Haloalkanes and Haloarenes for NEET UG
Haloalkanes and haloarenes are where organic reaction mechanisms begin. SN1, SN2 and elimination decide what product forms, and the conditions decide which mechanism wins.
ChemistryClass 1274 MCQs2 lessons13 flashcards3 memory tricks
Practise Haloalkanes and Haloarenes free
What you'll learn
- SN1 and SN2 reactions
Tell SN1 from SN2 and predict which mechanism a haloalkane follows. - Elimination and haloarenes
Apply Saytzeff's rule, and explain why haloarenes resist nucleophilic substitution.
Memory tricks
SN2: one step, backside attack, inversion; fastest for 1°. SN1: two steps through a carbocation, racemisation; fastest for 3°.
Reactivity R–I > R–Br > R–Cl > R–F, because the C–I bond is the weakest.
In light and air, CHCl₃ oxidises to phosgene (COCl₂), which is highly toxic.
Try 3 NEET-style questions
Q1. Bromoethane reacts with silver nitrite (AgNO₂). The main product is
- ethyl nitrite
- nitroethane
- ethylamine
- ethanol
Show answer
Answer: (2) nitroethane
AgNO₂ is mainly covalent, so attack occurs through nitrogen, giving nitroethane, CH₃CH₂NO₂. (KNO₂ gives mainly ethyl nitrite, through oxygen.)
Q2. Ethyl bromide reacts with alcoholic KOH to give mainly
- ethanol
- ethene
- diethyl ether
- ethane
Show answer
Answer: (2) ethene
Alcoholic KOH causes dehydrohalogenation (elimination), giving ethene.
Q3. An SN2 reaction at a chiral carbon atom results in
- retention of configuration
- inversion of configuration
- complete racemisation
- loss of the chiral carbon
Show answer
Answer: (2) inversion of configuration
The nucleophile attacks from the side opposite the leaving group, so the other three groups flip over like an umbrella in the wind: inversion of configuration.
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