Question

9. H NMR analysis - 2 points total a. Assign only the aldehyde proton (Ha) in the H NMR spectrum of 9-anthraldehyde (CDC13, 60 MHz). The aromatic protons are overlapping (9-7.5 ppm) and it is challenging to individually assign them all (0.25 point) Havo Aromatic protons 11.5297 POCO 6- -8.9060 -8.6782 -8.1397 -7002 2.001.02 6.03 90 115 11.0 105 10 0 95 45 40 35 3.0 85 80 751 7065 60 55 50 Chemical Shift (ppm)

Answer 1

The basic principle of NMR is to apply an external magnetic field and measure the frequency at which the nucleus resonate. Higher the electron density around the nucleus, greater is the opposing affect of the electrons to the applied magnetic field and thus called shielding effect. when the electron density is less around the nucleus, the opposing affect is also less.In such a case, the nucleus feels more the external magnetic field and it is called deshielding.

More the shielding effect lower is the chemical shift value and vice versa.

In aldehydes the proton is deshielded. Below is the pictorial representation of aldehyde shielding

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The blue cone shows the shielding area and thus, the proton is clearly deshielded. Therefore the chemical shift value for the aldehyde proton (Ha) increases having the value 11.5207.