Acidity trends in alpha,beta-unsaturated alkanes, silanes, germanes, and stannanes

The gas-phase acidity of ethyl-, vinyl-, ethynyl-, and phenyl-substituted s ilanes, germanes, and stannanes has been measured by means of FT-ICR techni ques. The effect of unsaturation on the intrinsic acidity of these compound s and the corresponding hydrocarbons was analyzed through the use of G2 ab initio and DFT calculations. In this way. it was possible to get a general picture of the acidity trends within group 14. As expected, the acid streng th increases down the group, although the acidity differences between germa nium and tin derivatives are already rather small. As has been found before for amines, phosphines, and arsines, the carbon, silicon, germanium, and t in alpha,beta -unsaturated compounds are stronger acids than their saturate d analogues. The acidifying effect of unsaturation is much larger for carbo n than for Si-, Ge-, and Sn-containing compounds. The allyl anion is better stabilized by resonance than its Si, Ge, and Sn analogues. [CH2-delta = CH +delta = CH2-delta]- vs [CH2-delta II = CH-delta III - XH2-delta IV]- (X = Si, Ge, Sn). The enhanced acid strength of unsaturated compounds is essenti ally due to a greater stabilization of the anion with respect to the neutra l, because the electronegativity of the alpha,beta -unsnturated carbon grou p increases with its degree of unsaturation. The phenyl derivatives are sys tematically weaker acids than the corresponding ethynyl derivatives by 15-2 0 kJ mol(-1). Experimentally, toluene acidity is very close to that of prop yne, because the deprotonation of propyne takes place preferentially at the =CH group rather than at the -CH3 group.