Question

The 1 H NMR chemical shifts of nitromethane, dinitromethane, and trinitromethane are at d 6.10, d 4.33, and d 7.52. Match each chemical shift with the compound. Explain how chemical shift correlates with pKa.

Short answer

Nitromethane – 4.33 ppm

Dinitomethane – 6.10 ppm

Trinitromethane – 7.52 ppm

Step-by-step solution

Electron-Withdrawing group

Anelectron-withdrawing group attached to a carbon chain causes its chemical shift high.[l1]

• Nitro-methane has one electron-withdrawing group, while dinitromethane has two electron-withdrawing groups, and trinitromethane has 3. Therefore, the chemical shift also increases accordingly.

[l1]This is not the proper definition of an Electron Withdrawing Group.

Relation of with pKachemical shift

The acidic hydrogen has a lowvalue, and the greater the acidity, greater the chemical shift.

Therefore, the order in the power of acid dissociation constant is:

${\text{CH}}_{\text{3}}{\text{NH}}_{\text{2}}>{\text{\hspace{0.17em}CH}}_{\text{3}}{\left({\text{NH}}_{\text{2}}\right)}_2>{\text{\hspace{0.17em}CH}}_{\text{3}}{\left({\text{NH}}_{\text{2}}\right)}_3$