Electrophilicity Lewis acidity

Carbocations are strongly electrophilic (Lewis acids) and react with nucleophiles (Lewis bases)... 

Electrophilic catalysis is catalysis by an electrophile (Lewis acid) acting as an electron-pair acceptor. For example, metal ions catalyze the decarboxylation of dimethyloxaloacetic acid. ... 

Before beginning a detailed discussion of alkene reactions, let s review briefly some conclusions from the previous chapter. We said in Section 5.5 that alkenes behave as nucleophiles (Lewis bases) in polar reactions. The carbon-carbon double bond is electron-rich and can donate a pair of electrons to an electrophile (Lewis acid), for example, reaction of 2-methylpropene with HBr yields 2-bromo-2-methylpropane. A careful study of this and similar reactions by Christopher Ingold and others in the 1930s led to the generally accepted mechanism shown in Figure 6.7 for electrophilic addition reactions. 

The first species produced in cationic polymerizations are carbocations, and these were unknown as such prior to World War II. It is now known that pure Lewis acids, such as boron trifluoride and aluminum chloride, are not effective as initiators. A trace of a proton-containing Lewis base, such as water, is also required. The Lewis base coordinates with the electrophilic Lewis acid, and the proton is the actual initiator. Since cations cannot exist alone, they are accompanied by a counterion, also called a gegenion. 

AROMATIC SUBSTITUTION BY ELECTROPHILES (LEWIS ACIDS, OR E) MECHANISM... 

The arylation reaction is not applicable to aryl bromides and iodides. Another line of experimental evidence also indicates that an arylpalladium halide complex, instead of an arylpalladium triflate complex, is not electrophilic (Lewis acidic) enough to cleave the cyclopropane ring. 

In the following procedures, the preparation and reactions are given for complexes that behave as strong electrophilic Lewis acids. Such complexes,... 

Figure 8.2. Reaction of a carbonyl compound with (a) an electrophile (Lewis acid) and (b) a nucleophile (Lewis base). The n bond is formally broken in the reaction with the nucleophile (b) but not in the reaction with the electrophile (a). Stereoisomeric forms may be generated in either case.

We will restrict our consideration to reactions of substituted benzenes and to nitrogen heteroaromatic systems in which the reaction takes place first with the n system. The simplest example of reaction of a monosubstituted benzene with an electrophile (Lewis acid) is shown in Scheme 11.1. The electrophile may attach itself to the n system (step A) in four distinct modes, ipso, ortho, meta, and para. The reactivity of the aromatic ring and the mode of attachment of the electrophile will be influenced by the specific nature of the substituent group, which may be X , Z, or C type. Detachment of the electro-... 

Similarly, relevant electrophiles (Lewis acids) including A-type metal cations (hard), bivalent transition metal ions (borderline), and B-type metal ions (soft) can be categorized (see Stumm and Morgan 1996). Note that in organic molecules, the atom where a nucleophile attacks (i.e., the electrophilic site) may possess harder (e.g., C=0, P=0) or softer (e.g., CH3-X) character. 

For carbocations, an electrophilicity (Lewis acidity) scale can be based on ions other than the hydroxide ion as is shown in general for X- in Equation (6), for which the equilibrium constant can be denoted A1R. Scales based on chloride ion, for example, have been used in the gas phase2,17,36 and are also appropriate for nonaqueous solvents. 

The boron atom in BH3 is sf hybridized with a vacant p orbital perpendicular to the plane of the three boron-hydrogen bonds. Thus borane and its derivatives are electrophilic (Lewis acidic) and combine readily with electron-rich species. For example, borane interacts with one of the lone pairs on the oxygen atom of tetrahydrofuran as shown below. 

Some of the terms associated with acids and bases have evolved specific meanings in organic chemistry. When organic chemists use the term base, they usually mean a proton acceptor (a Brpnsted-Lowry base). Similarly, the term acid usually means a proton donor (a Brpnsted-Lowry acid). When the acid-base reaction involves formation of a bond to some other element (especially carbon), organic chemists refer to the electron donor as a nucleophile (Lewis base) and the electron acceptor as an electrophile (Lewis acid). 

Notice that acetaldehyde acts as the nucleophile (Lewis base) in part (a) and as the electrophile (Lewis acid) in part (b). Like most organic compounds, acetaldehyde is both acidic and basic. It acts as a base if we add a strong enough acid to make it donate electrons or accept a proton. It acts as an acid if the base we add is strong enough to donate an electron pair or abstract a proton. 

Identify nucleophiles (Lewis bases) and electrophiles (Lewis acids), and write equations for Lewis acid-base reactions using curved arrows to show the flow of electrons. 

With only six electrons in the positive carbon s valence shell, a carbocation is a powerful electrophile (Lewis acid), and it may react with any nucleophile it encounters. Like other strong acids, carbocations are unlikely to be found in basic solutions. Carbocations are proposed as intermediates in many types of organic reactions, some of which we will encounter in Chapter 6. 

This polarization of the carbonyl group contributes to the reactivity of ketones and aldehydes The positively polarized carbon atom acts as an electrophile (Lewis acid), and the negatively polarized oxygen acts as a nucleophile (Lewis base). 

In acidic or electrophilic (Lewis acid) media, rearrangements are mainly centered at the oxetane ring (41,42), thus, taxol rearranges to the ortho ester 17 in the presence of ZnBr2 and to 18 with BF3. In the presence of ZnClj, a Wagner-... 

Initiation by Lewis acids requires the presence of a trace amount of a cocatalyst such as water or other proton or cation source. The Lewis base coordinates with the electrophilic Lewis acid, producing a proton, which is the actual initiator ... 

Polar reactions take place between electron-rich reagents (nucleophiles/ Lewis bases) and electron-poor reagents (electrophiles/Lewis acids). These reactions are heterolylic processes and involve species with an even number of electrons. Bonds are made when a nucleophile donates an electron pair to an electrophile bonds are broken when one product leaves with an electron pair. 

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. 

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