The Mistake Bank
Class 12 - Chapter 11: Alcohols, Phenols & Ethers
Don't let the mechanisms dehydrate your brain.
Williamson Synthesis Failure
EthersScenario: Prepare t-Butyl Methyl Ether using Williamson Synthesis.
$CH_3ONa + (CH_3)_3C-Br$
Student performs simple $S_N2$ substitution.
Product: t-Butyl Methyl Ether.
(Wrong! Tertiary halide = Elimination!)
Halide MUST be Primary!
Tertiary alkyl halides undergo elimination with strong bases (alkoxides).
Product: Isobutylene (Alkene).
Correct way: Use Sodium t-Butoxide + Methyl Bromide.
Reaction with HI (Anisole)
Cleavage of EthersScenario: Reaction of Anisole ($C_6H_5-O-CH_3$) with Hydroiodic Acid ($HI$).
Student breaks the bond randomly or attacks the benzene ring.
Product: Iodobenzene ($C_6H_5-I$) + Methanol ($CH_3OH$).
The "Aryl-Oxygen" Bond is Strong!
The $C_6H_5-O$ bond has partial double bond character due to resonance. It does not break.
The weaker Alkyl-Oxygen bond breaks.
Products: Phenol ($C_6H_5OH$) + Methyl Iodide ($CH_3I$).
Oxidation by Copper
DehydrogenationScenario: Pass vapors of t-Butyl Alcohol over heated Copper ($573K$).
Student thinks: "Cu converts alcohol to aldehyde/ketone."
Product: A ketone (somehow).
(Tertiary alcohols have no $\alpha$-H to remove!)
Dehydration instead of Dehydrogenation!
Since $3^\circ$ alcohols lack an $\alpha$-Hydrogen, they cannot be oxidized to ketones.
Instead, they undergo elimination (Dehydration).
Product: Isobutylene (Alkene).
Acidity: Phenol vs Alcohol
Acidic NatureScenario: Compare acidity of Phenol and Ethanol.
Student thinks: "Alkyl group releases electrons (+I), making O-H bond easier to break."
Conclusion: Ethanol is more acidic.
(Wrong logic!)
Resonance Stabilization!
The Phenoxide ion formed after losing $H^+$ is stabilized by resonance (negative charge disperses into the ring).
The Ethoxide ion is destabilized by the +I effect of the ethyl group.
Answer: Phenol is much more acidic.
Lucas Test Reactivity
IdentificationScenario: Which alcohol reacts fastest with Lucas Reagent ($Conc. HCl + ZnCl_2$)? $1^\circ$, $2^\circ$, or $3^\circ$?
Student thinks: "$1^\circ$ is less bulky (less steric hindrance), so it reacts fastest."
(Confusing $S_N2$ with $S_N1$ mechanism.)
Follow Carbocation Stability!
The reaction proceeds via a Carbocation intermediate ($S_N1$ character).
Since $3^\circ$ Carbocation is most stable, $3^\circ$ alcohols react instantly (turbidity appears immediately).
$1^\circ$ alcohols do not react at room temperature.
Nitration of Phenol
Electrophilic SubstitutionScenario: Phenol + Concentrated $HNO_3$.
Student assumes standard mixture of o-nitrophenol and p-nitrophenol.
Concentration Matters!
Dilute $HNO_3$: Mixture of ortho and para nitrophenol.
Concentrated $HNO_3$: Exhaustive nitration occurs at all activated positions (2,4,6).
Product: Picric Acid (2,4,6-Trinitrophenol).
Confess Your Sins!
"Did you break the wrong bond in the ether? Or dehydrate when you meant to oxidize?"
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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