The addition of dimethylarsine to hexafluoro-butyne-2 has previously been shown to give the predominantly trans 1:1 adduct, Me₂AsC(CF₃)=C(CF₃)H. In this work, a mechanistic study was made of the addition of dimethylarsine, as well as other Group V hydrides, to hexafluorobutyne-2. This was done to help shed light on the relatively unknown area of noncatalyzed nucleophilic hydride additions to acetylenes. In the course of the mechanistic investigation, the electronic properties of the product adducts became of interest, and consequently ultraviolet and photoelectron spectroscopy were used to study the arsine adducts as well as similiar Group V derivatives. The addition reaction was found to be kinetically controlled by the nucleophilic attack of the arsine molecule on the electron deficient carbon-carbon triple bond. A detailed discussion of the addition stereochemistry is necessary to explain many of the results found in this work, as well as those found by other investigators. Generally the noncatalyzed additions of Group V hydrides to acetylenes are predominantly, though not stereo-specifically, trans additions. Cis kinetic additions are possible under certain circumstances. The ultraviolet absorption found for the 1:1 adducts of Group V hydrides and hexafluorobutyne-2, can be assigned to a n-π* (or n-R) band. An unequivocal choice between the two possibilities cannot be made from the results obtained in this work. These assignments are suggested from experimental data about the upper ground states using photoelectron spectroscopy, the excited state using ultraviolet spectroscopy,and the intensities and solvent effects of the ultraviolet bands. / Science, Faculty of / Chemistry, Department of / Graduate