Chapter 16, Problem 16.29P

Interpretation Introduction

Interpretation:

Which carbon atom in benzyl chloride would have the highest chemical shift is to be predicted and the reason is to be explained.

Concept introduction:

${}^{13}\text{C}$ NMR provides valuable information about the carbon skeleton. Each signal in the ${}^{13}\text{C}$ NMR equals to the number of distinct carbon atoms in the given unknown. In most of the ${}^{13}\text{C}$ NMR spectra almost every time all the signals would appear as singlets. The ${}^{13}\text{C}$ absorptions are reported as chemical shifts with TMS as the reference compound (its carbon atom is assigned a chemical shift of $\text{0 ppm}$. In ${}^{13}\text{C}$ NMR, chemical shifts generally ranges from $\text{δ}\text{0 - 220 ppm}$. Aromatic carbon atoms have chemical shifts around $\text{δ}\text{128 ppm}$ because the aromatic rings are electron rich and withdraw the electron density from the carbon atom. The saturated carbon atoms appear in the range $\text{δ}\text{0-35 ppm}$ if it is a simple alkane fragment. If it is attached to any electronegative element such as halogens or nitrogen then the range is $\text{δ}\text{25-70 ppm}$. Nearby electron withdrawing substituents (such as oxygen, halogen, or nitrogen atoms) remove the electron density away from the carbon atom, thereby deshielding it. The result is a downfield shift. In addition, the ring current from aromatic rings or double bonds adds to the magnetic field experienced by the carbon atom, resulting in the downfield shift.