In the generic reaction, AB + CD →
AC + BD, B has changed position with C. An example is olefin
metathesis. “Olefin” is an older word for an alkene, a compound
with a CC double bond. Olefin metathesis is truly
an amazing reaction in which the strongest bond in an alkene,
the CC double bond, is broken and remade. Olefins are very
stable thermally in the absence of transition metal catalysts.