Unshared electrons, and

The general rule has been formulated (P) that the less substituted enamine is formed from unsymmetrical ketones such as the 2-alkylcyclohexanones. In enamine 21 the R, group and the N-alkyl groups would interfere with one another if overlap is to be maintained between the nitrogen unshared electrons and the double bond. There would be less repulsion if the isomeric enamine (22) were formed. 2-Phenylcyclohexanone and pyrrolidine with p-toluenesulfonic acid as catalyst in refluxing benzene gave enamine... 

The resonating-valence-bond theory of metals discussed in this paper differs from the older theory in making use of all nine stable outer orbitals of the transition metals, for occupancy by unshared electrons and for use in bond formation the number of valency electrons is consequently considered to be much larger for these metals than has been hitherto accepted. The metallic orbital, an extra orbital necessary for unsynchronized resonance of valence bonds, is considered to be the characteristic structural feature of a metal. It has been found possible to develop a system of metallic radii that permits a detailed discussion to be given of the observed interatomic distances of a metal in terms of its electronic structure. Some peculiar metallic structures can be understood by use of the postulate that the most simple fractional bond orders correspond to the most stable modes of resonance of bonds. The existence of Brillouin zones is compatible with the resonating-valence-bond theory, and the new metallic valencies for metals and alloys with filled-zone properties can be correlated with the electron numbers for important Brillouin polyhedra. 

The lOH has unshared electrons and is a nucleophile. Because of the polar nature of the C—Br bond,... 

Some atoms, even in covalent compounds, carry a formal charge, defined as the number of valence electrons in the neutral atom minus the sum of the number of unshared electrons and half the number of shared electrons. Resonance occurs when we can write two or more structures for a molecule or ion with the same arrangement of atoms but different arrangements of the electrons. The correct structure of the molecule or ion is a resonance hybrid of the contributing structures, which are drawn with a double-headed arrow () between them. Organic chemists use a curved arrow (O) to show the movement of an electron pair. 

Resonance must be used whenever more than one reasonable Lewis structure can be written for a molecule, provided that the Lewis structures have identical positions of all atoms. Only the positions of unshared electrons and multiple bonds are changed in writing different resonance structures. When a better picture of bonding is developed in Chapter 3, we will get a better understanding of what resonance means and when it must be used. 

The number of electrons owned by an atom is determined by its numba- of bonds and lone pairs. An atom owns all of its unshared electrons and half of its shared electrons. 

Formal charge (Section 1.3C) The electronic charge assigned to individual atoms in a Lewis structure. The formal chaige is calculated by subtracting an atom s unshared electrons and half of its shared electrons fiom the number of valence electrons that a neutral atom would possess. 

The formal charge on oxygen in 1-10 is —1. There are six unshared electrons and 2/2 = 1 electron from the pair being shared. Thus, the number of electrons is seven, which is one more than the number of valence electrons for oxygen. 

It is often necessary to adjust the reaction medium to Just the right acidity. Addition involves nucleophilic attack by the basic nitrogen compound on carbonyl carbon. Protonation of carbonyl oxygen makes carbonyl carbon more susceptible to nucleophilic attack in so far as the carbonyl compound is concerned, then, addition will be favored by high acidity. But the ammonia derivative, H2N—G, can also undergo protonation to form the ion, +H3N—G, which lacks unshared electrons and is no longer nucleophilic in so far-as the nitrogen compound is concerned, then, addition is favored by low acidity. The conditions under which... 

It is evident, however, that such intermediates cannot be wholly correct for free rotation would be expected about the carbon-carbon single bond in I, and this would lead to a mixture of meso and racemic products from either the cis or tram isomer. Roberts and Kimball, however, have pointed out that when there is an atom, such as bromine, whie.h has three pairs of unshared electrons near a carbonium ion, a covalent bond would almost surely be formed between the atom with unshared electrons and the carbon atom having an incomplete valence shell.11 Consequently, the true nature of the intermediate I is a three-membered bromoniuvi ion (II) which would indeed prevent rotation about the carbon-carbon bond ... 

This reaction can be considered as the sum of the processes J H B + H+, H+ + S - 11 S, the separation of B and H S and the changes in solvation that accompany these processes. In general the solvent molecules, especially if the solvent, like water, has unshared electrons and bonded hydrogen atoms, are strongly associated... 

A free radical, then, has an unshared electron and has no charge. It is very reactive and not solvated and does not develop high concentrations in solution. It will be demonstrated that radicals can be produced photochemically and in some cases by thermal processes, for example,... 

The molecular ion is a radical cation because it contains both an unshared electron and a positive charge. Using propane as an example, we can write the following equation to represent formation of its molecular ion by electron impact ionization ... 

In structure A, the dots represent the electrons from carbon, and the x s are the electrons from the oxygens. Structure B shows the bonds and oxygens unshared electrons, and structure C shows only the covalent bonds. Two electron pairs are shared between carbon and oxygen. Consequently, the bond is called a double bond. Each oxygen atom also has two pairs of nonbonding electrons, or unshared electron pairs. The loops in the following structures show that each atom in carbon dioxide has a complete valence shell of eight electrons ... 

Carbocations ould not be confused with free radicak or carbanions. A free radical is similar to a carbocation in that it has three bonds, but rather than having a deficit of two electrons, a free radical has one unshared electron and, as a result, is highly reactive. A carbanion has an unshared pair of electrons in addition to three bonds and carries a negative charge. A carbene carbon has two bonds and one unbared pair of electrons. Caibenes are rarely isolated, becauseof their prt Tensiiy to react with one another to form dimers. 

Radical cation (Section 9.14) A chemical species containing an unshared electron and a positive charge. 

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