Chemical Properties of Alkenes
Master the reactivity of the $\pi$ cloud. Decode Electrophilic Addition, the Hydration Triad, and the definitive rules for Ozonolysis cleavage.
Module Focus: The Nucleophilic $\pi$ Bond
Alkenes are characterized by the loosely held $\pi$ electrons extending above and below the plane of the molecule. This electron-rich cloud makes alkenes highly attractive to Electrophiles ($E^+$). Consequently, the signature reaction of alkenes is Electrophilic Addition. The key to mastering this topic is knowing exactly which intermediate forms during the reaction, as this dictates the regioselectivity and stereochemistry of the final product.
1. Addition of Hydrogen Halides ($HX$)
Adding $HCl, HBr, \text{ or } HI$ to an unsymmetrical alkene yields two possible products. The major product depends entirely on the reaction conditions and the intermediate formed.
Both pathways are driven by intermediate stability: $2^\circ$ Carbocation vs. $2^\circ$ Free Radical.
The Anti-Markovnikov (Kharasch) effect occurs ONLY with HBr in the presence of peroxides.
2. Addition of Water (The Hydration Triad)
Converting an alkene into an alcohol can be done via three different pathways. Memorizing the differences in their regiochemistry and stereochemistry is non-negotiable for NEET.
| Reagent / Method | Intermediate | Regiochemistry | Key Feature / Trap |
|---|---|---|---|
| 1. Acid-Catalyzed ($H_2O / H^+$) |
Carbocation | Markovnikov | Rearrangements Occur! (Shifts) |
| 2. Oxymercuration-Demercuration (OMDM) ($Hg(OAc)_2/H_2O$, then $NaBH_4$) |
Cyclic Mercurinium Ion | Markovnikov | No Rearrangements! (Anti-addition) |
| 3. Hydroboration-Oxidation (HBO) ($B_2H_6 / THF$, then $H_2O_2 / OH^-$) |
4-Membered Cyclic TS | Anti-Markovnikov | No Rearrangements (Syn-addition) |
3. Addition of Halogens ($X_2$)
Adding $Br_2$ or $Cl_2$ (dissolved in an inert solvent like $CCl_4$) yields a vicinal dihalide. The reddish-brown color of $Br_2$ rapidly discharges, serving as a classic test for unsaturation.
Unlike HX addition, halogenation does NOT form a standard planar carbocation. Instead, the lone pairs on the halogen form a bridged, 3-membered Cyclic Halonium Ion.
Consequence 2: The second halide ion must attack from the opposite face due to steric hindrance, forcing strict ANTI-ADDITION.
4. Oxidation Reactions
Baeyer's Reagent
Forms a vicinal diol (glycol) via syn-addition. The purple color of $KMnO_4$ is discharged to a brown precipitate of $MnO_2$.
Oxidative Cleavage
The double bond is completely cleaved.
- Terminal $=CH_2 \rightarrow CO_2 + H_2O$
- $=CHR \rightarrow$ Carboxylic Acid ($RCOOH$)
- $=CR_2 \rightarrow$ Ketone ($R_2C=O$)
Ozonolysis: The "Scissor" Trick
Reductive ozonolysis ($O_3$ followed by $Zn/H_2O$) cleaves the double bond to form aldehydes or ketones. The presence of Zinc prevents further oxidation of aldehydes into carboxylic acids.
NEET Grand Test: Alkene Reactions
15 High-Yield Questions testing the Hydration triad, ozonolysis reverse-engineering, and stereochemistry traps.
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