A reaction path Hamiltonian model is developed for describing chemical reaction dynamics in solution based on reference interaction site model self-consistent field (RISM-SCF) method. A systematic procedure is devised to determine all the quantities involved in the present Hamiltonian by the RISM-SCF electronic structure calculations for reaction systems combined with molecular dynamics simulations for bulk solvents. The present model is applied to the Menshutkin-type reactions between NH3 and methylhalides CH3X (X=Cl, Br, and I) in aqueous solution. The barrier height becomes lower with the increase of the size of leaving halogen atom, and the calculated activation energy for the CH3I reaction is in a good agreement with the experiment. The magnitude of the time dependent as well as the static solvent friction is CH3Cl