Tt bonding

Formation of a Tr-allylpalladium complex 29 takes place by the oxidative addition of allylic compounds, typically allylic esters, to Pd(0). The rr-allylpal-ladium complex is a resonance form of ir-allylpalladium and a coordinated tt-bond. TT-Allylpalladium complex formation involves inversion of stereochemistry, and the attack of the soft carbon nucleophile on the 7r-allylpalladium complex is also inversion, resulting in overall retention of the stereochemistry. On the other hand, the attack of hard carbon nucleophiles is retention, and hence Overall inversion takes place by the reaction of the hard carbon nucleophiles. 

Summarizing, the introduction of nitrogen at the place of C-3 in thiophene does not deeply disturb the electronic environment of the sulfur atom, but it induces in the rest of the molecule some alternating modification of the electronic density (Figs. 1-3 and 1-4). The perturbations induced by the nitrogen in the tt bond order of thiophene are... 

TABLE 1-12. SUBSTITUTION EFFECTS OUALITATIVE VARIATIONS OF tt-BOND ORDERS INDUCED BY A METHYL SUBSTITUTION ... 

TT-bonding (Scheme 84) (463). A series of simple thiazole complexes has... 

The higher reactivity of 2-halogenothiazoles with respect to halogenopyridines can be related to the different aromaticity of the two systems, less for thiazole than for pyridine, for example, the relatively stronger fixation of the tt bond in the thiazole than in the case of pyridine. As the data reported in Table V-1 (footnote a) indicates, the free thiophenol is more reactive than the thiolate anion toward the 2-halogenothiazoles. This fact should be considered when one prepares the thiazolyl sulfides. 

Identify the orbital overlaps involved in the indicated bond in the compound shown (propene) Is this a tt bond or a ct bond ... 

FIGURE 2 20 Bonding in acetylene based on sp hybridization of carbon The carbon-carbon triple bond is viewed as consisting of one cr bond and two tt bonds... 

The TT bond in ethylene generated by overlap of p orbitals of adjacent carbons... 

The triple bond of acetylene has a cr bond component and two tt bonds the two TT bonds are shown here and are perpendicular to each other... 

Does the overlap of two p orbitals in the fashion shown correspond to a a bond or to a tt bond" Explain... 

FIGURE 5 1 (a) The planar framework of u bonds in ethylene showing bond distances and angles (b) and (c) The p orbitals of two sp hybridized carbons overlap to produce a tt bond (d) The electrostatic potential map shows a region of high negative potential due to the tt elec trons above and below the plane of the atoms... 

The simplest arithmetic ap proach subtracts the C—C (j bond energy of ethane (368 kj/mol 88 kcal/mol) from the C=C bond energy of ethylene (605 kJ/mol 144 5 kcal/mol) This gives a value of 237 kJ/mol (56 5 kcal/mol) for the tt bond energy... 

FIGURE 9 2 The carbon atoms of acetylene are con nected by a cr + tt + tt triple bond (a) Both carbon atoms are sp hybridized and each IS bonded to a hydrogen by a (T bond The two tt bonds are perpendicular to each other and are shown sepa rately in (b) and (c)... 

FIGURE 9 3 Electro static potential maps of eth yiene and acetylene The region of highest negative charge (red) is associated with the TT bonds and lies between the two carbons in both This electron rich re gion IS above and below the plane of the molecule in ethylene Because acetylene has two TT bonds a band of high electron density encir cles the molecule... 

FIGURE 10 2 Electron delo calization in an allylic carbo cation (a) The tt orbital of the double bond and the vacant 2p orbital of the posi tively charged carbon (b) Overlap of the tt orbital and the 2p orbital gives an ex tended tt orbital that encom passes all three carbons The two electrons in the tt bond are delocalized over two car bons in part (a) and over three carbons in part (b)... 

Propadiene (H2C=C=CH2) also called allene, is the simplest cumulated diene The two tt bonds m an allene share an sp hybridized carbon and are at right angles to each other Certain allenes such as 2 3 pentadiene (CH3CH=C=CHCH3) possess a chirality axis and are chiral... 

In addition to its three sp hybrid orbitals each carbon has a half filled 2p orbital that can participate m tt bonding Figure >b shows the continuous rr system that encompasses all of the carbons that result from overlap of these 2p orbitals The six tt electrons of benzene are delocalized over all six carbons... 

Section 14 15 Coordination polymerization of ethylene and propene has the biggest eco nomic impact of any organic chemical process Ziegler-Natta polymer ization IS carried out using catalysts derived from transition metals such as titanium and zirconium tt Bonded and ct bonded organometallic com pounds are intermediates m coordination polymerization... 

Because the nng prevents linearity of the C—C=C—C unit and tt bonding m that unit IS weak benzyne is strained and highly reactive This enhanced reactivity is... 

The C—C single bond in vmylacetylene is a a bond generated by overlap of an sp hybridized orbital on one carbon with an sp hybndized orbital on the other Vmylacetylene has three u bonds and three tt bonds... 

TT bond (Section 2 20) In alkenes a bond formed by overlap of p orbitals in a side by side manner A tt bond is weaker than a u bond The carbon-carbon double bond in alkenes con sists of two sp hybridized carbons joined by a a bond and a TT bond... 

Steric Factors. Initially, most of the coUisions of fluorine molecules with saturated or aromatic hydrocarbons occur at a hydrogen site or at a TT-bond (unsaturated) site. When coUision occurs at the TT-bond, the double bond disappears but the single bond remains because the energy released in initiation (eq. 4) is insufficient to fracture the carbon—carbon single bond. Once carbon—fluorine bonds have begun to form on the carbon skeleton of either an unsaturated or alkane system, the carbon skeleton is somewhat stericaUy protected by the sheath of fluorine atoms. Figure 2, which shows the crowded hehcal arrangement of fluorine around the carbon backbone of polytetrafluoroethylene (PTFE), is an example of an extreme case of steric protection of carbon—carbon bonds (29). 

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

Silver ions form a number of complexes with both TT-bonding and non-TT-bonding ligands. Linear polynuclear complexes are known. The usual species are AgL and AgL2, but silver complexes up to AgL have been identified. Many of these complexes have commercial appHcation. 

Some simple zirconium organometaUic compounds, such as tetramethylzirconium [6727-89-5] are known. In general, these compounds are very unstable. It appears that zirconium must be TT-bonded to at least one moderately large ligand, such as a cyclopentadienyl group, for the compound to be stable. The abbreviation Cp is used here for the cyclopentadienyl group and Cp for [0 (0112)5]. 

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