Chapter 17, Problem 17.68P

Although benzene itself absorbs at $\text{128}\text{ppm}$ in its ${}^{13}\text{C}\text{NMR}$ spectrum, the carbons of substituted benzenes absorb either upfield or downfield from this value depending on the substituent. Explain the observed values for the carbon ortho to the given substituent in the monosubstituted benzene derivatives X and Y.