Answer to Question #59803 in Organic Chemistry for haley

Question

Consider the following questions about acetonitrile, fluoromethane, and chloromethane.

Acetonitrile has dipole moment of 3.92 and chemical shift of 1.97. Fluoromethane has a dipole of 1.85 and chemical shift of 4.26. chloromethane has a dipole of 1.87 and a chemical shift of 3.05.

a) Account for the observation that the dipole moments of fluromethane and chloromethane are almost identical even though fluorine is considerably more electronegative than fluorine.

b) explain why the dipole moment of acetonitrile is considerably greater than the dipole moment of either fluoromethane or chloromethane.

c) How do you account for the observation that the chemical shift of the methyl hydrogens in acetonitrile is considerably less than the chemical shift of the methyl hydrogens in either fluoromethane or chloromethane. Hint: Consider the differences between 1) induction effects and 2) anisotropy associated with pi bonds.