Organic Chemistry on Cold Molecular Films: Kinetic Stabilization of S_N1 and S_N2 Intermediates in the Reactions of Ethanol and 2‐Methylpropan‐2‐ol with Hydrogen Bromide

Abstract

We prepared thin molecular films of ethanol and 2-methylpropan-2-ol on Ru(001) substrates at temperature of 100 ± 150 K and examined their reactivity toward HBr. The reaction intermediates and products formed at the surfaces were unambiguously identified by the techniques of Cs reactive ion scattering (RIS) and low-energy sputtering. The reaction on the ethanol surface produced protonated ethanol, which is stabilized on the surface and does not proceed to further reactions. On the 2-methylpropan-2-ol surface, protonated alcohol [(CH 3 ) 3 COH 2 ] and carbocation [(CH 3 ) 3 C ] were formed with the respective yield of 20 and 78 %. Alkyl bromides, which are the final products of the corresponding reactions in liquid solvents, have extremely small yields on these surfaces (< 0.3 % for ethyl bromide and 2 % for tert-butyl bromide). The results indicate that the reactions on frozen films are characterized by kinetic control, stabilization of ionic intermediates (protonated alcohols and tert-butyl cation), and effective blocking of the charge recombination steps in S N 1 and S N 2 paths. The implication of these findings for the molecular evolution process in interstellar medium is also discussed.

Acid–Base Chemistry at the Ice Surface: Reverse Correlation Between Intrinsic Basicity and Proton‐Transfer Efficiency to Ammonia and Methyl Amines

“…The Q values shown in Figure 3 are much smaller than the equilibrium constants of proton-transfer reactions in the aqueous phase: K eq = 6.0 10 10 for (CH 3 ) 2 NH, 4.3 10 10 for CH 3 NH 2 , and 1.7 10 9 for NH 3 at 298 K. [4] As shown in previous studies, [14,21] such a behavior arises because reactions at ice surfaces often stop at intermediate states along the reaction coordinate rather than proceeding all the way to the thermodynamically equilibrated states. In this respect, we define the apparent free energy for proton transfer at the ice surface according to the equation DG* ice = ÀRT lnQ, in analogy to the relationship between free energy and equilibrium constant.…”

Reactive Ion Scattering of Low Energy Cs+from Surfaces. A Technique for Surface Molecular Analysis

“…Studies in this line of research have greatly contributed to understanding of physics and chemistry of ice surfaces in the past decade. They include the RIS and LES studies of surface diffusion of water molecules, 28 ionization of electrolytes, [29][30][31] acid-base chemistry, 12,[32][33][34][35] proton transfer, [36][37][38][39][40][41] and various types of reactions that are either of fundamental interest [42][43][44] or related to environmental chemistry [25][26] and astrochemistry. 45,46 These subjects have been reviewed recently.…”

“…17,18,64 In particular, reaction intermediates can be trapped on a surface at a cryogenic temperature and be identified with RIS. [23][24][25][26][42][43][44] Quantitative measurement of surface adsorbates is an important goal in surface analysis. In the following, a relationship between the surface coverage of adsorbates and their RIS signal intensity is discussed.…”

Electrophilic Addition Reaction of Ethene with Hydrogen Chloride on Cold Molecular Films: Evidence of Ethyl Cationic Intermediate

“…Recently, several research groups have investigated reactions occurring on the surface of ice or frozen molecular films at cryogenic temperatures. [7][8][9][10][11][12][13][14][15][16][17][18][19][20][21][22][23][24] These studies demonstrated that a cold molecular surface can halt a reaction at an intermediate stage and that the reaction intermediate trapped on the surface can be identified with spectroscopic methods. [17][18][19][20][21] For example, Park et al [17] reported that the nucleophilic substitution (S N 1) reaction of tert-butyl alcohol with hydrogen bromide yields protonated alcohol and tertbutyl cation as metastable products on frozen alcohol films.…”