Tetrahedral Electrophiles

Some of the same features affect the electrophilicity of tetrahedral electrophiles undergoing SN2 reactions. Thus some donor substituents on the carbon being attacked reduce the electrophilicity of alkyl halides, where the order of reactivity is methyl ethyl isopropyl terf-butyl. The transition structure of an SN2 reaction is inherently electron rich, and donor substituents slow it down. Equally, steric hindrance to attack, the traditional explanation for this reactivity order, is greater for the more substituted alkyl halides. 

The better the leaving group, the more electrophilic an alkyl halide or alkyl sulfonate, with alkyl iodides and trifluoromethanesulfonates exceptionally reactive. Several factors are at work here the strength of the 

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The lack of examples demonstrating the reactivity on C-2 may be the misleading impression that this atom is not electrophilic, contrary to what is indicated from charge diagrams. Such is not the case as the Cook s rearrangement demonstrates (209, 212). A logical mechanism for this reaction involves the tetrahedral intermediate (88) (Scheme 42). This... 

The boron atom in boron trifluoride is hybridized to the sp planar configuration and consequently is coordinatively unsaturated, ie, a Lewis acid. Its chemistry centers around satisfying this unsaturation by the formation with Lewis bases of adducts that are nearly tetrahedral sp [ The electrophilic properties (acid strengths) of the trihaloboranes have been found to increase in the order BF < BCl < BBr < BI (3,4). 

Electrophilic Substitution. The most common mechanism for electrophilic attack at an aromatic system involves the initial attack of an electrophile to give an intermediate containing a tetrahedral carbon atom loss of, usually a proton, from the intermediate, then gives the product ... 

There are large differences in reactivity among the various carboxylic acid derivatives, such as amides, esters, and acyl chlorides. One important factor is the resonance stabilization provided by the heteroatom. This decreases in the order N > O > Cl. Electron donation reduces the electrophilicity of the carbonyl group, and the corresponding stabilization is lost in the tetrahedral intermediate. 

The most common reaction of aldehydes and ketones is the nucleophilic addition reaction, in which a nucleophile, Nu , adds to the electrophilic carbon of the carbonyl group. Since the nucleophile uses an electron pair to form a new bond to carbon, two electrons from the carbon-oxygen double bond must move toward the electronegative oxygen atom to give an alkoxide anion. The carbonyl carbon rehybridizes from sp2 to sp3 during the reaction, and the alkoxide ion product therefore has tetrahedral geometry. 

As we saw in A Preview of Carbonyl Compounds, the most general reaction of aldehydes and ketones is the nucleophilic addition reaction. A nucleophile, Nu-, approaches along the C=0 bond from an angle of about 75° to the plane of the carbonyl group and adds to the electrophilic C=0 carbon atom. At the same time, rehybridization of the carbonyl carbon from sp2 to sp3 occurs, an electron pair from the C=0 bond moves toward the electronegative oxygen atom, and a tetrahedral alkoxide ion intermediate is produced (Figure 19.1). 

A Grignard reaction begins with an acid-base complexation of Vfg2+ to the carbonyl oxygen atom of the aldehyde or ketone, thereby making the carbonyl group a better electrophile. Nucleophilic addition of R then produces a tetrahedral magnesium alkoxide intermediate, and protonation by addition of water... 

Not all the reactions in this chapter are actually nucleophilic substitutions. In some cases the mechanisms are not known with enough certainty even to decide whether a nucleophile, an electrophile, or a free radical is attacking. In other cases (such as 10-79), conversion of one compound to another can occur by two or even all three of these possibilities, depending on the reagent and the reaction conditions. However, one or more of the nucleophilic mechanisms previously discussed do hold for the overwhelming majority of the reactions in this chapter. For the alkylations, the Sn2 is by far the most common mechanism, as long as R is primary or secondary alkyl. For the acylations, the tetrahedral mechanism is the most common. 

The first step is usually, but not always, rate determining. It can be seen that this mechanism greatly resembles the tetrahedral mechanism discussed in Chapter 10 and, in another way, the arenium ion mechanism of electrophilic aromatic substitution. In all three cases, the attacking species forms a bond with the... 

As is the case in the tetrahedral mechanism, it is also possible for the electrophilic species to attack first, in which case it goes to the heteroatom. This species is most... 

Triphenylphosphonium ylide reacts with the silylene complex 93 which has a highly electrophilic silicon center, to give the corresponding cationic adduct 94 [115]. The lengthening of the PC bond indicates a loss of the double bond character of the ylide and corresponds to the formation of a tetrahedral silicon center with four covalent bonds (Scheme 28). 

As we have seen (Section 4, p. 191) the range of effective molarities associated with ring-closure reactions is very much greater than that characteristic of intramolecular general acid-base catalysis the main classification is therefore in terms of mechanism. By far the largest section (I, Tables A-D) gives EM s for intramolecular nucleophilic reactions. These can be concerted displacements (mostly at tetrahedral carbon), stepwise displacements (mostly addition-elimination reactions at trigonal carbon), or additions, and they have been classified in terms of the nucleophilic and electrophilic centres. 

A nucleophile attacks the electrophilic carbon atom of the polar carbonyl group from a direction approximately perpendicular to the plane of sp hybridised orbitals of carbonyl carbon (Fig. 12.2). The hybridisation of carbon changes from sjf to s f in this process, and a tetrahedral alkoxlde intermediate is produced. This... 

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