Critical Misconceptions to Correct:
-
"SN2 gives racemization" — WRONG. SN2 gives Walden inversion (complete). Racemization is exclusively SN1. This is the single most common error in this chapter.
-
"C-F is most reactive because C-F is the shortest (and therefore weakest) bond" — WRONG. Shorter C-X = stronger C-X. C-F (135 pm) is the shortest AND the strongest (485 kJ/mol). C-F is the LEAST reactive haloalkane. C-I (214 pm, 213 kJ/mol) is the longest, weakest, and most reactive.
-
"The C-Cl bond in chlorobenzene is WEAKER than in chloroethane" — WRONG. The C-Cl bond in chlorobenzene (169 pm) is shorter AND stronger than in chloroethane (177 pm) due to partial double bond character from resonance. This is why haloarenes are LESS reactive (not more), and why extreme conditions are needed in the Dow process.
-
"Primary substrates undergo SN1 in polar protic solvents" — WRONG. Primary carbocations are too unstable to form. Primary substrates always prefer SN2. The polar protic solvent promotes SN1 only for substrates that can form relatively stable carbocations (secondary or tertiary).
-
"KOH (aqueous) and KOH (alcoholic) give the same products" — WRONG. Aqueous KOH gives SN2 → alcohol. Alcoholic KOH (ethanol, hot) gives E2 → alkene. The solvent controls the product.
-
"t-B (tert-butoxide) gives SN2 with secondary substrates" — WRONG. The bulky tert-butoxide cannot perform backside attack. It gives E2 (abstracts β-H from the periphery). Strong non-bulky nucleophile → SN2; strong bulky base → E2.
-
"DDT depletes the ozone layer" — WRONG. DDT causes food chain biomagnification (lipophilic, non-biodegradable). CFCs deplete ozone (UV cleaves C-Cl → Cl• radicals → destruction). Keep these environmental effects clearly separate.
-
"Rearrangements can occur in SN2" — WRONG. Rearrangements require a carbocation intermediate (SN1/E1 only). SN2 is concerted with no intermediate — zero opportunity for rearrangement.