No-bond resonance forms

Occasionally no-bond resonance forms are needed to describe the electron distribution in a molecule, and are usually associated with very acidic hydrogens. The polarized bond in hydrochloric acid can be represented by using resonance structures to show the partial ionic nature of the bond. The atoms do not move, only the electrons. 

As a final example of a hypervalent species, let us consider SiFg , whose Lewis structure is shown in Figure 1.6. As shown, silicon uses all four of its valence electrons in bonding in SiFs " and is therefore tetravalent. Indeed, the species may be represented by a no-bond resonance form consisting of SiF4 and two closed-shell F anions the latter do not contribute to the valence of the silicon. This is a general point coordination of closed-shell Lewis bases has no effect on the valence of an atom. For SiFg , we can also determine the valence of silicon more mechanically ... 

Polyacetylene retains multiple bonds along the chain. Electrons can be conducted through the series of tt-orbitals. A resonance form of the Lewis structure can be drawn, showing that the electrons may be delocalized along the polyacetylene chain. No such resonance form is possible for polyethylene. 

Quinone methides are strikingly different from the 1,2- and 1,4-isomers, because there is no direct orbital interaction between the meta-oxygen and carbon substituents at the benzene ring. Consequently, the neutral valence bond resonance form for the 1,3-quinone methide is a triplet biradical (Scheme 1). These 1,3-quinone methides are chemically more unstable and difficult to generate than their 1,2- and 1,4-isomers, which exist as stable neutral molecules.8... 

Finally some compounds related to the thiathiophthene no-bond resonance system may be regarded as containing contributions from isothiazolium forms (8 <-> 9).146... 

Figure 2.1 Preferred conformations of butane and 1,2-difluoroethane, illustrating the gauche effect. In the gauche conformation each C-F a orbital can overlap with a C-H a orbital on the vicinal carbon, corresponding to no-bond resonance as shown. Such no-bond resonance would be disfavoured if the C-F bonds were trans, since it would remove electron density from an electronegative element. The effect is strong enough in this case to overcome the electrostatic repulsion between the two C-F dipoles, which favours the trans form.

The no-bond resonance model of the anomeric effect predicts that, in a series of axial aryloxytetrahydropyran derivatives, the intracyclic bond should shorten and the extracyclic bond should lengthen as the parent phenol becomes more acidic and the ion-paired canonical form any effect should be much smaller in the equatorial case. Careful X-ray crystallographic studies of a series of tetrahydropyranyl ethers (structures in Figure 2.15) indeed showed that for seven such axial compounds, with the p a of ROH spanning 8 units, the lengths (A) of the extracyclic bond (x) and the intracyclic bond n) were given by eqns (2.2) and (2.3), respectively ... 

Scheme 6.94. Resonance forms for methylbenzene (toluene) suggesting that ortho- and para-positions may be particularly attractive for an incoming electrophile. The drawing is intended to illustrate the principle called no-bond-resonance or the theory of hyperconjngation. It is suggested that the o-bonding C-H electrons participate in resonance. It is important to recongize that the nuclei do not move.

This approach does not, however, account for a special type of heteroatoms that possesses expanded valence electron shells, i.e., those in their hypervalent state. A long-known and amply discussed example of the heteroaromatic compounds containing such a heteroatom is given by l,6,6aA -trithiapental-ene and related compounds 1. The structures of the parent trithiapentalene 1 (X = Y = S, A = B = CH, Z = C) and its symmetrically substituted derivatives and analogues have symmetry. Therefore, they cannot be represented by a single Lewis structure and have to be treated as either no bond resonance la la or resonance of the two forms lb lb containing three-center, four-electron Y—X—Y bonds. 

Simple resonance theory predicts that pentalene (48), azulene (49), and heptalene (50) should be aromatic, although no nonionic canonical form can have a double bond at the ring junction. Molecular orbital calculations show that azulene should be stable but not the other two, and this is borne out by experiment. Heptalene has been prepared but reacts readily with oxygen, acids, and bromine, is easily hydrogenated, and polymerizes on standing. Analysis of its NMR spectrum shows that it is... 

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