Question

Which one of the following orders is correct regarding the inductive effect of the substitutents

• A. -NR_2 < -NHR < -F
• B. -NR_2 > -NHR > -F
• C. -NHR < -NR_2 < -F
• D. -NR_2 > -F < -NHR

Answer 1

Answer: (A)

-NR_2 < -NHR < -F

Answer 2

The inductive effect (-I) is determined by electronegativity. Fluorine (EN ≈ 3.94) is more electronegative than Nitrogen (EN ≈ 3.04), making $-F$ a stronger electron-withdrawing group. For substituents with the same directly attached atom (Nitrogen), the effect is influenced by attached groups. Alkyl groups (R) are electron-donating (+I). $-NR_2$ has two R groups, making it a weaker electron-withdrawing group than $-NHR$, which has one R group. Thus, the order is $-NR_2 < -NHR < -F$.