Attached to a double bond

Table 7.44 Estimation of Chemical Shift of Proton Attached to a Double Bond 7.95...

Table 7.52 Estimation of Chemical Shifts of Carbon Attached to a Double Bond 7.103...

Rule 1 Considering the double-bond carbons separately, look at the two atoms directly attached to each and rank them according to atomic number. An atom with higher atomic number receives higher priority than an atom with lower atomic number. Thus, the atoms commonly found attached to a double bond are assigned the following order. Note that when different isotopes of the same element are compared, such as deuterium (2H) and pro tin m ( H), the heavier isotope receives priority over the lighter isotope. 

The twofold dehydrohalogenation takes place through a vinylic halide intermediate, which suggests that vinylic halides themselves should give alkynes when treated with strong base. (Recall A vinylic substituent is one that is attached to a double-bond carbon.) This is indeed the case. For example ... 

The ability of C—C to serve as a neighboring group can depend on its electron density. When the strongly electron-withdrawing CF3 group was attached to a double bond carbon of 37, the solvolysis rate was lowered by a... 

Several other types of photochemical reactions involving unsaturated carbohydrates have been reported. One of these is38 photochemical, E -Z isomerization of the groups attached to a double bond (see Scheme 5). A second is the internal cycloaddition between two double bonds connected by a carbohydrate chain.39-41 Although the carbohydrate portion of the molecule is not directly involved in this cycloaddition, its presence induces optical activity in the cyclobutane derivatives produced photochemically. Finally, a group of acid-catalyzed addition-reactions has been observed for which the catalyst appears to arise from photochemical decomposition of a noncarbohydrate reactant.42-44... 

Carbon substituents directly attached to a double bond experience pronounced shielding y-effects in the Z configuration (see Section 4.1.1.1.), as shown for (E)- and (Z)-3-hexene. In the Z-isomer the a-carbon experiences a typical y-syn effect from the other a-carbon, whereas the corresponding y-anti effect in the If-isomer is negligible. 

The principles described above may serve as a useful guide for unsaturated hydrocarbons, but substituents other than alkyl groups may involve attractive interactions with a catalytic site. A compound in which a phenyl group is attached to a double bond in a five- or six-membered cycloalkene and is also vicinal to an alkyl substituent yields a larger fraction of the cis stereoisomer than is formed from the related dialkylcycloalkene. Cis isomers are formed preferentially on Pd and Rh, and are believed to involve structures in which the phenyl group is bound to the surface through its ir-electrons. ... 

Olefin metathesis is the catalytic exchange of groups attached to a double bond. It presents a number of interesting possibilities for modifying the alkyl chain of fatty acids (Figure 17). 

The right side of Figure 12.13 shows what happens if a good pi donor is attached to a double bond. The pi donor HOMO interacts with both the double-bond HOMO and LUMO to produce a new ally lie system with a new HOMO and LUMO, which are raised... 

The left side of Figure 12.13 shows what happens if a good pi acceptor is attached to a double bond. The LUMO of the pi acceptor interacts with both the double bond HOMO and LUMO to produce a new ally lie system with a new HOMO and LUMO, which are lowered from those of the double bond. A pi acceptor bonded to a double bond makes the combination more electrophilic by lowering the LUMO of the system. When the energy of the LUMO of the electrophile is lower, then it is softer, and the stabilization resulting from overlap with the average nucleophile s HOMO is much greater (recall Fig. 2.14). 

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