Electrophilic and Free Radical Addition
Alkenes contain a carbon–carbon double bond, and this double bond works like the “reactive center” of the molecule. Most reactions involving alkenes happen at this double bond.
There are two major types of reactions:
- Addition reactions – double bond breaks and new atoms add across it.
- Reactions at the allylic position – double bond stays but still influences the reaction.
What is an Electrophile?
An electrophile is something that “loves electrons.” It is electron-poor and wants to accept electrons. Electrophiles are usually positively charged or neutral but electron-deficient.
Hydrogenation
Hydrogenation means adding hydrogen across a double bond. This reaction releases heat, known as the heat of hydrogenation. More heat released means the alkene was less stable.
Stability of Alkenes
Alkenes with more alkyl groups attached to the double-bonded carbon are more stable. So, stability order is often:
Tetra > Tri > Di > Mono-substituted alkenes
Markovnikov’s Rule
When an unsymmetrical reagent like HCl or HBr adds to an unsymmetrical alkene, the hydrogen goes to the carbon with more hydrogen atoms already. The other part of the reagent goes to the more substituted carbon.
Addition of Hydrogen Halides (HX)
Alkenes react with HCl, HBr, or HI to form alkyl halides. Their reactivity decreases as follows:
HI > HBr > HCl > HF
Mechanism of HX Addition
- HX ionises to H⁺ and X⁻.
- H⁺ attacks the double bond to form a carbocation (more stable carbocation forms faster).
- X⁻ attacks the carbocation to give the product.
Peroxide Effect (Anti-Markovnikov Addition)
Only HBr shows this special behaviour:
- In the presence of peroxides → Anti-Markovnikov addition (free radical pathway)
- In the absence of peroxides → Markovnikov addition (ionic pathway)
Peroxide effect was explained by Kharasch and Mayo. Peroxides create free radicals that change the direction of addition.
Electrophilic Addition – General Mechanism
- Reagent breaks into electrophile and nucleophile.
- Electrophile attacks the double bond, forming a carbocation.
- Nucleophile attacks the carbocation to form the final product.
Rearrangement
Since carbocations can rearrange (1,2-shifts), products often come from more stable carbocations.
Addition of Halogens
Cl₂ and Br₂ add across alkenes to form vicinal dihalides. The reaction involves formation of a halonium ion, so rearrangements generally do not occur.
Halohydrin Formation
Alkenes react with Cl₂ or Br₂ in water to form halohydrins. Water acts as the nucleophile, attaching to the more substituted carbon.
Free Radical Additions
In the presence of peroxides, HBr adds to alkenes by a free-radical mechanism. This gives anti-Markovnikov products. Steps include:
- Peroxide decomposition to radicals.
- Radical abstracts H from HBr → bromine radical forms.
- Bromine radical adds to alkene → alkyl radical forms.
- Alkyl radical reacts with HBr → product + new bromine radical.
Why Only HBr Shows Peroxide Effect?
- HCl bond is too strong → radicals cannot break it.
- HI forms radicals that recombine easily → reaction stops.
- HBr is “just right” → bond breaks easily, radicals survive.
Orientation in Free Radical Addition
Free radicals follow the stability order:
3° > 2° > 1° > CH₃
Therefore, bromine adds first to form the most stable radical.
Carbene Addition (Short Note)
Carbenes are highly reactive species with a divalent carbon. They can add across double bonds to form cyclopropane derivatives. This is another example of an addition reaction at alkenes.
Detailed Notes:
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PATH: PHARMD/PHARMD NOTES/ PHARMD FIRST YEAR NOTES/ ORGANIC CHEMISTRY/ PHARMACEUTICAL ORGANIC CHEMISTRY/ELECTROPHILLIC AND FREE RADICALS ADDITION.