Strengths of Multiple Bonds

Bond type Bond length Bond strength  

Single in C2H6 Double in CjHj Triple in C2H4 154 pm 134 pm 120 pm 

The preceding examples of bond type, length, and strength show that as the bond multiplicity increases, the bonds become shorter. Furthermore, the bond strength, expressed as the energy in kilojoules needed to break a mole of the bonds, increases. These trends are reasonable if a covalent bond is viewed as consisting of 

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The experimental values of AH0 for hydrogenation of a number of alkenes and alkynes are listed in Table 11-2. The AH0 calculated from average bond energies is —30 kcal mole-1 for a double bond and —69 kcal mole-1 for a triple bond. The divergences from these values reflect the influence of structure on the strengths of multiple bonds. Some important generalizations can be made ... 

The empirical finding that increased strength of multiple bonds over electron-pair bonds is mainly caused by an increase in single-bond strength at the closer interatomic approach that becomes possible due to screening of inter-nuclear repulsion, can now be examined more closely. Whenever the number of valence-electron pairs on an atom exceeds the number of electron-pair bonds to that atom, the excess density may screen the nucleus. Screening becomes effective when the excess density occurs in atomic s-type states with an appreciable s contribution. The first-period diatomics considered before illustrate this screening condition well. 

Heats of formation can be derived by reversing the Pauling definition of electronegativity, but empirical corrections are needed to allow for extra strength of multiple oxygen or nitrogen bonding, as in... 

Another consequence of Equation 9-3 is that if m1 and in, remain the same, the larger the value of k, the higher will be the vibrational frequency. Because k is expected to run more or less parallel to the bond strength, and because multiple bonds are stronger than single bonds, the absorption frequencies of multiple bonds are higher than for single bonds. Examples are the absorption of C=C at 2100 cm-1, C=C at 1650 cm-1, and C—C at 1000 cm-1. 

Each of the syntheses of seychellene summarized in Scheme 20 illustrates one of the two important methods for generating vinyl radicals. In the more common method, the cyclization of vinyl bromide (34) provides tricycle (35).93 Because of the strength of sjp- bonds to carbon, the only generally useful precursors of vinyl radicals in this standard tin hydride approach are bromides and iodides. Most vinyl radicals invert rapidly, and therefore the stereochemistry of the radical precursor is not important. The second method, illustrated by the conversion of (36) to (37),94 generates vinyl radicals by the addition of the tin radical to an alkyne.95-98 The overall transformation is a hydrostannylation, but a radical cyclization occurs between the addition of the stannyl radical and the hydrogen transfer. Concentration may be important in these reactions because direct hydrostannylation of die alkyne can compete with cyclization. Stork has demonstrated that the reversibility of the stannyl radical addition step confers great power on this method.93 For example, in the conversion of (38) to (39), the stannyl radical probably adds reversibly to all of the multiple bond sites. However, the radicals that are produced by additions to the alkene, or to the internal carbon of the alkyne, have no favorable cyclization pathways. Thus, all the product (39) derives from addition to the terminal alkyne carbon. Even when cyclic products might be derived from addition to the alkene, followed by cyclization to the alkyne, they often are not found because 0-stannyl alkyl radicals revert to alkenes so rapidly that they do not close. 

Density functional calculations (X) on the strengths of multiple metal-metal bonds in binuclear complexes such as M2Cl4(PR3)4 and M2X6, where the bonding primarily involves nd-orbitals rather than (n+l)s orbitals,reveal (XX) on the other hand only modest contributions from relativistic corrections (6-10 Kcal mol ) for the 5d homologues.The calculated order of stability for the M-M... 

The additivity of electron density at the HBCPs in multiple H-bonded (36 HBCPs) systems is clearly evident from Fig. 4. The correlation coefficient for the same is 0.99. Although, the connection between p(rc) and strength of H-bonded complex with single H-bond is already known, we have demonstrated the additivity of electron density at the HBCPs for the multiple H-bonded systems [244, 245]. 

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