The Mistake Bank
Chapter 12: General Organic Chemistry
Organic isn't about memorization. It's about electron flow.
The "Positive Means Electrophile" Myth
Reaction MechanismScenario: Which of these is an electrophile?
$NH_4^+$, $H_3O^+$, $NO_2^+$, $Na^+$
Student thinks: "Electrophiles love electrons. Electrons are negative. So, anything positive is an electrophile."
Student selects $NH_4^+$ or $Na^+$.
Look for Empty Orbitals!
An electrophile must be able to accept an electron pair.
- $NH_4^+$: Octet is complete (8e). No empty orbital. Not electrophile.
- $Na^+$: Octet complete. Inert gas config. Not electrophile (in organic sense).
- $NO_2^+$: N has incomplete octet. True Electrophile.
Carbocation Rearrangement
IntermediatesScenario: Reaction of $3,3$-dimethyl-1-butene with $HCl$. Identify the major product.
Student adds $H^+$ according to Markovnikov's rule to form a $2^\circ$ carbocation.
Adds $Cl^-$ immediately.
Result: 2-chloro-3,3-dimethylbutane. (Minor product!)
Watch for Methyl Shift!
1. $2^\circ$ Carbocation forms.
2. A methyl group shifts from the adjacent carbon (1,2-methyl shift).
3. A more stable $3^\circ$ Carbocation forms.
Major Product: 2-chloro-2,3-dimethylbutane.
Basicity of Amides
Acid & Base StrengthScenario: Compare basic strength of Aniline ($Ph-NH_2$) and Acetamide ($CH_3-CO-NH_2$).
Student thinks: "Resonance stabilizes the molecule."
Assumes since both have resonance, they might be similar, or Acetamide is stronger due to $+I$ of $CH_3$.
Availability of Lone Pair!
Basic strength depends on how easily N can donate its lone pair.
In Amides, the lone pair is involved in strong resonance with the carbonyl oxygen ($C=O$). It is not available.
Amides are extremely weak bases (practically neutral). Aniline is stronger.
Heat of Hydrogenation (HOH)
HyperconjugationScenario: Compare HOH of 1-butene and 2-butene.
Student thinks: "2-butene is more stable due to more $\alpha$-hydrogens."
Conclusion: "More stable means Higher Heat of Hydrogenation."
(Wrong relationship!)
Stability $\propto \frac{1}{\text{HOH}}$
A more stable alkene has lower energy to begin with, so it releases LESS heat when hydrogenated.
2-butene is more stable (6 $\alpha$-H), so it has Lower HOH than 1-butene.
The Ortho Effect
AcidityScenario: Compare acidity of Benzoic Acid and o-Methyl Benzoic Acid.
Student sees the Methyl group ($CH_3$).
Applies $+I$ effect (Electron Releasing).
Concludes: "Methyl destabilizes anion, so Ortho is Weaker."
The Ortho Effect Rules All!
In Benzoic acid derivatives, any group at the ortho position (electron donating OR withdrawing) usually increases acidity due to steric inhibition of resonance (SIR).
The ortho-isomer is the Strongest.
Aromaticity of Cation vs Anion
Hückel's RuleScenario: Which is aromatic: Cyclopentadienyl Cation or Anion?
Student sees the ring and conjugation in both.
Guesses or gets confused by electron counting.
Count the Pi Electrons ($4n+2$)
Cation ($+$): $4 \pi$ electrons (2 double bonds). Fits $4n$. Anti-aromatic (Highly Unstable).
Anion ($-$): $6 \pi$ electrons (2 double bonds + 1 lone pair). Fits $4n+2$. Aromatic (Stable).
Confess Your Sins!
"Organic Chemistry is 1% memorization and 99% understanding why electrons move."
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:
No comments:
Post a Comment