Triple bond strength

Understand the relationship between bond type (single, double, and triple), bond strength (or enthalpy), and bond length. (Section 8.8)... 

The length of the carbon-carbon triple bond in acetylene is 120 pm, and the strength is approximately 835 kj/mol (200 kcal/mol), making itthe shortest and strongest known carbon-carbon bond. Measurements show that approximately 318 kj/mol (76 kcal/mol) is needed to break a rbond in acetylene, a value some 50 kj/mol larger than the 268 kj/mol needed to break an alkene n bond. 

Of the eight nonmetals listed in Table 21.1, nitrogen is by far the least reactive. Its inertness is due to the strength of the triple bond holding the N2 molecule together (B.E. N=N = 941 kj/mol). This same factor explains why virtually all chemical explosives are compounds of nitrogen (e.g., nitroglycerin, trinitrotoluene, ammonium nitrate,... 

FIGURE 2.17 The strengths (in kilojoules per mole) of single and multiple bonds between two carbon atoms. Note that, for bonds between carbon atoms, a double bond is less than twice as strong as a single bond and a triple bond is less than three times as strong as a single bond, as shown by the fourth column. 

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. 

In the discussion of metallic radii we may make a choice between two immediate alternative procedures. The first, which I shall adopt, is to consider the dependence of the radius on the type of the bond, defined as the number (which may be fractional) of shared electron pairs involved (corresponding to the single, double, and triple bonds in ordinary covalent molecules and crystals), and then to consider separately the effect of resonance in stabilizing the crystal and decreasing the interatomic distance. This procedure is similar to that which we have used in the discussion of interatomic distances in resonating molecules.7 The alternative procedure would be to assign to each bond a number, the bond order, to represent the strength of the bond with inclusion of the resonance effect as well as of the bond type.8... 

A range of symbolic conventions is used in representing atomic and molecular stractures at the electronic level. So for example double and triple lines are used for multiple bonds. This seems a clear convention, which helps keep check of valency rales. However the symbol = for a double bond is not intended to imply two equal bonds (which the symmetry of the symbol could seem to suggest) as u and TT components have different geometries, contributions to bond strength , and consequences for chemical properties. The novice learner may well find interpreting such representations a considerable challenge. 

The order of a bond may be defined as the number of electron pairs that constitute the bond. Thus the bond orders of single, double, and triple bonds are respectively 1, 2, and 3. As the number of electron pairs forming the bond increases, the attraction of the bonding electrons for the two atomic cores increases, so the bond strength increases and the bond length decreases. 

The nature of the bonding in this molecule has been the cause of considerable discussion. Its short length (112.8 pm) and its great strength (bond dissociation enthalpy 1072 kJ mol- ) are consistent with the usual triple-bond Lewis structure... 

An aromatic ring and a double or triple bond in the a-position relative to the C—H bond weaken this bond by virtue of the delocalization of the unpaired electron in its interaction with the iT-bond. The weakening of the C—H bond is very considerable for example, D(C—H) is 422 kJ mol-1 in ethane [27], 368 kJ mol-1 in the methyl group of propene [27] (AD = 54 kJ mol-1), and 375 kJ mol-1 in the methyl group of toluene [27] (AD = 47 kJ mol-1). Such decrease in the strength of the C—H bond diminishes the enthalpy of the radical abstraction reaction and, hence, its activation energy. This effect is illustrated below for the reactions of the ethylperoxyl radical with hydrocarbons ... 

The physical conditions to effect a satisfactory extent of reaction are fairly severe, but are needed to overcome the enormous strength of the nitrogen triple bond. The role of the iron is essential chemisorptive adsorption of nitrogen occurs on the surface of the iron, with charge being donated from the N=N bond to the surface of the iron. As a result, less electron density remains between... 

The force constant K relates with the strength of the bond. For a single bond, it is approximately 5 x 105 dynes cm . It becomes double and triple for double and triple bonds respectively. 

The characteristic chemical feature of N2 is its stability. Like acetylene, N2 has a triple bond N=N. This triple bond has great strength and vigorous conditions are required to convert N2 into ammonia, NH3, in the chemistry laboratory. This conversion is of great importance as ammonia is employed in enormous quantities for human purposes. 

The differences in selection rules between Raman and infrared spectroscopy define the ideal situations for each. Raman spectroscopy performs well on compounds with double or triple bonds, different isomers, sulfur-containing and symmetric species. The Raman spectrum of water is extremely weak so direct measurements of aqueous systems are easy to do. Polar solvents also typically have weak Raman spectra, enabling direct measurement of samples in these solvents. Some rough rules to predict the relative strength of Raman intensity from certain vibrations are [7] ... 

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