Conjugated 7T bonds

Protonation occurs on the nitrogen atom having the sp2 lone pair because both lone pairs contribute and the resulting delocalized cation is still aromatic. Pyridine is also protonated on its sp2 lone pair (it is the only one it has ) and the pyridinium ion is also obviously aromatic—it still has three conjugated 7t bonds in the ring. 

The anisotropy of fhe purine nucleobases is larger than that of fhe pyrimidines, which is attributable to the larger physical extension in the direction of the molecular plane. The mean excitation energy of fhe nucleobases is also more anisotropic than in the case of fhe amino acids, which is caused by the fact that the nucleobases include planar heterocycles with conjugated 7T-bonds. 

We have seen that the stereochemistry of an electrocyclic reaction depends on the mode of ring closure, and the mode of ring closure depends on the number of conjugated 7T bonds in the reactant and on whether the reaction is carried out under thermal or photochemical conditions. What we have learned about electrocyclic reactions can be summarized by the selection rules listed in Table 29.1. These are also known as the Woodward-Hoffmann rules for electrocyclic reactions. 

Cyanodecapentayne has been identified in intergalactic space by radio astronomers. How many conjugated 7t bonds are in this compound ... 

The best way to understand how orbital symmetry affects pericyclic reactions is to look at some examples. Let s look first at a group of polyene rearrangements called electrocyclic reactions. An electrocyclic reaction is a pericyclic process that involves the cycli/ation of a conjugated polyene. One 7r bond is broken, the other 7t bonds change position, a new cr bond is formed, and a cyclic compound results. For example, a conjugated triene can be converted into a cyclohexa-diene, and a conjugated diene can be converted into a cyclobutene. 

The first class was found when the olefin substituents are highly conjugated with the double bond, the second class when there is no resonance between the substituents and the 7t-bond and the last class when the substituents are moderately conjugated with the double bond. 

The optical properties of conjugated polymers primarily arise from the 7t-bonds in the polymer backbone. Alignment of 7i-bonds leads to electron delocalization, also referred to as conjugation or -conjugation. Theoretically, backbone 7i-electrons can be delocalized along the entire length of the polymer if the 7i-bonds are all aligned however, twists in the backbone and defects in the polymer structure at... 

As conjugated systems with alternating 7T-charges, the polymethine dyes are comparatively highly reactive compounds (3). Substitution rather than addition occurs to the equalized 7T-bond. If the nucleophilic and electrophilic reactions are charge-controlled, reactants can attack regiospecifically. 

Dewar and coworkers39 have pointed out that an adequate description of polyenes has to be based on a set of reference molecules and reference bonds that comprises not only a C(sp3)—C(sp3) bond, but also C(sp2)—C(sp3), C(sp2)—C(sp2) reference bonds in order to separate normal cases of bond conjugation from exceptional cases which show bond delocalization. When this extended set of reference bonds is used, polyenes as well as radialenes and many cyclopolyenes are described as normal conjugated 7t-systems without any significant extra stabilization from bond delocalization39. 

Examine the lowest-unoccupied molecular orbital (LUMO) for the most stable conjugate acid of each compound (include protonated acetonitrile). Which atom makes the largest contribution to this orbital Is this the site of H20 attack Will adding electrons to the LUMO strengthen or weaken the C=0 (C=N) 7t bond Explain. 

UV region. This absorption is due to the long conjugated double bond system of carotenoids. The conjugated unsaturated part of the carotenoid molecule containing delocalized 7t-elec-trons is called the chromophore and is re-... 

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