Lewis acids reacting with lone pairs

Silicon reacts directly with chlorine to form silicon tetrachloride, SiCl4 (this reaction was introduced in Section 14.17, as one step in the purification of silicon). This compound differs strikingly from CC14 in that it reacts readily with water as a Lewis acid, accepting a lone pair of electrons from H20 ... 

In this reaction, ferrous iron, a Lewis acid, reacts with the lone pair electrons on water molecules, which are Lewis bases, to form a hydrated cation, a Lewis adduct. Although the sulfate ion also interacts with water molecules, this interaction is much weaker than the Lewis acid-base interaction of the ferrous iron. Many, perhaps all, chemical reactions can be viewed as Lewis acid-base interactions, making this theory very useful for developing models of reaction mechanisms. 

The idea of the proton as the acidic entity was extended by G. N. Lewis. Protons can accept electron pairs from bases Lewis acids are generalized electron-pair acceptors. For example, aluminium trichloride (1.60) behaves as a Lewis acid and reacts with the chloride ion (a Lewis base). Boron trifluoride may react with the lone pair of the oxygen of diethyl ether to form boron trifluoride etherate (1.61). A Lewis base is an electron pair donor, in this case the ether oxygen. 

Describe the following reactions with structural formulas and curly arrows. Label the reacting species as either a Lewis acid or a Lewis base. Show the lone pairs that form the coordinate bond, a Boron trifluoride reacting with ethoxyethane (0(C2H5)2>. 

Notice in the list of Lewis bases just given that some compounds, such as carboxylic acids, esters, and amides, have more than one atom ivith a lone pair of electrons and can therefore react at more than one site. Acetic acid, for example, can be protonated either on the doubly bonded oxygen atom or on the singly bonded oxygen atom. Reaction normally occurs only once in such instances, and the more stable of the two possible protonation products is formed. For acetic add, protonation by reaction with sulfuric acid occurs on... 

Push-spectator carbenes of the type 31 (R, R = alkyl) were synthesized and reacted with various Lewis Acids to compare the reactivity of the phosphorus and carbene centers. Two such reactions are shown in Scheme 7.11. From an X-ray structural analysis, the phosphorus substituent was shown to act as a spectator, leaving its lone pair available to react in a Lewis basic manner. When carbene 31 was reacted with BF3, only the carbene adduct 32 was formed. By contrast, when 31 was reacted with the softer Lewis Acid BH3, it was the phosphorus that reacted to yield adduct 33. These types of carbenes exhibited C-NMR shifts in the range of 320-348 ppm, a P-C-N angle of 116.5° a short C-N distance of 1.296 A, and a long C-P distance of 1.856 A. The latter is very similar to that of a typical C-P single bond. 

Ammonia reacts with boron trichloride to form a molecule called an adduct or Lewis acid base complex in which the lone pair on the ammonia molecule is shared with the boron atom to form a covalent bond and completing an octet on boron (Figure 1.16) ... 

From the resonance description you might conclude that although the primary site for electrophilic attack is at N-1, reactions at carbon C-3(5) might be possible, even if not as likely. However, an important point to remember is that the N atom of pyridine carries a lone pair of electrons these electrons are NOT part of the jc-system. As a result, pyridine is a base 5.2), reacting with acids, Lewis acids and other electrophiles (E ) to form stable pyridinium salts (Scheme 2.2), in which the heterocycle retains aromatic character. 

Because of the presence of a lone pair and a vacant orbital, singlet carbenes are supposed to be able to react with both Lewis bases and acids. Transient electrophilic carbenes are known to react with Lewis bases to give normal ylides (Scheme 8.19). For example, carbene-pyridine adducts have been spectroscopically characterized and used as a proof for the formation of carbenes,and the reaction of transient dihalogenocarbenes with phosphines is even a preparative method for C-dihalogeno phosphorus ylides. Little is known about the reactivity of transient carbenes with Lewis acids. 

According to the acid-base concept of Pearson, A -phosphorins can be viewed as soft bases the lone electron pair at phosphoms is much more delocalized than the lone pair at nitrogen in pyridine. Thus, such soft Lewis acids as Hg ions are more likely to react with A -phosphorins (see p. 84). 

The lone pair of electrons on the N atom of amines accounts for their base strength and nucleophilicity. They abstract protons from water, react with Lewis acids, and attack electrophilic sites such as carbonyl carbon. 

Such a bond, in which the donor molecule (or anion) provides both bonding electrons and the acceptor cation provides the empty orbital, is called a coordinate or dative bond. The resulting aggregation is called a complex. Actually, any molecule with an empty orbital in its valence shell, such as the gas boron trifluoride, can in principle act as an electron pair acceptor, and indeed BF3 reacts with ammonia (which has a lone pair, NH3) to form a complex H3N ->BF3. Our concern here, however, is with metal cations, and these usually form complexes with from 2 to 12 donor molecules at once, depending on the sizes and electronic structures of the cation and donor molecules. The bound donor molecules are called ligands (from the Latin ligare, to bind), and the acceptor and donor species may be regarded as Lewis acids and Lewis bases, respectively. 

Since they have both a vacant low-energy orbital and a lone pair, silylenes might behave either as electron pair donors or acceptors. There is scant evidence for silylenes reacting as Lewis bases, but complexes of silylenes acting as Lewis acids are now well-established these complexes can also be described as silaylides, R2>Si —B+262. Trinquier has calculated that even SiLL should form a weak complex with 112S i , in which a silane hydrogen binds to the p-orbital of the silylene263. 

These five-membered rings have lone pair(s) delocalised from the heteroatom round the ring and are electron-rich . They react all too easily with electrophiles and are unstable in acid whether protic or Lewis. We have to find reactions that can be used in neutral or only weakly acidic solution. The synthesis of tolmetin 99 illustrates the two most important reactions.14 The disconnection of the ketone would lead naturally to an AICI3-catalysed Friedel-Crafts reaction between the acid chloride 100 and the pyrrole 101. 

The lone pair (see Lone Pair) of electrons on the nitrogen atom of bound cyanide can react with Lewis acids (see Lewis Acids Bases) to form adducts of the type =N L. Often, strong Lewis acids such as BF3 are used... 

Halogen molecules are not strong electrophiles and, fluorine excepted, do not react with benzene. However, in the presence of a Lewis acid, reaction occurs readily. The role of the catalyst is to accept a lone pair of electrons from the halogen molecule, which then becomes electron deficient at one of the halogen atoms. The actual electrophile is probably the complex formed from the halogen and the catalyst, rather than a halonium ion, e.g. Cr or Br. Bromination of benzene serves as a good example of halogenation (Scheme 2.4). 

Ethers are relatively unreactive substances, which is why diethyl ether and tetrahydrofuran are widely used as solvents for organic reactions. However, the lone pairs on the oxygen atom are a source of reactivity. The oxygen atom may be protonated, and it reacts with Lewis acids. The increased polarity of the C-O bond then makes the neighbouring carbon atoms sensitive to nucleophilic attack. 

The conversion of amines to amides is useful in the synthesis of substituted anilines. For example, aniline itself does not undergo Fiiedel-Crafts reactions (Section 18.1 OB). In.stead, its basic lone pair on N reacts with the Lewis acid (AICI3) to form a deactivated complex that does not undergo further reaction. 

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