The Mistake Bank
Chapter 13: Hydrocarbons
Reaction mechanisms are logical, until you forget the catalyst.
The "Universal" Peroxide Effect
AlkenesScenario: $CH_3-CH=CH_2 + HCl$ in the presence of Peroxide ($R_2O_2$).
Student sees "Peroxide" and thinks "Anti-Markovnikov".
Attaches $Cl$ to the terminal carbon.
Product: 1-Chloropropane. (Wrong!)
Peroxide Effect is Unique to HBr!
The H-Cl bond is too strong to be broken by free radicals generated by peroxide.
Reaction follows standard Markovnikov Addition.
Major Product: 2-Chloropropane.
Friedel-Crafts Rearrangement
BenzeneScenario: Benzene + n-Propyl Chloride ($CH_3CH_2CH_2Cl$) + Anhydrous $AlCl_3$.
Student simply attaches the propyl group where the Cl was.
Product: n-Propylbenzene.
(Carbocation stability ignored!)
Look for Hydride Shift!
1. $AlCl_3$ removes $Cl^-$ creating a primary carbocation ($CH_3CH_2C^+H_2$).
2. Unstable $1^\circ$ carbocation rearranges via 1,2-hydride shift to stable $2^\circ$ carbocation ($CH_3C^+HCH_3$).
Major Product: Isopropylbenzene (Cumene).
Ozonolysis: Oxidative vs Reductive
AlkenesScenario: $CH_3-CH=CH-CH_3$ reacted with $O_3$, followed by $H_2O$ (Zinc is ABSENT).
Student breaks the double bond and adds oxygen.
Product: 2 moles of Ethanal ($CH_3CHO$).
(This only happens if Zn is present!)
Check for Zinc Dust!
With Zn (Reductive): Stops at Aldehyde/Ketone.
Without Zn (Oxidative): The $H_2O_2$ formed oxidizes aldehydes further into Carboxylic Acids.
Major Product: Ethanoic Acid ($CH_3COOH$).
Light vs Dark Chlorination
BenzeneScenario: Benzene + $3Cl_2$ in the presence of Sunlight ($h\nu$).
Student assumes electrophilic substitution.
Product: Chlorobenzene + $HCl$.
(That requires a Lewis Acid and darkness!)
Light favors Free Radicals!
In sunlight, Chlorine radicals add across the double bonds (Addition Reaction).
The ring loses aromaticity.
Product: Benzene Hexachloride (BHC) / Gammaxene.
Preparing Propane (Wurtz)
AlkanesScenario: Can you prepare pure Propane ($C_3H_8$) using Wurtz reaction?
Student says: "Yes, mix Methyl chloride + Ethyl chloride + Na."
$$ CH_3Cl + C_2H_5Cl \xrightarrow{Na} C_3H_8 $$
Avoid Odd Carbons!
The reactants will also react with themselves:
- $CH_3 + CH_3 \to \text{Ethane}$
- $C_2H_5 + C_2H_5 \to \text{Butane}$
- $CH_3 + C_2H_5 \to \text{Propane}$
You get a mixture that is hard to separate. Wurtz is best for Symmetric alkanes.
Hydration of Alkynes
AlkynesScenario: Propyne ($CH_3-C \equiv CH$) + $H_2O$ ($Hg^{2+}/H^+$).
Student adds OH to terminal carbon (Anti-Markovnikov) or stops at Enol.
Product: Propanal ($CH_3CH_2CHO$).
Markovnikov + Tautomerism
1. OH adds to the middle carbon (Markovnikov).
2. Enol forms: $CH_3-C(OH)=CH_2$.
3. Tautomerizes to stable Keto form.
Product: Acetone (Propanone).
Note: Only Ethyne yields an Aldehyde.
Confess Your Sins!
"Hydrocarbons are the fuel of life, but getting the mechanism wrong will burn your grade."
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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