No linear free energy relationship (LFER) exists that can predict reduction rates of munitions compounds (MCs). We measured the reduction kinetics of aromatic, nitramine, and charged MCs by hydroquinones and computed electron transfer (ET) and hydrogen atom transfer (HAT) energies to develop LFERs for rate prediction. Three hydroquinones – lawsone, AQDS, and AQS – reduced by dithionite were used to reduce aromatic (2,4,6-trinitrotoluene (TNT) and 2,4-dinitroanisole (DNAN)), nitramine (hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX) and nitroguanidine (NQ)) and charged (3-nitro-1,2,4-triazol-5-one (NTO) and 3,4-dinitropyrazole (DNP)) MCs and three nitroaromatics (NACs), 2-amino-4,6-dinitrotoluene (2-A-DNT), 4-amino-2,6-dinitrotoluene (4-A-DNT), and 2,4-dinitrotoluene (DNT). All MCs/NACs were reduced by the hydroquinones except NQ. pH strongly affected reaction rates by controlling the speciation of both hydroquinones and MCs, permitting determination of their second-order rate constants. The intrinsic reactivity of MCs (oxidants) decreased upon deprotonation, while the opposite was true for hydroquinones (reductants). The rate constants spanned ~6 orders of magnitude. That the rate constants for structurally diverse MCs can be described by ET and HAT LFERs suggests all the MCs/NACs (except NQ) are reduced by hydroquinones through the same rate-limiting step. These results increase the applicability of LFERs for predicting the fate and half-lives of MCs and related compounds in subsurface environments.

Electron transfer energy and hydrogen atom transfer energy-based LFERs for predicting reaction rate constants of munitions compounds with hydroquinones

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