Question

explain the observed absorption maxima of each of the following molecules:

a) nitrobenzene, 252 nm

b) 2-nitrotoluene, 250 nm

c) 2-isopropylnitrobenze, 247 nm

d) 2-t-butylnitrobenzene, no peak

They are all being observed in UV-vis spectroscopy (200 - 800 nm)

In addition to this, the question also asks to provide 3D structures which i think i have a grasp of. I dont know, however, if they are necessary to answer this question but i am lost on explaining the absorption maximas.

Thank you in advance!

Answer 1

a) nitrobenzene shows UV absorption maxima at 252 nm due to transition. The electrons present in orbital gets excited to * on absorbing the UV light.

b) In 2-nitro toluene, the benzene is disubstituted, one group is nitro and other is methyl. The nitro group is electron withdrawing and methyl is electron releasing. There will be drift of electrons from electron releasing to electron withdrawing group through pi-bond of benzene ring. As the two groups are ortho to each other, so there will be shift in the spectrum. The value of same transition becomes 250 nm.

c) In 2-isopropyl nitrobenzene, along with drift in electrons, another effect comes into play, which is steric hindrance. As the two groups are close to each other, they cause steric hindrance, which shifts the absorption maxima a little bit more and it becomes 247 nm.

d) the group, t-butyl is so much bulkier that it does not allow the transition to occur. So, there is no peak observed for 2-t-butyl nitrobenzene.