Leaving group ability dictates SN2 rate for primary alkyl halides

Steps:

• Primary alkyl halides like 1-chlorobutane and 1-iodobutane favor SN2 with aqueous NaOH due to backside attack preference.
• SN2 rate depends on substrate and nucleophile; leaving group affects bond breaking ease.
• I- is better leaving group than Cl- as C-I bond weaker (bond dissociation energy: C-I 238 kJ/mol vs. C-Cl 351 kJ/mol).
• Thus, 1-iodobutane has greatest rate via SN2.

Why D is correct:

• 1-Iodobutane follows SN2 with fastest rate, as I- exceeds Cl- in leaving group order (I > Br > Cl > F) from polarizability and stability.

Why the others are wrong:

• A: 1-Chlorobutane follows SN2, not SN1; primary halides resist carbocation formation.
• B: Identical to D but not selected; chlorobutane slower despite same mechanism.
• C: 1-Chlorobutane SN2 correct mechanism but slower rate due to poorer Cl- leaving group.

Final answer: D