7586
J. Am. Chem. SOC.1981, 103, 7586-7589
Gas-Phase Ion-Molecule Reactions of Cyclopentadienyl Anion (c-C5H5-) Richard N. McDonald,* A. Kasem Chowdhury, and D. W. Setser Contribution from the Department of Chemistry, Kansas State University, Manhattan, Kansas 66506. Received June 15, I981
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Abstract: Several gas-phase ion-molecule reactions of c-C5Hs- (CH30- + c-CSH6 c-C5HS-+ HOCH3) have been studied in a flowing afterglow apparatus at 298 K. In its reactions with alcohols, c-CSH< was rapidly protonated by (CF3)zCHOH in an exoergic proton transfer reaction. However, the alcohols CH30H, CzH50H,n-C3H70H,and (CH3)3COHslowly formed the clusters c-C5HS-(HOR) when allowed to react with c-C5HS-;direct H+ transfers are strongly endoergic in these reactions. The rate constants for cluster formation were roughly proportional to the AZfacido of ROH. The reaction of c-C5HS-with CF3CH20Hoccurs by the sequence c-CsH5- + CF3CHz0H4c-CsH5-(HOCHzCF3)4c-C5H6+ CF3CHz0-(HOCH2CF3) 4CF3CH20-(HOCHzCF3)2,where steps 2 and 3 involve bimolecular reactions with another CF3CHzOHmolecule, and kl = (2.7 f 0.2) X cm3 molecule-' s-* = k3 = 1 / z k 2 .Although the direct H+ transfer between c-C5H5-and CF3CHz0H is endoergic (8 kcal mol-'), this sequential process of "indirect" H+ transfer is related to the role of solvation on acid-base equilibria in the gas and condensed phases. c-CSH< is shown to be a poor nucleophile in SN2reactions with CH3X molecules and in Michael (conjugate) addition reactions with HzC=CHCN and HZC=CHCO2CH3. Michael addition of c-C5HS-with HzC=CHCN produces the adduct c-C5HSCHzCHCN-( m / z 118), while addition of c-C5H5-to H2C=CHC02CH3 yields the (M- 2) adduct c-C5H4CHCHCOzCHc( m / z 149). These different results from apparently similar reactions are rationalized in terms of conformationally allowed rearrangement followed by elimination (-H2) reactions from the adduct cC5HSCH2CHCOZCH3which are not possible from c-C5HSCH2CHCN-. The basic and nucleophilic properties of c-CSH5are compared with those of the hypovalent anion radical species, c-C5H4-*.The upper limit of EA(C6F6)
