Thursday, January 17, 2019

inorganic chemistry - Are any bonds broken when NO2 becomes N2O4?


This reversible reaction occurs in room condition:


2NOX2NX2OX4(ΔH=57.23 kJ/mol)


This is the Lewis structure of NOX2 (courtesy Ben Mills via Wikipedia):



This is the Lewis structure of NX2OX4 (courtesy ChemSpider):




To me, it seems that no bonds need to be broken for the forward reaction (2NOX2NX2OX4) to occur, which is strange indeed.



Answer



No bonds are broken, but there are several isomers of N2O4. The unpaired electron can be on either the N or the O, for each NO2 molecule. The long bond between the monomers involves these two unpaired electrons.


See sections 7-1 to 7-3 of Bonding in Electron-Rich Molecules: Qualitative Valence-Bond Approach via Increased-valence Structures for more information.


According to First-Principles Study of the Role of Interconversion Between NO2, N2O4, cis-ONO-NO2, and trans-ONO-NO2 in Chemical Processes J. Am. Chem. Soc., 2012, 134 (31), pp 12970–12978:


there is no barrier to the bonding of two monomers to form the planar isomer with the N-N bond.


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