Tt orbitals

We can consider the hydroboration step as though it involved borane (BH3) It sim phfies our mechanistic analysis and is at variance with reality only m matters of detail Borane is electrophilic it has a vacant 2p orbital and can accept a pair of electrons into that orbital The source of this electron pair is the rr bond of an alkene It is believed as shown m Figure 6 10 for the example of the hydroboration of 1 methylcyclopentene that the first step produces an unstable intermediate called a tt complex In this rr com plex boron and the two carbon atoms of the double bond are joined by a three center two electron bond by which we mean that three atoms share two electrons Three center two electron bonds are frequently encountered m boron chemistry The tt complex is formed by a transfer of electron density from the tt orbital of the alkene to the 2p orbital... 

Acetylene is linear and alkynes have a linear geometry of their X—C=C—Y units The carbon-carbon triple bond m alkynes is com posed of a CT and two tt components The triply bonded carbons are sp hybridized The ct component of the triple bond contains two electrons m an orbital generated by the overlap of sp hybndized orbitals on adja cent carbons Each of these carbons also has two 2p orbitals which over lap m parrs so as to give two tt orbitals each of which contains two electrons... 

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)... 

One of molecular orbital theories early successes came m 1931 when Erich Huckel dis covered an interesting pattern m the tt orbital energy levels of benzene cyclobutadiene and cyclooctatetraene By limiting his analysis to monocyclic conjugated polyenes and restricting the structures to planar geometries Huckel found that whether a hydrocarbon of this type was aromatic depended on its number of tt electrons He set forth what we now call Huckel s rule... 

A molecular orbital description of benzene has three tt orbitals that are bonding and three that are antibonding Each of the bonding orbitals is fully occupied (two electrons each) and the antibonding orbitals are vacant... 

UV-VIS Aldehydes and ketones have two absorption bands in the ultraviolet region Both involve excitation of an electron to an antibonding tt orbital In one called a TT TT transition the electron is one of the tt electrons of the C=0 group In the other called an n ir transition it is one of the oxygen lone pair electrons Because the tt electrons are more strongly held than the lone parr electrons the transition is of... 

A common example of the Peieds distortion is the linear polyene, polyacetylene. A simple molecular orbital approach would predict S hybddization at each carbon and metallic behavior as a result of a half-filled delocalized TT-orbital along the chain. Uniform bond lengths would be expected (as in benzene) as a result of the delocalization. However, a Peieds distortion leads to alternating single and double bonds (Fig. 3) and the opening up of a band gap. As a result, undoped polyacetylene is a semiconductor. 

All lation of Phenols. The approach used to synthesize commercially available alkylphenols is Friedel-Crafts alkylation. The specific procedure typically uses an alkene as the alkylating agent and an acid catalyst, generally a sulfonic acid. Alkene and catalyst interact to form a carbocation and counter ion (5) which interacts with phenol to form a 7T complex (6). This complex is held together by the overlap of the filled TT-orbital of the aromatic... 

The bonding between carbon monoxide and transition-metal atoms is particularly important because transition metals, whether deposited on soHd supports or present as discrete complexes, are required as catalysts for the reaction between carbon monoxide and most organic molecules. A metal—carbon ( -bond forms by overlapping of metal orbitals with orbitals on carbon. Multiple-bond character between the metal and carbon occurs through formation of a metal-to-CO TT-bond by overlap of metal-i -TT orbitals with empty antibonding orbitals of carbon monoxide (Fig. 1). 

Other types of bonding include donation by Ligand TT-orbitals, as in the classical Zeiss s salt ion [Pt( 7 -CH2=CH2)Cl3] [12275-00-2] and sandwich compounds such as ferrocene. Another type is the delta (5) bond, as in the Re2Clg ion, which consists of two ReCl squares with the Re—Re bonding and echpsed chlorides. The Re—Re 5 bond makes the system quadmply bonded and holds the chlorides in sterically crowded conditions. Numerous other coordination compounds contain two or more metal atoms having metal—metal bonds (11). 

Charge-Transfer Forces. An electron-rich atom, or orbital, can form a bond with an electron-deficient atom. Typical examples are lone pairs of electrons, eg, in nitrogen atoms regularly found in dyes and protein and polyamide fibers, or TT-orbitals as found in the complex planar dye molecules, forming a bond with an electron-deficient hydrogen or similar atom, eg, —0 . These forces play a significant role in dye attraction. 

The UV absorption of pyrimidine occurs in two bands centred at 243 and 298 nm in cyclohexane. The second band is ascribed to the electronic transition from a nitrogen lone pair non-bonding orbital to an empty ring tt-orbital, in short an n transition, on... 

Anti-bonding tt orbital, 293 Atomic orbital resonance, 295 16/3-Azido- 17a-iodoandrostanes, 24 16p-Azido-17a-iodo-5a-androstan-3 -ol acetate, 28... 

The mechanism of the Diels-Alder reaction is best understood on the basis of a molecular orbital approach. To understand this approach we need to take a more detailed look at the tt orbitals of alkenes and dienes. 

As is tr-ue for all orbitals, a tt orbital may contain a maximum of two electrons. Ethylene has two tt electrons, and these occupy the bonding tt molecular- orbital, which is the HOMO. The antibonding tt molecular orbital is vacant, and is the LUMO. 

The picture of benzene as a planar- framework of a bonds with six electrons in a delocalized TT orbital is a useful, but superficial, one. Six electrons cannot simultaneously occupy any one orbital, be it an atomic orbital or a molecular- orbital. We can fix this with the more accurate molecular- orbital picture shown in Figure 11.4. We learned in Section 2.4 that when atomic orbitals (AOs) combine to give molecular- orbitals (MOs), the final number of MOs must equal the original number of AOs. Thus, the six 2p AOs of six 5/r -hybridized carbons combine to give six tt MOs of benzene. 

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