Influence of steric and intramolecular inductive effects on the variable trends in R-X (R=Alkyl) bond dissociation energy

• 来源期刊: 中国科学B辑(英文版)
• 年，卷（期）: 2009,（7）
• 页码范围: 943-951
• 页数: 9页
• 语种: 英文
• DOI: 10.1007/s11426-009-0031-1

摘要

The question "why are the variation trends of R-X bond dissociation energy different?" is answered.The R-X bond dissociation energy (BDE) may be influenced by three main factors:the C-X intrinsic bond energy,the 1,3 geminal repulsion,and the intramolecular charge-induced dipole.In the presence of atom X,the variation trend of BDE in R-X (R=Me,Et,i-Pr,t-Bu) is dominated by two factors,the 1,3 geminal repulsion and the intramolecular charge-induced dipole.The former decreases the R-X BDE,and the latter either increases or decreases the R-X BDE.For the series of R-X with the R-C bond (such as R-Me,R-CH-CH2,R-C-CH,and R-CN),the 1,3 geminal repulsion decreases the R-X BDE,and the variation trends of R-C BDE decrease from Me to t-Bu.As regards the series of R-X (such as R-H,R-BH2,and R-SiH2) in which the electronegativity of atom X is smaller than that of the carbon atom,the above two factors decrease the R-X BDE,and the variation trends of the R-X BDE decrease from Me to t-Bu.As to the series of R-X (such as R-F,R-OH,R-CI,R-Br,R-I,and R-NH2) in which the electronegativity of atom X is larger than that of the carbon atom,the 1,3 geminal repulsion decreases the R-X BDE,while the intramolecular charge-induced dipole increases the R-X BDE.In this case,the variation trends of R-X BDE depend on the competition of the two factors.As a result,some of them (e.g.,R-F,R-OH) increase from Me to t-Bu,some (e.g.,R-I) decrease from Me to t-Bu,and some (e.g.,R-Br) change very little.