Chemical reactions of organic molecules adsorbed at ice: 2. Chloride substitution in 2-methyl-2-propanol

A new reaction is reported on the ice surface, namely the conversion of ads orbed 2-methyl-2-propanol [(CH3)(3)COH] to adsorbed 2-chloro-2-methylpropan e [(CH3)(3)CCl] in the presence of absorbed HCl. The reaction was studied u sing temperature-programmed desorption mass spectrometry on ultrathin ice f ilms under ultrahigh vacuum. Formation of (CH3)(3)CCl occurs at two tempera tures: at or below 155 K and at 185 K. The lower-temperature process occurs at the surface of the film, but the origin of product at 185 K is unclear. The yield of the surface reaction product is low, approximately 0.04 monol ayers. Importantly, adsorbed HCl is unreactive toward a;methyl-a-propanol; reactive chlorine is derived from a dissociatively ionized state in the ver y near surface region of the film. The lower-temperature pathway for chlori de substitution is observed only at the surface of an amorphous film, an ob servation that may have implications regarding the nature of surface reacti on sites. Possible mechanisms of the surface reaction are discussed.