Reactive species Lewis structures

Write the Lewis structure of each of the following reactive species, all of which are found to contribute to the destruction of the ozone layer, and indicate which are radicals (a) chlorine monoxide, CIO (b) dichloroperoxide, Cl—O—O—Cl ... 

Trigonal-bipyramidal species and nucleophilic displacement reactivity The 3c/4e cu-bonding motif can also be achieved in nonlinear polyatomics by backside attack of a nucleophile X - on a polar Y—Z bond of a conventional Lewis-structure molecule,... 

The splitting of a Cl2 molecule is an initiation step that produces two highly reactive chlorine atoms. A chlorine atom is an example of a reactive intermediate, a short-lived species that is never present in high concentration because it reacts as quickly as it is formed. Each Cl- atom has an odd number of valence electrons (seven), one of which is unpaired. The unpaired electron is called the odd electron or the radical electron. Species with unpaired electrons are called radicals or free radicals. Radicals are electron-deficient because they lack an octet. The odd electron readily combines with an electron in another atom to complete an octet and form a bond. Figure 4-1 shows the Lewis structures of some free radicals. Radicals are often represented by a structure with a single dot representing the unpaired odd electron. 

Fig. 6.1 Lewis structures of auxiliary oxidants and of reactive oxygen species.

The Lewis structure concept may superficially appear to lose its usefulness for open-shell species. The electrons of radical or excited-state species cannot be strictly paired as in a conventional closed-shell Lewis structure diagram. Moreover, the concept of structure itself seems to lose its validity in many radical species, which tend to be characterized by floppiness, large-amplitude vibrations, and general lack of structural rigidity compared with closed-shell species. The reactivity and instability of radical species might seem to preclude a useful role for the Lewis structural concepts. 

The unstable and highly reactive OH molecule is a free radical, a substance with one or more unpaired electrons, as seen in the Lewis structure -O — H. The OH molecule is also called the hydrox) l radical, and the presence of the unpaired electron is often emphasized by writing the species with a single dot, OH. In cells and tissues, hydroxyl radicals can attack biomolecules to produce new free radicals, which in turn attack yet other biomolecules. Thus, the formation of a single hydroxyl radical via Equation 2131 can initiate a large number of chemical reactions that are ultimately able to disrupt the normal operations of cells. 

The molecule benzyne (CgH4) is a very reactive species. It resembles benzene in that it has a six-membered ring of carbon atoms. Draw a Lewis structure of the molecule and account for the molecule s high reactivity. 

Experimental details are provided for key reactions to introduce the reaction. The point is to show that the experiment shows a result and then we try to understand that result in the context of the structural featrues of the reactive species. Mechanisms are given as part of the discussion of key reactions. The mechanism is discussed in most cases first as a walk-through of the reaction to understand how the transformation occurred, and then structures for the walkthrough are provided that constitute the mechanism. Many of these mechanistic steps will involve acid-base reactions—both Brpnsted-Lowry and Lewis. 

An ylide is a neutral species whose Lewis structure contains opposite charges on adjacent atoms. The atoms involved are carbon and an element from either group 15 (VA) or 16 (VIA) of the periodic table, such as N, P, or S. The Wittig reaction uses phosphorus ylides, which are obtained by deprotonation of a phosphonium salt with a strong base. Phosphorus ylides are relatively stable, but reactive species, for which the following resonance structures may be written the phosphorus atom can exceed an octet by accommodating electron donation into its 3d orbitals. 

These exceptions include odd-electron species, which necessarily have Lewis structures with only seven electrons around an atom. Such molecules, called free radicals, tend to be unstable and chemically reactive. 

Students sometimes assume (mistakenly) that chemical bonding is completed once the electrons are maximally paired up in a closed-shell species of valid Lewis structural form. The error of this assumption was recognized nearly a century ago with discovery of numerous complexes that defied Lewis structural formulation, unless written as two (or more) distinct species. Such complexes therefore appear to violate the valence rules that usually govern chemical stmcture and reactivity, apparently involving some type of extra-valence Nebenvalenz, in the phrase of German inorganic chemist Alfred Werner) that demands significant extension of Lewis structural concepts. Nowadays, the term hypervalency is commonly used to describe species that have too many bonds for conventional Lewis structural depiction, or seem to require chemical association mechanisms beyond those of closed-shell Lewis structure formation. 

Despite their central role in chemical reaction theory, TS species challenge conventional structural characterization by experimental means. Modern ab initio methods therefore provide a uniquely valuable source of detailed TS information that can significantly advance understanding of chemical reactivity. Given the fact that accurate TS wavefunctions and IRC profiles are now routinely available for a variety of chemical reactions, our aim is to extend NBO/NRT-based analysis techniques to characterize TS and other IRC species in simple Lewis structural and resonance theoretic terms, analogous to those found useful for equilibrium species. 

Exceptions to the Octet Rule—There are often exceptions to the octet rule. (1) Odd-electron species, such as NO, have an unpaired electron and are paramagnetic. Many of these species are reactive molecular fragments, such as OH, called free radicals. (2) A few molecules have incomplete octets in their Lewis structures, that is, not enough electrons to provide an octet for every atom. (3) Expanded valence shells occur in some compoimds of nonmetals of the third period and beyond. In these, the valence shell of the central atom must be expanded to 10 or 12 electrons in order to write a Lewis structure. 

Raman spectroscopy has provided information on catalytically active transition metal oxide species (e. g. V, Nb, Cr, Mo, W, and Re) present on the surface of different oxide supports (e.g. alumina, titania, zirconia, niobia, and silica). The structures of the surface metal oxide species were reflected in the terminal M=0 and bridging M-O-M vibrations. The location of the surface metal oxide species on the oxide supports was determined by monitoring the specific surface hydroxyls of the support that were being titrated. The surface coverage of the metal oxide species on the oxide supports could be quantitatively obtained, because at monolayer coverage all the reactive surface hydroxyls were titrated and additional metal oxide resulted in the formation of crystalline metal oxide particles. The nature of surface Lewis and Bronsted acid sites in supported metal oxide catalysts has been determined by adsorbing probe mole-... 

---