Lewis acids electrophilicity

Aminating agents 3.14 Other Lewis acids Electrophilic Attack at Carbon... 

Recall that an electron-pair acceptor is a Lewis acid. Electrophiles are Lewis acids. 

There is no fundamental difference between trimethylamine acting as a Br0nsted base or as a Lewis base, except that in the Br0nsted concept it donates its electrons to a proton electrophile, whereas as a Lewis base it donates its electrons to a Lewis acid electrophile. 

The thermodynamic tendency of a substance to act as a Lewis acid. The strength of a Lewis acid depends on the nature of the base with which the Lewis acid forms a Lewis adduct. Hence, comparative measures of Lewis acidities are given by equilibrium constants for the formation of the adducts by a common reference base. See Lewis Acid Electrophilicity Hard Acids Soft Acids Acceptor Number... 

A Lewis acid (electrophile) shares an electron pair furnished by a Lewis base (nucleophile) to form a covalent (coordinate) bond. The Lewis concept is especially useful in explaining the acidity of an aprotic acid (no available proton), such as BFj. 

The normal course of reaction of alkenes involves addition of Lewis acids (electrophiles) yielding an intermediate carbocation which is trapped by a weak nucleophile [114]. The most common electrophilic addition reactions are summarized in Ligure 6.1. If the olefin is unsymmetrically substituted, the question of regioselectivity arises. We begin by examining the effects on the olefin n system of three classes of substituents as... 

When nitronium tetrafluoroborate was attempted to react with the trityl cation in CH2CI2 or sulfolane, no nitration occurred due to the deactivating effects of the carbenium ion center in 215. Nitration of deactivated substrates is also readily accomplished by reaction with NO2CI with three mole excess AICI3 suggesting Lewis acidic electrophilic solvation of the nitronium cation (217, eq 62).105... 

Model Definition. The HSAB model classifies Lewis acids (electrophiles) and bases (nucleophiles) as either "hard" or "soft." Hard acids and bases are relatively small, and exhibit low polarizability and a comparatively low tendency to form covalent bonds. Soft acids and bases have the opposite characteristics (24). Stated simply, the model postulates that hard acids react most readily with hard bases, and soft acids react most readily with soft bases (26). 

Label the reactants in these acid-base reactions as Lewis acids (electrophiles) or Lewis bases (nucleophiles). Use curved arrows to show the movement of electron pairs in the reactions. 

An electrophile is a compound that has a relatively low energy empty orbital available for bond-making. An electrophile may be neutral or positively charged. There are three classes of electrophiles Lewis acid electrophiles, 77-bond electrophiles, and (j-bond electrophiles. 

Lewis acid electrophiles have an atom E that lacks an octet and has a low-energy nonbonding orbital, usually a p orbital. A pair of electrons from the nucleophile is used to form a new bond to E, giving it its octet. The formal charge of E decreases by 1. Carbocations, boron compounds, and aluminum compounds are common Lewis acid electrophiles. 

Lewis acid electrophiles are Lewis acids, so they exist under acidic conditions only. 

The 77 electrons in a C=C 77 bond can react with a Lewis acidic electrophile to give a carbocation. The simplest example is the reaction of an alkene with H+. Note that one of the C atoms of the 77 bond forms the bond to H+ using the electrons of the 77 bond, whereas the other C becomes electron-deficient and gains a formal positive charge. Other cationic electrophiles (carbocations, acylium ions, and Br+) can react with C=C 77 bonds too. 

Electrophilic reagents are Lewis acids. Electrophilic catalysis is catalysis by Lewis acids. 

The mechanism of S02 insertion has been investigated extensively for 18-electron iron complexes.30 Scheme 8.5 outlines the general mechanism for insertion. Note that S02 is a Lewis acid (electrophile), and it does not attack the metal in this case.31... 

Again, the reaction sites are multiple bonds (double and triple bonds) polar bonds and Lewis acids (electrophiles) and Lewis bases (nucleophiles). 

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