Effects of packing and orientation on the hydrolysis of ester monolayers on gold

Two novel nitrophenyl disulfide esters, di(4-nitrophenyl)-4,4'-dithiobisben zoate (pNBD) and di(4-nitrophenyl)-3,3'-dithiobisbenzoate (mNBD), were prep ared, adsorbed spontaneously as thiolate monolayers on gold surfaces, and u sed as models for examining factors affecting rates of interfacial reaction s. These monolayers were characterized by infrared reflection spectroscopy (IRS), X-ray photoelectron spectroscopy, single sweep voltammetry, and cont act angle goniometry. The adsorbed form of mNBD (mNBT) undergoes a pseudo-f irst-order base-catalyzed hydrolysis at a rate that is 7 x 10(4) slower tha n that for mNBD in solution. The monolayer of the adsorbed para isomer of t he ester is even less reactive; that is, it is not detectably hydrolyzed af ter 3 days of immersion in 0.5 M KOH. The lower packing density of the meta isomer ester accounts for its faster rate of hydrolysis as compared with t hat of the para isomer ester. The progression of the IRS data suggests that the orientation of the nitrophenyl ring in the mNBT monolayer undergoes a significant change during the early stages of hydrolysis but that this chan ge in orientation does not detectably affect the rate of hydrolysis. Reacti on of the mNBT monolayer with n-butylamine, which forms the corresponding a mide via pseudo-first-order kinetics, does not cause a change in the orient ation of the nitrophenyl ring. Thus, the change in orientation during hydro lysis reflects an increase in the free volume of the adlayer that arises fr om loss of the nitrophenylate product to solution.