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Chemical shifts

Hi, can anyone help me with this question, I am a bit confused, why at chemical shift 4 we have CH group? According to the data booklet the chemical shift for alkane is 0,9-1,7 , I would actually switch 2,7 and 4 in the mark scheme, can anyone explain why is it so?

This is quite a tricky question! The presence of the phenyl group (benzene ring) alters things a lot. Protons near a benzene ring are deshielded because the ring has a sort of electron withdrawing effect. Because of this, protons near it feel the magnetic field of the NMR machine more strongly. As a result they appear further downfield than you might expect. Remember 'downfield = deshielded'.

I would have expected the hydrogens on the ring itself to be most deshielded. Then comes the single H atom at 4.0 followed by the O-H proton at 2.7. These OH protons move around a lot in so the chemical shift is not much help. This is where your D₂O shake is useful because the O-H signal disappears. The methyl group is least affected by the benzene ring so it has the smallest chemical shield (upfield).

I have attached the spec. of the compound

(edited 1 year ago)

Screenshot 2022-05-11 at 20.58.14.png22.9KB

Reply 4

PolinaChepurnova

Thank you!