The Mistake Bank
Class 12 - Chapter 10: Haloalkanes & Haloarenes
Substitution or Elimination? The conditions decide, not you.
Aqueous vs Alcoholic KOH
Reagent ConditionsScenario: $CH_3CH_2Cl$ reacted with Alcoholic KOH ($KOH_{alc}$).
Student sees OH group and assumes substitution.
Product: Ethanol ($CH_3CH_2OH$).
(Wrong! This happens with Aqueous KOH.)
Alcoholic = Elimination!
In alcohol, the ethoxide ion ($C_2H_5O^-$) acts as a strong base, not just a nucleophile.
It abstracts a $\beta$-hydrogen, causing Dehydrohalogenation.
Product: Ethene ($CH_2=CH_2$).
Grignard + Moisture
Grignard ReagentScenario: Prepare Phenyl Magnesium Bromide ($PhMgBr$) in a flask containing moist air.
Student writes the formation reaction: $PhBr + Mg \rightarrow PhMgBr$.
Assumes product is stable.
Grignard hates Protons!
Grignard reagents are extremely strong bases. If there is any moisture ($H_2O$), they abstract the proton immediately.
$$ PhMgBr + H_2O \rightarrow Ph-H + Mg(OH)Br $$
Product: Benzene (The reagent is destroyed).
KCN vs AgCN
Ambident NucleophilesScenario: Reaction of Ethyl Chloride ($C_2H_5Cl$) with $AgCN$ (Silver Cyanide).
Student sees "CN" and attaches Carbon.
Product: Ethyl Cyanide ($C_2H_5CN$).
(This happens with ionic KCN!)
AgCN is Covalent!
In AgCN, the Carbon is bonded to Silver covalently, so only the Nitrogen lone pair is available for attack.
Result: Attack occurs via Nitrogen.
Product: Ethyl Isocyanide ($C_2H_5NC$).
Reactivity of Chlorobenzene
Nucleophilic SubstitutionScenario: React Chlorobenzene with aqueous NaOH at room temperature.
Student applies SN mechanism.
Product: Phenol.
(Haloarenes are stubborn!)
Partial Double Bond Character!
Due to resonance, the C-Cl bond acquires partial double bond character. It is very strong.
No Reaction occurs at room temperature.
Requires drastic conditions (623K, 300atm) to force the reaction (Dow's Process).
Stereochemistry of SN2
MechanismScenario: Draw the product of SN2 reaction on (R)-2-Bromobutane.
Student replaces Br with OH while keeping the same orientation.
Product: (R)-2-Butanol.
(Retention of configuration?)
Walden Inversion (Umbrella Flip)!
In SN2, the nucleophile attacks from the backside.
This flips the configuration of the molecule.
Product: (S)-2-Butanol (Inversion of configuration).
SN1 Stability Check
MechanismScenario: Which undergoes SN1 faster? Benzyl Chloride ($C_6H_5CH_2Cl$) or Chlorobenzene?
Student thinks: "Both have benzene rings, so they are similar."
Or thinks Chlorobenzene is $2^\circ$ (it's not) and Benzyl is $1^\circ$.
Look at the Carbocation!
Benzyl Cation: Stabilized by Resonance ($Ph-CH_2^+$). Very fast SN1.
Phenyl Cation: Positive charge on $sp^2$ carbon. Highly unstable. No SN1.
Answer: Benzyl Chloride is much faster.
Confess Your Sins!
"Organic reactions have conditions. If you ignore them, they will ignore you."
Did one of these catch you? Or do you have a different horror story from your last exam?
Scroll down to the comments section below and tell us:
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