Original sentence: Addition of singlet carbenes to olefinic double bonds is stereospecific but that of triplet carbenes is stereoselective.Addition reactions with alkenes can be used to determine whether the singlet or triplet carbene is involved.For instance the reaction of methylene generated from photolysis of diazomethane with cis-2-butene and trans-2-butene is stereospecific which proves that in this reaction methylene is a singlet.Reactions of singlet methylene are stereospecific whereas those of triplet methylene are not
Edited sentence: Due to these two modes of reactivity, reactions of singlet methylene are stereospecific whereas those of triplet methylene are stereoselective.For example, the reaction of methylene generated from photolysis of diazomethane with cis-2-butene or with trans-2-butene each give a single diastereomer of the 1,2-dimethylcyclopropane product: cis from cis and trans from trans, which proves that the methylene is a singlet.If the methylene were a triplet, one would not expect the product to depend upon the starting alkene geometry, but rather a nearly identical mixture in each case.This difference can be used to probe the nature of a carbene