Are loc reactia de aditie, cand la masa unui mol de alchena se adauga masa unui mol de clor:
CnH2n+Cl2= CnH2nCl2
M.alchena= (12n+2n )g/mol; M, derivat bromurat= (14n+71)g/mol
M,clor= 71 g/mol
in 100g derivat bromurat 62,8g clor
in(4n+71) g 71g ----->7100= 879,2n+4458,8--->n=3 propena