Dated on structures of A. B. and C. arrange chemical shifts of pointed CH_2 prot

Question

Dated on structures of A. B. and C. arrange chemical shifts of pointed CH_2 protons in the decreasing order of ppm. and explain the reasons for such order Assume benzene proton shows a peak at 7.34ppra. Compare aromatic protons of the following molecules with that of benzene, predict which aromatic protons have the downfield shit) or up field shift Stale the reason (s) for the shift (downfield shift means to have signals shifted towards the direction of 10 ppm. and up field shift docs towards the direction of 0 ppm)

Explanation / Answer

A>B>C

Electronegativity of atoms increases, the lower will be the electron density and hence a higher magnetic field is experienced. Hence it will have higher ppm. Among the atoms attached to methylene protons, oxygen will have greatest electronegativity, followed by nitrogen and then carbon. Hence it follows the above order in ppm values.

2)In 1,4 dimethoxybenzene, the OCH3 group are electron releasing group and hence increases the electron density in the ring as well as the protons attached to it. Hence the proto will be upfielded relative to benzene protons.

Nitro groups are electron withdrawing groups and hence removes the electron density from ring and attached protons. Therefore in 1,4-dinitrobenzene, the aromatic protons will be downshifted.

In 4- nitroaniline, protons next to nitro group ( ortho protons of nitro group) experience a downfield shift due to the electron withdrawing nature of nitrogroup. While ortho protons of NH2 group experience an upfield shift due to electron releasing nature of NH2 group

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