Resonance Theory Revisited

Allyl bromide (3-bromo-l-propene) forms a carbocation readily. For example, it undergoes SnI reactions. Explain this observation. 

STRATEGY AND ANSWER Ionization of allyl bromide (at right) produces an aUyl cation that is unusually stable (far more stable than a simple primary carbocation) because it is resonance stabilized. 

Resonance structures exist only on paper. Although they have no real existence of their own, resonance structures are useful because they allow us to describe molecules, radicals, and ions for which a single Lewis structure is inadequate. Instead, we write two or more Lewis structures, calling them resonance structures or resonance contributors. We connect these structures by double-headed arrows ( — ), and we say that the hybrid of all of them represents the real molecule, radical, or ion. 

In writing resonance structures, we are only allowed to move electrons. The positions of the nuclei of the atoms must remain the same in all of the structures. Structure 3 is not a resonance structure for the allylic cation, for example, because in order to form it we would have to move a hydrogen atom and this is not permitted  

Resonance is an important tool we use frequently when discussing structure and reactivity. 

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G.J. Daniels, R.D. Jenkins, D.S. Bradshaw, D.L. Andrews, Resonance Energy Transfer The Unified Theory Revisited. J. Chem. Phys. 119 (2003) 2264. 

The UV resonance Raman spectrum of thymine was revisited in 2007, with a slightly different approach, by Yarasi, et al. [119]. Here, the absolute UV resonance Raman cross-sections of thymine were measured and the time-dependent theory was used to experimentally determine the excited-state structural dynamics of thymine. The results indicated that the initial excited-state structural dynamics of thymine occurred along vibrational modes that are coincident with those expected from the observed photochemistry. The similarity in a DFT calculation of the photodimer transition state structure [29] with that predicted from the UV resonance Raman cross-sections demonstrates that combining experimental and computational techniques can be a powerful approach in elucidating the total excited-state dynamics, electronic and vibrational, of complex systems. 

P. Bering, Structure and vibrations of the phenol-ammonia cluster, J. Chem. Phys. 102, 9197-9204 (1995). (c) S. Tanabe, T. Ebata, M. Fujii, and N. Mikami, OH stretching vibrations of phenol-(H20) (n = 1-3) complexes observed by IR-UV double-resonance spectroscopy, Chem. Phys. Lett. 215, 347-352 (1993). (d) D. Michalska, W. Zierkiewicz, D. C. Bien ko, W. Wojciechowski, and T. Zeegers-Huyskens, Troublesome vibrations of aromatic molecules in second-order Moller-Plesset and density functional theory calculations infrared spectra of phenol and phenol-OD revisited, J. Phys. Chem. A 105, 8734-8739 (2001). 

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