Which of the following alkenes cannot undergo hydrogenation?

Question

a) cyclohexene 

b)benzene

c) 1-propene 

d) 2-propenal 

Answer 1

I agree with Carol, but I think your teacher is actually asking a bogus question.  Even aromatic compounds can undergo hydrogenation!  It just takes more pressure and heat. 

 

So the question (in my opinion) should really ask "which of the following does not readily undergo hydrogenation?"  ;)

Answer 2

Start by drawing the structure of each choice.  One of them should stand out as being different from the others by virtue of being aromatic.   What is special about aromatic compounds and hydrogenation?