one of the most synthetically important cheletropic reactions is the addition of a singlet carbene to an alkene to make a cyclopropane (see figure at left). a carbene is a neutral molecule containing a divalent carbon with six electrons in its valence shell. because of this, carbenes are highly reactive electrophiles and generated as reaction intermediates. a singlet carbene contains an empty "p" orbital and a roughly "sp""2" hybrid orbital that has two electrons. singlet carbenes add stereospecifically to alkenes, and alkene stereochemistry is retained in the cyclopropane product. the mechanism for addition of a carbene to an alkene is a concerted [2+1] cycloaddition (see figure). carbenes derived from chloroform or bromoform can be used to add cx2 to an alkene to give a dihalocyclopropane, while the simmons-smith reagent adds ch2.