Unshared pairs of electrons

In the equation shown the base uses an unshared pair of electrons to remove a proton from an acid The base is converted to its conjugate acid, and the acid is converted to... 

An unshared pair of electrons from the Lewis base is used to form a covalent bond between the Lewis acid and the Lewis base The Lewis acid and the Lewis base are shown as ions m the equation but they need not be If both are neutral molecules the analogous equation becomes... 

Thus with dihalocarbenes we have the interesting case of a species that resem bles both a carbanion (unshared pair of electrons on carbon) and a carbocation (empty p orbital) Which structural feature controls its reactivity s Does its empty p orbital cause It to react as an electrophile s Does its unshared pair make it nucleophilic s By compar mg the rate of reaction of CBi2 toward a series of alkenes with that of typical electrophiles toward the same alkenes (Table 14 4) we see that the reactivity of CBi2... 

The unshared pairs of electrons on hydroxyl oxygens seek electron deficient centers. Alkylphenols tend to be less nucleophiUc than aUphatic alcohols as a direct result of the attraction of the electron density by the aromatic nucleus. The reactivity of the hydroxyl group can be enhanced in spite of the attraction of the ring current by use of a basic catalyst which removes the acidic proton from the hydroxyl group leaving the more nucleophiUc alkylphenoxide. 

The unshared pair of electrons on the nitrogen atom provides the basic character to the fatty amines. Basicity of amines has been deterrnined as... 

Most heterocyclic anions may be considered to be derived by loss of a proton from a parent compound, which is therefore the conjugate acid. Such anions have at least one unshared pair of electrons at the anionic site. They are named by appending the suffix -ide , with elision of a terminal e (lUPAC recommendation RC-83.1.1), as in (190)-(193). The site may be specified by a locant placed immediately before the suffix, and so chosen as to be as low as possible consistent with the numbering of the skeleton of the parent compound. The locant may be omitted in order to designate an equilibrating mixture of positionally isomeric anions, which is what one usually obtains in practice. The anion of piperidine is often informally referred to as piperidide . 

Another significant structural effect that inrparts high nucleophilicity is the alpha effect. It is observed that atoms which are directly bonded to an atom with one or rhore unshared pairs of electrons tend to be stronger nucleophiles than would othermse be expected. Examples in Table 5.7 include H02, which is more nucleophilic than HO , and... 

Since Stork et al. introduced as a new synthetic method the alkylation and acylation of carbonyl compounds via enamines, this class of compounds has been the subjeet of intensive studies 1-3). The exceptional physical and chemical behavior of the enamine structure can be ascribed to resonance by conjugation of the unshared pair of electrons of the nitrogen atom with the 77 electrons of the double bond ... 

An unshared pair of electrons, owned entirely by one atom, is shown as a pair of dots on that atom. (An unshared pair is often referred to, more picturesquely, as a lone pair.)... 

Strategy Draw the Lewis structure as a first step. Then decide what type (AX AX3, and so on) the molecule is, focusing on the central atom. Remember, X represents a terminal atom, F an unshared pair of electrons. 

In principle, any molecule or anion with an unshared pair of electrons can act as a Lewis base. In other words, it can donate a lone pair to a metal cation to form a coordinate covalent bond. In practice, a ligand usually contains an atom of one of die more electronegative elements (C, N, O, S, F, Cl, Br, I). Several hundred different ligands are known. Those most commonly encountered in general chemistry are NH3 and HzO molecules and CN , Cl-, and OH- ions. 

Some ligands have more than one atom with an unshared pair of electrons and hence can form more than one bond with a central metal atom. Ligands of this type are referred to as chelating agents the complexes formed are referred to as chelates (from the Greek chela, crab s claw). Two of the most common chelating agents are the oxalate anion (abbreviated ox) and the ethylenediamine molecule (abbreviated en), whose Lewis structures are... 

In reaction (18), one of the unshared pairs of electrons on the oxygen atom of the MOH group has accepted a proton, and so MOH can act as a base without actually releasing hydroxide ion. 

An interesting question is the effect of unshared pairs of electrons on an axial atom. Pauling states that they appear to contribute little to three fold barriers, basing this on the observed fact that methyl amine with two N—H bonds and one pair has about 2/3 the barrier of ethane while methyl alcohol with one O—H bond and two pairs has approximately one third the ethane value. He explains this by pointing out that the unshared pairs, not forming bonds, have not the same reason to acquire / character, without which they will not contribute to a threefold barrier. 

Since it is clear that the presence of an unshared pair of electrons on the sulfur of the sulfoxide group leads to no special instability in the case of the known thiirene oxides (i.e., 18a, 28a,b and the first alkyl-substituted thiirene oxide 30 recently synthesized60), the reduced antiaromatic properties of the thiirene oxides relative to that of thiirenes have been manifested experimentally. As far as the possibility of electron-attracting conjugative stabilization involving the sulfur atom in thiirene oxides is concerned, the experimental evidence accumulated so far is not decisive. Thus, the chemical shift of the vinylic carbon of... 

The contraction to 105° is explained by postulating that the repulsion between the unshared pairs of electrons is greater than between the shared pairs, thus opening-out the angle between the former and contracting the angle between the latter. 

Dicyclopentadienyltin(II) is readily prepared from cyclopentadienyl-lithium or -sodium and tin(II) chloride, and is the best known, monomeric, RtSn compound. The planes of the 7)-cyclopentadienyl groups subtend an angle of 55°, and the unshared pair of electrons can act as a ligand towards such Lewis acids as BF3 and AICI3 i319). 

Aromatic sextets can also be present in five- and seven-membered rings. If a five-membered ring has two double bonds and the fifth atom possesses an unshared pair of electrons, the ring has five p orbitals that can overlap to create five new orbitals— three bonding and two antibonding (Fig. 2.6). There are six electrons for these orbitals the four p orbitals of the double bonds each contribute one and the filled orbital contributes the other two. The six electrons occupy the bonding orbitals and constitute an aromatic sextet. The heterocyclic compounds pyrrole, thiophene, and... 

Compounds With Tervalent Chiral Atoms. Atoms with pyramidal bonding might be expected to give rise to optical activity if the atom is connected to three different groups, since the unshared pair of electrons is analogous to a fourth group, necessarily different from the others. For example, a secondary or tertiary amine where X, Y, and Z are different would be expected to be chiral and thus resolvable. Many attempts have been made to resolve such compounds, but until 1968 all of them failed because of pyramidal inversion, which is a rapid oscillation of the unshared pair from one side of the XYZ... 

By definition, every carbanion possesses an unshared pair of electrons and is therefore a base. When a carbanion accepts a proton, it is converted to its conjugate acid (see Chapter 8). The stability of the carbanion is directly related to the strength of the conjugate acid. The weaker the acid, the greater the base strength and the lower the stability of the carbanion. Here, by stability we mean stability toward a proton donor the lower the stability, the more willing the carbanion is to accept a... 

Soft Acids. The acceptor atoms are large, have low positive charge, and contain unshared pairs of electrons (p or d) in their valence shells. They have high polarizability and low electronegativity. 

In all cases, Y must have an unshared pair of electrons, so that all nucleophiles are Lewis bases. When Y is the solvent, the reaction is called solvolysis. Nucleophilic substitution at an aromatic carbon is considered in Chapter 13. 

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