well i have just now started learning chemical bonding so I am not very sure about the answer but anyway..
para-dichlorobenzene most probably doesnt have a dipole moment because of the oppositely positioned Chlorine atoms. Ortho and meta dichlorobenzene would definitely have a dipole moment(greater in the case of ortho). But I have no clue about dihydro-benzene. I dont even have any idea about its strucutre... Can you please help?
NOTE: The post previously had 1,3 cyclohexadiene mislabeled as 1,2 cyclohexadiene. Mistake noted by Cesium.
dihydrobenzene comes in two forms. These are also known as:
1) 1,3 cyclohexadiene
2) 1,4 cyclohexadiene
(or if you wanted you could do the ortho,para naming)
but the key difference between these compounds and dichlorobenzene is that these, while still a derivative of benzene, do not have 3 alternating double bonds and are not aromatic.
If you are confused of the exact structure I found them at sigmaaldrich.com searching for dihydrobenzene.
The dipole moment of the 1,3 cyclohexadiene form makes sense considering the molecule is no longer symetrical (as in the para-dichlorobenzene or the 1,4 cyclohexadiene). As for the dipole moment of the 1,4 cyclohexadiene I really have no idea, it shouldn't have much, if any dipole moment.
you are correct, It was my mistake, I mislabeled my first post. The 1,2 cyclohexadiene should read as 1,3 cyclohexadiene. When I was working the nomenclature out by hand (in attempt to answer your question) I had mistakenly used the numbers as the carbon positions where the hydrogen is attached, not the bond position. So YES, 1,3 cyclohexadiene has a dipole moment, I don't think 1,4 cyclohexadiene does. And I'll probably be corrected again, but I don't think 1,2 cyclohexadiene exists. Good call.