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An investigation of the regioselectivity and mechanisms of the photochemical addition of NH 3 , PH 3 , and SiH 4 to olefinic and acetylenic nitriles is described. The photolysis of NH 3 in the presence of acrylonitrile led to the aaddition product 2-aminopropanenitrile (2), propanenitrile, and 2,3-dimethylbutanedinitrile (3). When NH 3 was photolyzed in the presence of substituted derivatives (crotononitrile, methacrylonitrile, or 1-cyclohexenecarbonitrile), the a-addition products were still obtained. However, under similar reaction conditions, only the b-addition products, 7 and 8, were obtained from acrylonitrile and PH 3 , or acrylonitrile and SiH 4 , respectively. On the other hand, the photolysis of 2-butynenitrile and NH 3 gave the b-addition products, (Z)-and (E)-3-aminocrotononitrile (10). The photolysis of these acetylenic nitriles with PH 3 or SiH 4 also gave the b-adducts (12) and (13). The a-addition of NH 3 proceeds by the stepwise addition of H . and. NH 2 , respectively, to the a,b-unsaturated nitriles. The b-addition products are formed by a radical chain mechanism initiated by photochemically generated radicals. The radical chain pathway provides an explanation for a number of previously described photochemical additions to olefins and acetylenes. Photochemical processes similar to the addition of ammonia and phosphine to unsaturated organic compounds may have played a role in the evolution of the atmosphere of the primitive Earth, and may even be currently occurring in the atmospheres of other planets.

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