • The electrophilic character of the carbonyl carbon, which is manifested by organometallic addition reactions, may be transmitted to a β-carbon through a conjugated double bond, as demonstrated by a variety of useful conjugate addition reactions.
  • Here, the cyanohydrin conjugate base acts as a surrogate acyl anion, yielding a 1,2-addition intermediate that generates the final product.
  • Since the benzoin condensation is reversible, the 1,4-addition product is favored.
  • Examples 1 & 2 show how such intermediates may serve as effective α-electrophiles.

• Substitution Reactions of Benzene and Other Aromatic Compounds

  • The chemical reactivity of benzene contrasts with that of the alkenes in that substitution reactions occur in preference to addition reactions, as illustrated in the following diagram (some comparable reactions of cyclohexene are shown in the green box).
  • 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.
  • These include SN1 and E1 reactions of alkyl halides, and Brønsted acid addition reactions of alkenes.
  • The cation may bond to a nucleophile to give a substitution or addition product.

• Oxidation of Alcohols by DMSO

  • The reaction is operationally easy: a DMSO solution of the alcohol is treated with one of several electrophilic dehydrating reagents (E).
  • The alcohol is oxidized; DMSO is reduced to dimethyl sulfide; and water is taken up by the electrophile.
  • Because so many different electrophiles have been used to effect this oxidation, it is difficult to present a single general mechanism.
  • Most of the electrophiles are good acylating reagents, so it is reasonable to expect an initial acylation of the sulfoxide oxygen.
  • In some cases triethyl amine is added to provide an additional base.