Homework #18 Wk 7+8

[305920501]

Why don't all of the Hydrogen atoms of all 3 molecules lie on the same plane? The correct answer to one of the subquestions of this problem is that only H2CCH2 and H2CCCCH2 have H atoms that lie on the same plane, but I don't understand why H2CCCH2 H atoms don't like on this plane as well.

[Annabelle Jeon 3K]

There's an explanation posted in the solutions. I don't think it was really emphasized in lecture, but if we look at the Lewis structure, HCCCH2 lies on a different plane because the H atoms of H2CCCH2 are perpendicular to each other as a result of the double C pi bond. The general rule for the shape of the hydrocarbons is that if there's an odd number of carbons, the terminal H-atoms usually lie perpendicular to each other while if there's an even number of carbons, the terminal H-atoms lie parallel which is why H2CCH2 and HCCCCH2 both lie on the same parallel plane.