Top: C H 2 double bonded to C H single bonded to C H 3 plus C L isotope goes to C H 2 double bonded to C H single bonded to C H 2 isotope plus H C L. Bottom: C H 2 double bonded to C H single bonded to C H 2 isotope plus C L 2 goes to C H 2 double bonded to C H single bonded to C H 2 C L plus C L isotope.