[Kaladiscope]

Hi,

As a starting molecule I have 2 nitro benzyl alcohol. If I compare it to 4,5 dimethoxy 2 nitro benzyl alcohol, can I say that the presence of methoxy group make the ring more rich in terms of electrons..compare to nitrobenzyl alcohol which should be poor due to the only presence of a nitro group?

[Dan]

Are the OMe groups donating or withdrawing electrons to/from the ring?

So will their presence increase or decrease electron density in the ring?

[Kaladiscope]

They are a donating group, so they push electrons to the system (ring). Therefore they will increase the electron density in the ring.

But do u think that the presence of both could cancel this effect?

[fledarmus]

A nice question, and one that has gotten research attention in terms of both reactivity and NMR chemical shifts. The presence of a substituent doesn't just affect the ring itself in terms of electron density, but affects different positions on the ring differently, with stronger effects typically found in the ortho and para positions. A number of researchers have published tables of substituents that allow you to calculate the effect on electron density on benzenes substituted with different patterns of substituents. Here is one recent article that might help guide you into the literature on the subject:

http://144.206.159.178/ft/621/47303/844469.pdf

[AC Prabakar]

Yes it is!
I just want to add some points...
Electron density of the ring system also can be calculated/predicted by taking/predicting simple 1H-NMR as we know that the electron cloud around the proton will give difference in shielding and deshielding effects.
This method can be employed for the prediction of active reaction sites in complex structures with reasonable precision!!!