Halides electron pair bases

A series of Rh and Ir complexes is also basic enough to add boron halides, although the metal-boron bonds are weak, e.g., dppe and PPh3-Rh complexes serve as electron-pair bases when they react with BF3 and BCI3 . Table 1 summarizes the results. 

Despite this, they are good solvents for chloride-ion transfer reactions, and solvo-acid-solvo-base reactions (p. 827) can be followed conductimetri-cally, voltametrically or by use of coloured indicators. As expected from their constitution, the trihalides of As and Sb are only feeble electron-pair donors (p. 198) but they have marked acceptor properties, particularly towards halide ions (p. 564) and amines. 

The alkynide anion is a Lewis base and reacts with the alkyl halide (as an electron pair acceptor, a Lewis acid). 

As has already been mentioned, boron halides are electron-deficient molecules. As a result, they tend to act as strong Lewis acids by accepting electron pairs from many types of Lewis bases to form stable acid-base adducts. Electron donors such as ammonia, pyridine, amines, ethers, and many other types of compounds form stable adducts. In behaving as strong Lewis acids, the boron halides act as acid catalysts for several important types of organic reactions (see Chapter 9). 

As recognized in the pioneering work of Meerwein, Ingold, and Whitmore,10 12 18 19 744 trivalent alkyl cations (C H2 +i+) play important roles in the acid-catalyzed transformations of hydrocarbons as well as various electrophilic and Friedel-Crafts-type reactions. Trivalent alkyl cations can directly be formed by the ionization of lone-pair (nonbonded electron pair) containing precursors (w-bases) such as alkyl halides, alcohols, thiols, and so on, or by protonation of singlet carbenes or olefins. 

First identify the base as the amide ion (H2N-) portion of potassium amide (KNH2). Amide ion is a strong base and uses an unshared electron pair to abstract a proton from /3 carbon of the alkyl halide. The pair of electrons in the C—H bond becomes the tt component of the double bond as the C—Br bond breaks. The electrons in the C—Br bond become an unshared electron pair of bromide ion. 

O The aluminum trichloride bonds with an electron pair on the chlorine of the alkyl halide to form a Lewis acid-base adduct. This changes the leaving group to AICI4, which is a weaker base and a better leaving group than chloride anion. 

The structure of the complex between a pair of hydrogen halide molecules is depicted in Fig. 2.7 where three lone electron pairs are placed on the proton-accepting molecule. In the classical case of sp hybridization, one might expect an angle p of some 109°. a. measures the nonlinearity of the H-bond as in the above cases, A nonzero value of a might be expected based on the direction of the dipole moment of the acceptor molecule. 

Because the.se substitution reactions involve electron-rich nucleophiles, they are called nucleophilic substitution reactions. Examples are shown in Equations [l]-[3]. Nucleophilic substitutions are Lewis acid—base reactions. The nucleophile donates its electron pair, the alkyl halide (Lewis acid) accepts it, and the C-X bond is heterolytically cleaved. Curved arrow notation can be used to show the movement of electron pairs, as shown in Equation [3]. 

Here, the alkyl halide acts as a base, while the aluminium trichloride acts as the electron pair acceptor. 

Alkylation involves treating ammonia or an amine with an alkyl halide. The amine, as a Lewis base with a non-bonding electron pair, is a good nucleophile and displaces the halide ion from the alkyl halide the reaction is nucleophilic substitution with a neutral nucleophile. SN2 reactions are common. Since alkylation tends to continue until four groups are bonded to the nitrogen, it has limited synthetic utility. 

A number of years ago G. N. Lewis extended our understanding of acid-base behavior to include reactions other than proton transfers. According to Lewis, an acid is an electron-pair acceptor and a base is an electron-pair donor. Thus, carbocations are electron-pair acceptors and are Lewis acids. Halide anions are electron-pair donors and are Lewis bases. It is generally true that electrophiles are Lewis acids, and nucleophiles are Lewis bases. 

If the reactions of organic halides and Mg metal are performed in hydrocarbons with the addition of 1 or a few equiv of an electron-pair donor base, they occur much more... 

The choice of acid or base for solvent is simplified appreciably for melts containing complex ions (as a rule, they are anions), which are prone to the acid-base dissociation. Dissociation of this ion is assumed as the intrinsic acid-base equilibrium of a melt of such kind. In this case, the simpler eliminated anion will be considered as the base of the solvent and the coordinationally unsaturated residue will be the acidic particle of the solvent. Naturally, the division of particles formed by the auto-dissociation into acids and bases is made on the basis of the Lewis definition [13] an acid is the acceptor of an electron pair and a base is the donor of this electron pair. Ionic melts based on complex halides of gallium(III) [28], aluminium(III) [29] and boron(III) [30,31] may serve as examples of successful application of the above approach. The electron-deficient covalent halide (e.g. A1C13, BF3) in these melts is the solvent acid, and the corresponding halide ion is the base of the solvents ... 

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