CHEMCA
EXAM MASTER REVIEW SHEET
Hydrocarbons: Alkenes (Olefins)
1. General Properties & Stability
Unsaturated hydrocarbons containing at least one $C=C$ double bond (Hybridization: $sp^2$).
Stability $\propto$ Number of $\alpha$-hydrogens (Hyperconjugation).
Tetrasubstituted > Trisubstituted > Disubstituted > Monosubstituted
Energy released when 1 mole of alkene is hydrogenated.
$\Delta H_{hydro} \propto \frac{1}{\text{Stability of Alkene}}$
2. Important Preparations
Dehydration of Alcohols
Reagent: Conc. $H_2SO_4$ or $H_3PO_4$ / $\Delta$.
• Follows Saytzeff Rule (More substituted alkene is major).
• Carbocation rearrangement possible.
Dehydrohalogenation
Reagent: Alcoholic $KOH$.
• Mechanism: $E2$ (Anti-periplanar transition state).
• Hofmann Product if base is bulky (e.g., t-BuOK).
3. Electrophilic Additions (The Core)
"The negative part of the addendum attaches to the carbon with fewer hydrogen atoms." (i.e., via the most stable carbocation).
Via Carbocation. Rearrangement possible.
Note: $HBr + \text{Peroxide}$ follows Anti-Markovnikov (Free Radical Mechanism).
Via Cyclic Halonium Ion.
Stereochemistry: ANTI-Addition.
Test for unsaturation: Bromine water decolourisation.
4. Hydration & Oxidative Cleavage
| Reaction | Reagents | Regioselectivity | Stereochemistry |
|---|---|---|---|
| Acid Hydration | $H^+ / H_2O$ | Markovnikov (Rearranges) | Mixed |
| Oxymercuration | $Hg(OAc)_2, H_2O$ then $NaBH_4$ | Markovnikov (No Rearrange) | ANTI |
| Hydroboration | $B_2H_6, THF$ then $H_2O_2/OH^-$ | Anti-Markovnikov | SYN |
Reductive ($Zn/H_2O$)
Aldehydes & Ketones
Oxidative ($H_2O_2$)
Acids & Ketones
5. Addition Stereochemistry (Mnemonic)
// The CAR-TAM Rule
• Cis + Anti \(\rightarrow\) Racemic
• Trans + Anti \(\rightarrow\) Meso
// The SYN Addition Rules
• Cis + Syn \(\rightarrow\) Meso (CSM)
• Trans + Syn \(\rightarrow\) Racemic (TSR)
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