Butadiene charge distribution

MO calculations at the 6-3IG level have been done on both acrolein and aminoethylene. The resulting MOs were used to calculate charge distributions. Figure 1.26 gives the 7t-electron densities calculated for butadiene, acrolein, and aminoethylene. Inclusion of the hydrogen and a orbitals leads to overall charges as shown. These charge distributions result from a polarization which is counter to the n polarization. 

Neutral dienes have been reacted with a large variety of ions in the gas phase. Besides the cases concerning the same reactants discussed above but with reversed charge distribution, e.g. those of neutral 1,3-butadiene with ionized alkenes, there are interesting studies of reactions of 1,3-dienes with even-electron cations and studies on ion/molecule... 

Charge distribution on butadiene one unit model and % 1,2 structure in polybutadiene. 19... 

Bywater and coworkers (5,6) employed 1JC nmr to identify the the shift positions and to estimate the charge distribution over the various carbon positions in several organoalkali metal model compounds of butadiene and isoprene. They concluded that all of the compounds are delocalized ionic compounds in the solvents, and several ethers. 

Formation of both 1,2- and 1,4-addition products occurs not only with halogens, but also with other electrophiles such as the hydrogen halides. The mechanistic course of the reaction of 1,3-butadiene with hydrogen chloride is shown in Equation 13-1. The first step, as with alkenes (Section 10-3A), is formation of a carbocation. However, with 1,3-butadiene, if the proton is added to C1 (but not C2), the resulting cation has a substantial delocalization energy, with the charge distributed over two carbons (review Sections 6-5 and... 

TABLE 5. MINDO/3 calculated charge distribution for 1-dimethylamino- and 2-dimethylamino-1,3-butadienes"... 

With its concerted mechanism implying little charge distribution change along the pathway, the Diels-Alder reaction has been understood to have little rate dependence on solvent choice. For example, the relative rate of cyclopentadiene dimerization increases only by a factor of 3 when carried out in ethanol. The relative rate for the Diels-Alder reaction of isoprene with maleic anhydride (Table 7.1) varies by only a factor of 13 with solvents whose dielectric constants vary by almost a factor of 10, but the rate acceleration is not a simple function of the solvent polarity. Furthermore, the dimerizations of cyclopentadiene and 1,3-butadiene proceed at essentially identical rates in the gas and solution phases. ... 

Proceeding as we did for the allyl radical, it is easily seen that the electron charge distribution is uniform (one n electron onto each carbon atom, alternant hydrocarbon) and the spin density is zero, as expected for a state with S = Ms = 0 since the two bonding MOs are fully occupied by electrons with opposite spin. The delocalization (or conjugation) energy for linear butadiene is ... 

Charge distributions and bond orders of the cation and anion of 2//-benz[crf]azulene, on the other hand, are dissimilar. The seven-membered ring of the cation is delocalized, containing most of the positive charge, while that of the anion 87 is localized and the negative charge resides mainly in the delocalized indenyl moiety. A delocalized indolizine part with a localized butadiene bridge in the isoelectronic cycl-... 

The magnetic symmetry of 1,3-butadiene [5C 1,4) 116.6 ppm 5C 2,3) 137.2 ppm] is broken by the introduction of an amino function at C(l) or C 2). The MINDO/3 calculated charge distribution for 1-dimethylamino- and 2-dimethyl amino-1,3-butadiene in the j-cis and -fran conformation appears in Table 5. The alternation of charge, typical of the enamine system, is maintained when the conjugation is extended, as is 97, the odd-numbered carbons bearing positive charge, and the nitrogen and even-numbered... 

Fig. 2-8). We note from Fig. (2-8) that, in the ground state, v, = v2 = 2 and vj = v4 = 0 hence, ail we require from equations (2-67) are cu, r = 1, 2,..., 4 and c2r, r = 1, 2,..., 4. The application of equation (4-4) to these data is then summarised in Table 4-1. Thus we find that the n-electron charge-densities on all four carbon atoms of butadiene are unity. This is by no means fortuitous and is always the case for a certain class of molecules (called alternant hydrocarbons and dealt with in Chapter Six) to which butadiene belongs. The charge distributions in excited-state species will also be discussed in detail in the context of the Coulson-Rushbrooke Pairing-Theorem in 6.5. 

In the diagram above there are two identical structures having opposite charge distributions and there is no net separation of charge. The importance of resonance structures to the composite structure increases with the stability of the individual structures, so structures B and C are less important than A, as they have separation of charge and only one rather than two tt bonds. By applying resonance criteria 3a and 3b, we conclude that these two structures contribute less stabilization to butadiene than the two equivalent benzene resonance structures. Therefore, we expect the enhancement of electron density between C(2) and C(3) to be small. 

Fig. 1.21. Charge distribution in butadiene, acrolein, and aminoethylene based on HF/6-31G calculations. From J. Org. Chem., 59, 4506 (1994).

Of course, we cannot expect the description to reflect all the details of the charge distribution in the butadiene molecule, but one may expect this approach to be able to reflect at least some rough features of the yr electron distribution. If the results of more advanced calculations contradicted the rough particle-in-box results, then we should take a closer look at them and... 

From the 3 values we can argue that butadiene could well be more reactive to neutral nonpolar reagents, such as free radicals at the 1 and 4 carbons, than at the 2 and 3 carbons. Neutral nonpolar reagents are specified here so as to avoid commitments that might have to be modified later by consideration of charge distribution effects. 

Fig. 1.26. Charge distribution in butadiene, acrolein, and aminoethy-lene. Data are from K. B. Wiberg, R. E. Rotlienberg, and P. R. Rablen,...

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