3 Match the chemical shifts with the protons labeled in structure X below. (1 ma

Question

3 Match the chemical shifts with the protons labeled in structure X below. (1 mark each) Explain briefly the reason the that one signal has a negative value. Note the relative intensities of the peaks at the end of your tree. (2 marks) Hd 0 Hb Chemical shifts: 11,0 9.55 7.2 -1.56(yes it is minus 1.58) Solution and explanations Ha has a chemical shift of -1.5 ppm. The hydrogen lies in the cone of shielding of the aromatic ring (see below for more discussion). Eb has a chemical shift of 7.2 ppm. The hydrogen is on the outside of a benzene ring. Such hydrogens absorb at 7 because of anisotropic deshielding effect of the induced magnetic field generated from the delocalized electrons. He has a chemical shift of 9.5 ppm. This hydrogen is part of an aldehyde unit, which typically absorbs at ~9.5 because of a combination of electron-withdrawing inductive effects and anisotropic effects from the induced magnetic field ofthe CO bond. Hd has a chemical shift of 11 ppm. This hydrogen is part of the carboxylic acid, which typically absorbs at 11.08 because of a combination of electron-withdrawing inductive effects and anisotropic effects from the induced magnetic field of the C-O bond. The reason that Ha absorbs at a negative value is that it is in the cone of shielding, or the shielding domain, which is formed by the induced magnetic field of the aromatic electrons Hydrogens on the periphery of benzene rings are deshielded by the induced magnetic field, but Ha lies in the inside of the ring. Thus, it is shielded, as shown below Induced magnetic field Shielding area Deshielding Deshielding area Induced electric current

Explanation / Answer

In compound X, The proton Ha present on top of benzene ring, therefore that proton completely shielded by pi electrons of benzene and hence will go upfield.As a result, it will show negative value.

Acid proton generallycomes at around 11(downfield) and aromatic hydrogen Hb comes at around 7.2

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