Bond distances 1,3-butadiene

At 146 pm the C 2—C 3 distance m 1 3 butadiene is relatively short for a carbon-carbon single bond This is most reasonably seen as a hybridization effect In ethane both carbons are sp hybridized and are separated by a distance of 153 pm The carbon-carbon single bond m propene unites sp and sp hybridized carbons and is shorter than that of ethane Both C 2 and C 3 are sp hybridized m 1 3 butadiene and a decrease m bond distance between them reflects the tendency of carbon to attract electrons more strongly as its s character increases... 

An important contribution for the endo selectivity in the carho-Diels-Alder reaction is the second-order orbital interaction [1], However, no bonds are formed in the product for this interaction. For the BF3-catalyzed reaction of acrolein with butadiene the overlap population between Cl and C6 is only 0.018 in the NC-transi-tion state [6], which is substantially smaller than the interaction between C3 and O (0.031). It is also notable that the C3-0 bond distance, 2.588 A, is significant shorter than the C1-C6 bond length (2.96 A), of which the latter is the one formed experimentally. The NC-transition-state structure can also lead to formation of vinyldihydropyran, i.e. a hetero-Diels-Alder reaction has proceeded. The potential energy surface at the NC-transition-state structure is extremely flat and structure NCA (Fig. 8.6) lies on the surface-separating reactants from product [6]. 

Complexes 17-19 can be written in one valence structure as a, /3-unsaturated carbonyl compounds in which the carbonyl oxygen atom is coordinated to a BF2(OR) Lewis acid. The C=C double bonds of such organic systems are activated toward certain reactions, like Diels-Alder additions, and complexes 17-19 show similar chemistry. Complexes 17 and 18 undergo Diels-Alder additions with isoprene, 2,3-dimethyl-1,3-butadiene, tram-2-methyl-l,3-pentadiene, and cyclopentadiene to give Diels-Alder products 20-23 as shown in Scheme 1 for complex 17 (32). Compounds 20-23 are prepared in crude product yields of 75-98% and are isolated as analytically pure solids in yields of 16-66%. The X-ray structure of the isoprene product 20 has been determined and the ORTEP diagram (shown in Fig. 3) reveals the regiochemistry of the Diels-Alder addition. The C-14=C-15 double bond distance is 1.327(4) A, and the... 

The amount of high precision experimental structural data on conjugated polyenes is limited. Some structure results are presented in Table 5. In gas electron diffraction studies it is difficult to determine closely spaced bond distances accurately, because these parameters are highly correlated with the corresponding vibrational amplitudes. Today it is possible to calculate the vibrational amplitudes accurately, if the vibrational frequencies are known. This was, however, not the case when the GED studies presented in Table 5 were carried out. The observed differences between the terminal and central C=C bonds in the GED studies of traw.s-l,3,5-hexatriene and c/s-l,3,5-hexatricne are probably too large29. A very accurate X-ray study of traw.s-l,3,5-hexatriene has, however, been carried out also in connection with the preparation of this chapter4. Figure 4 shows the molecular structures of trans-1,3-butadiene and trans-l,3,5-hexatriene as found in the crystal lattice. 

Dimethylenecyclobutane, with the exocyclic double bonds as depicted in Table 18, should be comparable with butadiene. The double bond distances are virtually the same... 

The shorter C-C single bond distance in butadiene comes about because the two carbon atoms in the bond are sp hybridized. 1 point for correct explanation. 

The epoxidations of propylene and isobutylene with peroxyformic acid proceed in a concerted way via slightly unsymmetrical Markovnikov-type transition stmctnres where the differences in the bond distances between the donble-bond carbons and the spiro oxygen are only 0.021 and 0.044 A at the QCISD/6-31G level. In contrast, the more polarizable natnre of the carbon-carbon double bond of o ,/ -unsaturated systems results in a highly nnsymmetrical transition structure for the epoxidation of 1,3-butadiene with an order-of-magnitnde difference in the carbon-oxygen bond distances of 0.305 A at the QCISD/6-31G level. A highly unsymmetrical transition structure has been also found for the epoxidation of acrylonitrile. 

Comparison of the calculated C—C bond distances (MP2) of 105 [C(1)=C(2) 1.357, C(2)—C(3) 1.439, C(3)=C(4) 1.371, C(l)—C(7) 1.497 A] with trans-butadiene [C(1)=C(2) 1.341,C(2)—C(3) 1.461 A]244 shows that a considerable degree of bond equalization is present in 105. Cremer and Kraka suggest that this is the consequence of homo-conjugative electron delocalization238. This conclusion was confirmed by these authors from the values of the calculated bond orders and -character indices. These results are contrary to predictions obtained at lower, less reliable levels of theory239-243. 

Fig. 3 Optimized geometric features of the transition structure associated with the cyclobutene-1,3-butadiene interconversion (a) and with the second transition structure of the reaction between ketene and methanimine to yield azetidin-2-one. Bond distances and angles are given in A and deg., respectively, co denotes the dihedral angle between the C4, Nl, C2, and C3 atoms...

Fig. 2. IRC of the electrocyclic ring opening of cyclobutene to 1,3-butadiene, a Energy profile, h Evolution of bond distances and angles...

In the carbonyl compounds FC(0)Y the C—F bond distances show a strong dependence on the substituent X and shorten from 136.2(2) pm in acetyl fluoride (Y = Me) to 131.6(1) pm carbonyl fluoride (Y = F). In formyl fluoride (Y = H) and carbonyl chloride fluoride (Y = Cl) the C—F bond lengths are intermediate. When CF2 groups are double-bonded to C, O, S or Se, the C—F bond distances are remarkably constant and range only from 131.5 to 131.9 pm. Such bond lengths, however, are shorter by ca 1.5 pm in compounds with C=N double bonds, such as methanimine or 2,3-diaza-l, 3-butadiene. In both compounds only mean values for the C—F bond distances can be derived from the ED experiments. Ab initio calculations for perfluoromethanimine predict that the C—F bond trans to N—F is longer by 0.4 pm than the cis bond. 

Several chlorinated 1,3-butadienes have been investigated in the gas phase with the primary interest in the influence of chlorination on the conformational properties. In the dichloro derivatives the C—Cl bond distances vary by about 2 pm, depending on the positions of the chlorine atoms. C—Cl bonds which eclipse a geminal C—H bond, such as in 23, are shortest [172.4(2) pm], C—Cl bonds which eclipse the central C—C bond in 24 are intermediate [173.1(3) pm] and C—Cl bonds which eclipse a C=C double bond are longest... 

Figure 10. Schematic illustration of a tendency of each benzene fragment in naphthalene to retain its aromaticity by producing cis 1,3-butadiene partial localization in its twin-ring as described by the resonance structures (7a) and (7b) yielding the resulting predominant canonical structure (7c). This intuitive argument is supported by the (HF/6-31G ) bond distances and the corresponding 7r-bond orders given within parentheses.

The observed bond distances can be explained by looking at hybridization. Each carbon atom in 1,3-butadiene is sp hybridized, so the central C-C single bond is formed by the overlap of two sp hybridized orbitals, rather than the sp hybridized orbitals used to form the C—C bond in CH3CH3. 

The binuclear iron complex (C8H8)Fe2(CO)6 had been expected from the reaction, but the chair conformation (XXXV), which was subsequently found for this substance, was entirely unexpected (65j 66j 67). In this complex, each end of the cyclooctatetraene ligand behaves as a butadiene-type (n = 4) ligand, and bond distance measurements indicate very little tt-tt interaction between the two halves of the ring. The proton NMR spectrum of the complex in solution exhibits two resonances of equal intensity, while the infrared spectrum is very similar to the spectrum of butadiene-iron tricarbonyl and similar diene complexes (105). 

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