Rr-Bonding

The polarizing influence of an electronegative atom decreases with the number of inteiwening rr-bonds. This is called the inductive effect and is indicated in Figure 3-6b by a progression of 6 symbols, (t is generally accepted that the inductive effect is attenuated by a factor of 2-3 by each intervening bond. The inductive ctlcct is not... 

Neighboring group participation (a term introduced by Winstein) with the vacant p-orbital of a carbenium ion center contributes to its stabilization via delocalization, which can involve atoms with unshared electron pairs (w-donors), 7r-electron systems (direct conjugate or allylic stabilization), bent rr-bonds (as in cyclopropylcarbinyl cations), and C-H and C-C [Pg.150]

As portrayed m Figure 2 20 the two carbons of acetylene are connected to each other by a 2sp-2sp cr bond and each is attached to a hydrogen substituent by a 2sp-ls CT bond The unhybndized 2p orbitals on one carbon overlap with their counterparts on the other to form two rr bonds The carbon-carbon triple bond m acetylene is viewed as a multiple bond of the ct + rr + rr type... 

Section 2 20 Carbon is sp hybridized in ethylene and the double bond has a ct com ponent and a rr component The sp hybridization state is derived by mix mg the 2s and two of the three 2p orbitals Three equivalent sp orbitals result and their axes are coplanar Overlap of an sp orbital of one car bon with an sp orbital of another produces a ct bond between them Each carbon still has one unhybridized p orbital available for bonding and side by side overlap of the p orbitals of adjacent carbons gives a rr bond between them... 

The double bond in ethylene is stronger than the C—C single bond in ethane but It IS not twice as strong Chemists do not agree on exactly how to apportion the total C=C bond energy between its ct and rr components but all agree that the rr bond is weaker than the ct bond... 

Although both stereoisomers yield 4 tert butylcyclohexene as the only alkene they do so at quite different rates The cis isomer reacts over 500 times faster than the trans The difference in reaction rate results from different degrees of rr bond develop ment in the E2 transition state Since rr overlap of p orbitals requires their axes to be parallel rr bond formation is best achieved when the four atoms of the H—C—C—X unit he in the same plane at the transition state The two conformations that permit this are termed syn coplanar and anti coplanar... 

Bonding m alkenes is described according to an sp orbital hybridization model The double bond unites two sp hybridized carbon atoms and is made of a ct component and a rr component The ct bond arises by over lap of an sp hybrid orbital on each carbon The rr bond is weaker than the CT bond and results from a side by side overlap of p 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... 

An orbital overlap description of electron delocalization mil dimethylallyl cation H2C=CH—C(CH3)2 is given m Figure 10 2 Figure 10 2a shows the rr bond and the vacant p orbital as independent units Figure 10 2b shows how the units can overlap to give an extended rr orbital that encompasses all three carbons This permits the two rr electrons to be delocalized over three carbons and disperses the positive charge... 

Calculated rr-bond orders are summarized in Table 1. These calculations are supported by chemical evidence that the S—N bond is the one most easily cleaved. Attempts have been made to relate bond orders and electron densities to NMR coupling constants (74CJC833, 77aC6i9> and CNMR (75CJC596, 75CJC1677> and NNMR (78JOC4693> chemical shifts, with limited success. 

Typical coupling constants for isothiazoles are given in Table 5. The electronegative nitrogen atom reduces 3,4 and V3.5 from the values of 3.50 Hz and 0.27 Hz, respectively, in thiophene. The V values correlate quite well with rr-bond orders calculated by MO methods (74CJC833). 

Reactivity and orientation in electrophilic aromatic substitution can also be related to the concept of hardness (see Section 1.2.3). Ionization potential is a major factor in determining hardness and is also intimately related to the process of (x-complex formation when an electrophile interacts with the n HOMO to form a new a bond. In MO terms, hardness is related to the gap between the LUMO and HOMO, t] = (sujmo %omo)/2- Thus, the harder a reactant ring system is, the more difficult it is for an electrophile to complete rr-bond formation. 

Table 23.2 lists the oxidation states of the elements along with representative examples of their compounds. Coordination numbers as high as 12 can be attained, but those over 7 in the case of Cr and 9 in the cases of Mo and W involve the presence of the peroxo ligand or rr-bonded aromatic rings systems such as rj -CsHs or rj -C(,H(,. 

I osmyl complexes (a) 3-cenire rr bond formed hy overlap of ligand p, and nilar tv>nd is produced hy p, and d,. ovcrlapi, and Ihi MO diagram Isee lexij,... 

The mechanism of this reaction is not straightforward but the crucial step appears to be nucleophilic attack by water or OH" on the eoordinated ethene to give rr-bonded —CH2CH2OH which then rearranges and is eventually eliminated as MeCHO with loss of a proton. 

Figure 1.16 The structure of acetylene. The two sp-hybridized carbon atoms are joined by one sp-sp rr bond and two p-p 7t bonds.

You might recall from Section 1.9 that a carbon-carbon triple bond results from the interaction of two sp-hybridized carbon atoms. The two sp hybrid orbitals of carbon lie at an angle of 180° to each other along an axis perpendicular to the axes of the two unhybridized 2py and 2pz orbitals. When two sp-hybridized carbons approach each other, one sp-sp a bond and two p-p -rr bonds are... 

Figure 3-3. A schematic picture of a (irin.v-polyaceiylenc chain, showing the 2p electronic orbitals, which are perpendicular to the plane formed hy the rr-bonds (solid lines).

Explanations of the trans-effect and trans-influence have considered a- and rr-bonding, often to the point of mutual exclusion. 

A carbon-carbon double bond is stronger than one carbon-carbon single bond but weaker than the sum of two single bonds (Section 2.15). A carbon-carbon triple bond is weaker than the sum of three carbon-carbon single bonds. Recall that a single C—C bond is a o-bond, but the additional bonds in a multiple bond are TT-bonds. One reason for the difference in strength is that the side-by-side overlap of p-orbitals that results in a rr-bond is not as great as the end-to-end overlap that results in a o-bond. 

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