Why Are Addition Reactions to N2 Thermodynamically Unfavorable?

• Published in: The journal of physical chemistry. A, Molecules, spectroscopy, kinetics, environment, & general theory Vol. 121; no. 5; pp. 1140 - 1144
• Main Author: Borden, Weston Thatcher
• Format: Journal Article
• Language: English
• Published: American Chemical Society 09.02.2017
• ISSN: 1089-5639; 1520-5215
• DOI: 10.1021/acs.jpca.6b11728

Thermochemical data are used to show that, of the 89.9 kcal/mol difference between the endothermicity of H2 addition to N2 (ΔH = 47.9 kcal/mol) and the exothermicity of H2 addition to acetylene (ΔH = −42.0 kcal/mol), less than half is due to a stronger π bond in N2 than in acetylene. The other major contributor to the difference of 89.9 kcal/mol between the enthalpies of hydrogenation of N2 and acetylene is that the pair of N–H bonds that are created in the addition of H2 to N2 are significantly weaker than the pair of C–H bonds that are created in the addition of H2 to acetylene. The reasons for this large difference between the strengths of the N–H bonds in E-HNNH and the C–H bonds in H2CCH2 are analyzed and discussed.