Question:
Which of the two: $\mathrm{O}_{2} \mathrm{NCH}_{2} \mathrm{CH}_{2} \mathrm{O}^{-}$or $\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{O}^{-}$is expected to be more stable and why?
Solution:
$\mathrm{NO}_{2}$ group is an electron-withdrawing group. Hence, it shows -l effect. By withdrawing the electrons toward it, the $\mathrm{NO}_{2}$ group decreases the negative charge on the compound, thereby stabilising it. On the other hand, ethyl group is an electron-releasing group. Hence, the ethyl group shows +I effect. This increases the negative charge on the compound, thereby destabilising it. Hence, $\mathrm{O}_{2} \mathrm{NCH}_{2} \mathrm{CH}_{2} \mathrm{O}^{-}$is expected to be more stable than $\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{O}^{-}$.