Can you explain what you mean when you say that there is a trivalent C in H3C - CH = CH - CH - CH3.

Answer 1

As you know, carbon has four bonds (it is tetravalent) in organic compounds. The C that is second from the rightmost C has only three bonds (trivalent) so that is not a real organic compound. I don't know how to name an organic compound that has a trivalent carbon atom because trivalent carbon atoms don't exist. If I did name that one in an earlier post, that happened because I mentally added another H atom to make the C atom tetravalent and named that compound accordingly.