[orgoclear]

Q1. See the attachment. Which of the structures will have maximum dipole moment?
Q2. See the attachment (it contains two figs). Which of the canonical structures contributes maximum to the structure of the compound?

My attempts
For the first question, I can say that the first one will have the highest dipole moment because -NO2 is a strong -I group. Further all three compounds are ortho. So the effect of the angle between the two vectors is same. So the one with higher magnitude will have higher dipole moment.
But i am confused over the moments caused by -Cl and -CH3 groups.

For the second question, The 2nd and third structures are equivalent. The first structure is symmetrical so it ought to have the maximum contribution. However, as the second and third ones are equivalent, then they should have the most contribution. (No. of covalent bonds are same, all atoms satisfy octet rule.) Further the charge delocalisation is most in the first structure. So it should be the most contributing.
So my ans. is A>B=C
But the answer was given the opposite i.e. B=C>A

[Arctic-Nation]

For question 1, halogen atoms have a stronger polarization effect than alkyl groups because of their higher electronegativity. This means that o-dichlorobenzene has a larger dipole moment than o-dimethylbenzene.

As for the second problem, based on what I can find on google, I would say that you are right and the answer you were given is wrong. These two links give a limited explanation:
http://www.cem.msu.edu/~reusch/VirtTxtJml/intro3.htm#strc7
http://books.google.com/books?id=ocKWuxOur-kC&pg=PA486&lpg=PA486&dq=azide+resonance+structure&source=bl&ots=M7xXHjdCDY&sig=NmNw502Gq8gtBTC1ehdufQkKWQ8&hl=nl&ei=dkeNSoDDOsGQsAbpoK2vDA&sa=X&oi=book_result&ct=result&resnum=6#v=onepage&q=azide%20resonance%20structure&f=false

[orgoclear]

Thanks for confirming my answer for the second question. 

But do

For question 1, halogen atoms have a stronger polarization effect than alkyl groups because of their higher electronegativity. This means that o-dichlorobenzene has a larger dipole moment than o-dimethylbenzene.

But do you have any reference regarding that. I would like to know the reason for that one.

[orgoclear]

can you please tell me the reason for this

For question 1, halogen atoms have a stronger polarization effect than alkyl groups because of their higher electronegativity. This means that o-dichlorobenzene has a larger dipole moment than o-dimethylbenzene.

[macman104]

You mean a reference for the fact that chlorine is more electronegative than a methyl group?  You are comparing a C(sp²)-C(sp³) bond and a C(sp²)-Cl bond.  Look at any reference for a table of electronegativities and you will see that chlorine has a higher electronegativity than carbon.

[orgoclear]

I agree that -Cl is more electronegative.
But -Cl has also a +M(or +R) effect while for -CH3 there's +I and +R effect. So I thought that the polarization would be more for the -Me group

[Arctic-Nation]

The most important factor in polarization is electronegativity. When the two atoms in a bond are the same and bear similar subsitutuents, the polarization across that bond will be very small. The +I and +M effects of methyl in this case are limited compared to the -I effect of chlorine (which is, don't forget, one of the most electronegative elements). And when you have two chlorine atoms tugging on the same electrons in about the same direction, the effect is even stronger.

[orgoclear]

thanks a lot

[orgoclear]

I have got another reason (which might or might not be correct).
The 2 Cl groups are bulky, so we might expect steric repulsion between them which will increase the angle between the two vectors.
So the resultant dipole moment might become less than in case of -Me

Any comments on this view?

[Arctic-Nation]

Nah. A methyl group is about the same size as a bromine atom, so, if anything, o-xylene should have a larger angle between the substituents. But you need groups larger than that to give an appreciable steric repulsion.