Electron-deficient atoms

Charge-Transfer Forces. An electron-rich atom, or orbital, can form a bond with an electron-deficient atom. Typical examples are lone pairs of electrons, eg, in nitrogen atoms regularly found in dyes and protein and polyamide fibers, or TT-orbitals as found in the complex planar dye molecules, forming a bond with an electron-deficient hydrogen or similar atom, eg, —0 . These forces play a significant role in dye attraction. 

Delocalisation takes place (cf. 1,3-dienes, p. 13), so that an electron-deficient atom results at C3, as well as at C, as in a simple carbonyl compound. The difference between this transmission via a conjugated system, and the inductive effect in saturated system, is that here the effect suffers much less diminution by its transmission, and the polarity at adjacent carbon atoms alternates. 

The term hetero rearrangement is not limited to those reactions in which only the initially electron-deficient atom is something other than carbon, but may be applied to rearrangements in which any of the carbon atoms of formula L have been replaced with other elements. 

The electron deficient atom is stabilized by conjugation with the unshared electrons of the adjacent atom. 

Hydrolysis Reactions. Hydrolysis reactions involve cleavage of a single bond by reaction with water, a hydronium, or a hydroxide ion (78). The bond is typically polarized between an electron-deficient atom (C in carbonyl, P in organophosphates) and an electron-rich atom (0, Cl, Br). The reaction may be neutral, base-, or acid-promoted, depending on the substrate properties and the reaction conditions, such as pH, temperature, and ionic strength (78, 79). 

In their continued efforts, McBreen and co-workers selected boron, an electron-deficient atom, as the core to build a series of new anion receptors using the same tactics with electron-withdrawing substituents. These new additives can be classified roughly into thesethreesubcategories borate, borane, and boronate. Selected representatives from each category are also listed in Table 8. 

The most common examples of rearrangements involve an electron-deficient atom, and pre-eminent amongst these are carbocations. Since carbocations are a feature of the SnI and El mechanisms, it follows that rearrangements can be side-reactions of these types of transformation. The driving force in carbocation rearrangements is to form a more stable carbocation. 

For example, the hydrogen atoms of the strongly polarized bonds in hydrides LiH and BeH2 or BH4 can be electron donors, and the electron-deficient atoms Li, Be, or B can accept electrons to form inverse hydrogen-bonded complexes Li-H Li-H, H-Be-H Li-H, and others [3]. Similar to classical hydrogen bonds, the electronic distribution in these inverse hydrogen bonds, analyzed in the framework of AIM theory, shows that the hydrogen atom is bound to both the electron donor and the electron acceptor by closed-shell interactions. In addition, the bond critical points correspond to all the characteristics associated... 

Electrophiles, the most common type of reactive intermediate, are molecules with an electron-deficient atom, so having a full or partial positive charge. 

Some atoms are more electronegative than others that is, they more strongly attract electrons. The relative electronegativities of atoms encountered in this text are F > O > N > 0 S > P 11. For example, the two electron pairs making up a 0=0 (carbonyl) bond are not shared equally the carbon is relatively electron-deficient as the oxygen draws away the electrons. Many reactions involve an electron-rich atom (a nucleophile) reacting with an electron-deficient atom (an electrophile). Some common nucleophiles and electrophiles in biochemistry are shown at right. 

The increase in ligancy of the carbon atom illustrates the principle, mentioned in Section 10-6, that an electron-deficient atom causes adjacent atoms to increase their ligancy. The platinum atom in the monomer, Pt(CHf)4, would make use of only seven of its nine valence orbitals four for bonds to the four carbon atoms and three for the three unshared pairs of bd electrons. The electron deficiency of this atom then permits the increase in ligancy of carbon. 

There is some evidence that the form of the chemical carcinogen that ultimately reacts with cellular macromolecules must contain a reactive electrophilic center, that is. an electron-deficient atom that can attack the numerous electron-rich centers in polynucleotides and proteins. As examples, significant electrophilic centers include free radicals, carbonium ions, epoxides, the nitrogen in esters of hydroxylamines and hydroxamic acids, and some metal cations. It is believed that carcinogens, which in themselves are not electrophiles, are metabolized to electrophilic derivatives that then become the ultimate" carcinogens. 

All of these groups possess a positively charged atom or an electron deficient atom (i.e. an electrophilic centre) directly attached to the aromatic ring. Since this atom is electron deficient, it has an electron-withdrawing effect on the ring. 

Similar [1,2] migrations involving other electron-deficient atoms are also known. The Beckmann rearrangement is a reaction of an oxime to produce an amide ... 

The hydrolytic attack on an ester linkage in the presence of water at high pH is an important example of this mode of degradation. The attack initiates on the electron-deficient atom in a highly polarized bond, as on carbonyl carbon ... 

The Br0nsted-Lowry definition of acids and bases depends on the transfer of a proton from the acid to the base. The base uses a pair of nonbonding electrons to form a bond to the proton. G. N. Lewis reasoned that this kind of reaction does not need a proton. Instead, a base could use its lone pair of electrons to bond to some other electron-deficient atom. In effect, we can look at an acid-base reaction from the viewpoint of the bonds that are formed and broken rather than a proton that is transferred. The following reaction shows the proton transfer, with emphasis on the bonds being broken and formed. Organic chemists routinely use curved arrows to show the movement of the participating electrons. 

Curly arrows also predict the same electron distribution for all these intermediates, whether the electrophile is a proton or any of the other reagents we will meet in this chapter. The cation can be represented as three different delocalized structures that show clearly the electron-deficient atoms, or by a structure with partial bonds that shows the delocalization but is of no use for drawing mechanisms. 

Almost all compounds polymerizing by the radical mechanism belong to the classical monomers with a double or triple bond. Radicals of relatively low reactivity formed from the initiators do not usually attack the bonds of electron-rich atoms (with an excess of electrons). They react readily with electron-deficient atoms. Thus the anionically polymerizing monomers usually also polymerize by a radical mechanism. Typical cationic monomers do not undergo radical polymerization. The quite neutral ethylene forms a transition between the two groups. It polymerizes reluctantly by the radical and ionic mechanisms cationically it only yields oligomers. 

The electron-deficient atom with which the alcohol reacts may not be a carbon atom. It may be sulfur as in thionyl chloride (SOCI2) or toluene-4-sulfonyl chloride (MeC H4S02Cl), nitrogen as in nitrosyl chloride (NOCl), phosphorus as in phosphorus oxychloride (POCI3) or phosphorus pentachloride (PCI5) or chromium(Vr) as in chromium trioxide (CrOj). In each case, esters of the corresponding inorganic acids are formed. Many of these esters are very reactive and form intermediates in reaction sequences. 

The electron-deficient atom to which the nitrogen adds may be another heteroatom, as in toluene-4-sulfonyl chloride. The product is then a sulfonamide. Some sulfonamides have useful biological activity as antibacterial agents. 

When the electron-deficient atom is the nitrogen atom of nitrous acid or nitrosyl chloride, the products will depend on whether the amine is a primary or secondary amine. Primary aliphatic amines give rise to dia-zonium salts (RNj X) which readily decompose, sometimes with rearrangement. Aromatic diazonium salts (ArNj X ) are very important in aromatic transformations, and are discussed later. Secondary amines, on the other hand, give rise to nitrosamines... 

Draw a curved arrow from the electron pair of the base to the electron-deficient atom of the acid. 

Bases attack protons. Nucleophiles attack other electron-deficient atoms (usually carbons). 

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