Electrons, unpaired “lone

The table gives the computed spin densities for each atom (the value in parenthese following the substituent is its electronegativity). The illustrations are Lewis doi structures showing the primary resonance form for each structure and indicatinj unpaired electrons and lone pairs. 

In a substitution reaction a part X of a molecule R—X is replaced by a group Y (Figure 1.1). The subject of this chapter is substitution reactions in which a part X that is bound to an ip3-hybridized C atom is replaced by a group Y via radical intermediates. Radicals are valence-unsaturated and therefore usually short-lived atoms or molecules. They contain one or more unpaired ( lone ) electrons. From inorganic chemistry you are familiar with at least two radicals, which by the way are quite stable NO and Oz. NO contains one lone electron it is therefore a monoradical or simply a radical. Oz contains two lone electrons and is therefore a biradical. 

Organic chemists tend to be pragmatists when faced with rival MO and VB descriptions of molecular electronic structure. Many will use whichever model seems most convenient for the problem at hand. MO descriptions are widely employed in frontier orbital approaches, as in the Woodward-Hoffmann rules, and tend to be favoured when predicting excited states or photoelectron spectra. On the other hand, it is customary to represent reaction mechanisms in terms of resonance between classical VB structures with single, double etc. bonds (plus any unpaired electrons or lone pairs) and then to indicate by means of curly... 

The one unpaired electron leads to one bond, and the three pairs of electrons give oxygen atoms with an extra electron three lone pairs. 

Consider a molecule formed by an element A in main-group N of the Periodic Table (N> 4). What does the polarity have to be to keep within an octet Answer An A ion has iV- x valence electrons, comprising / lone pairs and A - x-2l unpaired electrons. The latter can be shared with an equal number of unpaired electrons on other atoms to form covalent bonds. We want the total number of electrons (= 2N - 2x - 2/) to be <8. We must therefore have x > N- 4-1. [There will heN-x-2l covalent bonds and x ionic ones, giving a total a N-2l = V). The maximum value of Fis A (when / = 0).]... 

There are two equivalent resonance structures for NO2 for the purposes of VSEPR, we can use either one. NO2 is different from the examples considered up to now in that it has an odd number of electrons and the Lewis structure has an unpaired lone electron on the nitrogen. For the purposes of determining the basic VSEPR... 

Add multiple bonds to eliminate unpaired electrons. Draw the remaining nonbonding electrons as lone pairs. 

The valence electrons in molecular compounds are shown using an electron-dot formula, also called a Lewis structure. The shared electrons, or bonding pairs, are shown as two dots or a single line between atoms. The nonbonding pairs of electrons, or lone pairs, are placed on the outside. For example, a fluorine molecule (F2) consists of two fluorine atoms, Group 7A (17), each with seven valence electrons. In the F2 molecule, each F atom achieves an octet by sharing its unpaired valence electron. 

Both the oxygen and sulfur atoms have two lone pairs while the C/ carbon has ar unpaired electron, and in both cases the double bond shifts from the two carbor atoms to the carbon and the substituent. In acetyl radical, the electron density i centered primarily on the C2 carbon, and the spin density is drawn toward the lattei more than toward the former. In contrast, the density is more balanced between thf two terminal heavy atoms with the sulfur substituent (similar to that in allyl radical with a slight bias toward the sulfur atom. These trends can be easily related to th< varying electronegativity of the heavy atom in the substituent. 

This difference between these two carbenes can be attributed to the presence of the lone pair of electrons on nitrogen in 4b. Two-center, three-electron bonding in the molecular plane of 4b, involving the lone pair, produces some unpaired a spin density on nitrogen. Coulombic repulsion between the a and tt electrons of... 

The CHO radical is a o-radical and the main qualitative feature of the comparison between Tables 2 and 3 is the tendency for the a GHOs to be less contracted in CHO than is found in CH20. In particular, the sp2 hybrid on carbon which nominally contains the unpaired electron is considerably expanded (exponent 1.5923 compared to 1.8660 in CH20). The hydrogen orbital and the aoc orbital are also noticeably expanded while the lone pairs on oxygen are largely unaffected. 

It is obvious that one-electron oxidation has to proceed at the expense of the lone electron pair at the group NH2. Consequently, the 6-NH2 deprotonation is expected. Nevertheless, the real process occurs at the expense of both 6-NH2 and N(9)-H bond (Dias and Vieira 1996). In the N(9)-deprotonated radical, the unpaired electron is delocalized over the purine system, incorporating four nitrogen and five carbon atoms (see Scheme 1.23). 

Electron-donating functional groups, e.g. ethers, also stabilize radicals via their lone pair orbitals. However, electron-withdrawing groups can also stabilize radicals, so that radicals next to carbonyl or nitrile are more stable than even tertiary alkyl radicals. This is because these groups possess a jr electron system and the unpaired electron can take advantage... 

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