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Chemical Properties of Alkenes | chemca

Hydrocarbons

Chemical Properties of Alkenes

Reactions: Addition, Oxidation, and Polymerization.

By chemca Team • Updated Jan 2026

Alkenes are highly reactive due to the presence of the pi ($\pi$) bond. The loosely held $\pi$-electrons attract electrophiles, making Electrophilic Addition the most characteristic reaction of alkenes.

1. Addition of Hydrogen Halides (HX)

Markovnikov's Rule

When an unsymmetrical alkene reacts with HX (HCl, HBr, HI), the negative part of the addendum ($X^-$) attaches to the carbon atom possessing fewer hydrogen atoms.

$$ CH_3-CH=CH_2 + HBr \xrightarrow{\text{Major}} CH_3-CH(Br)-CH_3 $$

Product: 2-Bromopropane (Major)

Mechanism Note: The reaction proceeds via the formation of the most stable Carbocation intermediate ($3^\circ > 2^\circ > 1^\circ$). Rearrangements (Hydride/Methyl shift) are possible.

Anti-Markovnikov Addition (Peroxide Effect)

Specifically for HBr in the presence of Organic Peroxide ($R_2O_2$), the addition occurs opposite to Markovnikov's rule (Kharasch Effect).

$$ CH_3-CH=CH_2 + HBr \xrightarrow{(PhCO)_2O_2} CH_3-CH_2-CH_2-Br $$

Product: 1-Bromopropane (Major) | Mechanism: Free Radical

Note: This effect is observed ONLY with HBr, not with HCl or HI.

2. Addition of Water (Hydration)

Alkenes react with water in the presence of acid catalysts (dil. $H_2SO_4$) to form alcohols according to Markovnikov's rule.

$$ CH_3-CH=CH_2 + H_2O \xrightarrow{H^+} CH_3-CH(OH)-CH_3 $$

3. Oxidation Reactions

A. With Cold Dilute KMnO4 (Bayer's Reagent)

Alkenes react with cold, dilute, alkaline $KMnO_4$ to form Vicinal Glycols (Syn-hydroxylation). The purple color of $KMnO_4$ decolorizes, serving as a test for unsaturation.

$$ CH_2=CH_2 + H_2O + [O] \xrightarrow{dil. KMnO_4, 273K} \underset{\text{Ethane-1,2-diol}}{CH_2(OH)-CH_2(OH)} $$

B. Ozonolysis

Alkenes react with Ozone ($O_3$) to form an Ozonide, which upon cleavage with $Zn/H_2O$ yields carbonyl compounds (Aldehydes/Ketones). This reaction locates the position of the double bond.

$$ CH_3-CH=CH_2 \xrightarrow{1. O_3} \text{Ozonide} \xrightarrow{2. Zn/H_2O} CH_3CHO + HCHO $$

C. With Acidic KMnO4 or K2Cr2O7

Strong oxidation cleaves the C=C bond completely.

$=CH_2$ converts to $CO_2 + H_2O$.
$=CHR$ converts to Carboxylic Acid ($RCOOH$).
$=CR_2$ converts to Ketone ($R_2C=O$).

4. Polymerization

At high temperature and pressure, alkene molecules (monomers) join together to form giant molecules (polymers).

$$ n(CH_2=CH_2) \xrightarrow{\text{High Temp/Press}} -(CH_2-CH_2)_n- $$

Product: Polythene

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