Organic Chemistry on Cold Molecular Films: Kinetic Stabilization of S N 1 and S N 2 Intermediates in the Reactions of Ethanol and 2‐Methylpropan‐2‐ol with Hydrogen Bromide

• Published in: Chemistry : a European journal 2003-04, Vol.9 (8), p.1706-1713
• Main Authors: Park, Seong‐Chan, Maeng, Kye‐Won, Kang, Heon
• Format: Article
• Language: English
• ISSN: 0947-6539; 1521-3765
• DOI: 10.1002/chem.200390194

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.