7.7 Electrophilic Addition Reactions of Alkenes

Electrophilic Addition Reactions of Alkenes

Electrophilic addition is the most characteristic reaction of alkenes. The electron-rich $\pi$ bond acts as a nucleophile, attacking an electron-poor species (the electrophile), which triggers a sequence of bond-breaking and bond-forming steps that add new atoms across the double bond. Understanding this mechanism unlocks the logic behind regioselectivity, stereochemistry, and energy diagrams for a whole family of reactions.

Mechanism of Electrophilic Addition

The $\pi$ electrons in an alkene are loosely held and sit above and below the plane of the double bond, making them an easy target for electrophiles. The general mechanism has two key steps (with an optional third):

1. Electrophilic attack and carbocation formation. The $\pi$ electrons donate into the electrophile (e.g., the $H$ of $HBr$), forming a new $\sigma$ bond to one of the alkene carbons. The other carbon loses its share of the $\pi$ electrons and becomes a carbocation intermediate. This carbocation forms at the carbon that can best stabilize the positive charge (tertiary > secondary > primary > methyl).

2. Nucleophilic capture. A nucleophile (e.g., $Br^-$, $H_2O$) donates a lone pair to the electron-deficient carbocation, forming a second new $\sigma$ bond and completing the addition.

3. Proton transfer (if needed). When the nucleophile is a neutral molecule like water, the initial product carries a positive charge. A proton transfer to solvent or base restores a neutral product (this is how acid-catalyzed hydration gives an alcohol rather than an oxonium ion).

Regioselectivity and Markovnikov's Rule

Because step 1 is where the carbocation forms, the regiochemistry of the whole reaction is set at that point. Markovnikov's rule states that the electrophile (usually $H$) adds to the less substituted carbon of the double bond, placing the positive charge on the more substituted carbon. This isn't an arbitrary rule; it follows directly from carbocation stability. A tertiary carbocation is stabilized by hyperconjugation and inductive effects from three alkyl groups, so the pathway through that intermediate has a lower activation energy.

Energy Diagrams for Addition Reactions

An energy diagram plots free energy on the y-axis against reaction progress on the x-axis. For a two-step electrophilic addition:

• Reactants sit at the starting energy level on the left.
• First transition state ($TS_1$) is the highest-energy point. It corresponds to the moment the $\pi$ bond is partially broken and the new bond to the electrophile is partially formed. Because this step generates the carbocation, it is the rate-determining step, and its activation energy ($E_a$) controls how fast the reaction proceeds.
• Carbocation intermediate appears as a local energy minimum (a valley) between the two transition states. A more stable carbocation sits in a deeper valley, meaning a lower $E_a$ for step 1 and a faster reaction.
• Second transition state ($TS_2$) is typically lower in energy than $TS_1$ because nucleophilic attack on a carbocation is fast.
• Products sit at the final energy level. The overall enthalpy change ($\Delta H$) is the difference between the energy of the products and the reactants. Most electrophilic additions to alkenes are exothermic because two strong $\sigma$ bonds replace one $\pi$ bond and one $\sigma$ bond.

Common Electrophiles for Alkenes

Hydrogen halides ($HX$: $HF, HCl, HBr, HI$)

The $H\text{–}X$ bond is polarized with hydrogen as the electrophilic end. The $\pi$ electrons attack $H$, and the halide ion acts as the nucleophile in step 2. Reactivity order is $HI > HBr > HCl > HF$ because weaker $H\text{–}X$ bonds break more easily. These reactions follow Markovnikov's rule and produce alkyl halides.

Water with acid catalyst (acid-catalyzed hydration)

A strong acid (e.g., $H_2SO_4$) protonates the alkene to form a carbocation. Water then attacks the carbocation as the nucleophile, and a final proton transfer yields an alcohol. The acid is regenerated, so it's truly catalytic. Products follow Markovnikov's rule ($OH$ ends up on the more substituted carbon).

Halogens ($X_2$: $Cl_2, Br_2$)

Halogen addition works differently from the reactions above. As $Br_2$ approaches the electron-rich $\pi$ bond, it becomes polarized, and the alkene donates into the nearer bromine. Instead of an open carbocation, a three-membered bromonium ion (cyclic halonium ion) forms, with the halogen bridging both carbons. The halide ion ($Br^-$) then attacks from the opposite face of the ring in an $S_N2$-like step. The result is a vicinal dihalide with anti addition stereochemistry (the two halogens end up on opposite faces of the original double bond).

Other electrophilic reagents you may encounter include $H_2SO_4$ (gives alkyl hydrogen sulfates), mercury(II) acetate $Hg(OAc)_2$ (oxymercuration-demercuration, a milder route to Markovnikov alcohols), and borane $BH_3$ (hydroboration, which gives anti-Markovnikov addition of water after oxidation).

Reaction Mechanism and Stereochemistry

The stereochemical outcome of an addition reaction depends on the type of intermediate formed:

• Open carbocation intermediate (e.g., $HBr$ addition): The carbocation is $sp^2$-hybridized and planar, so the nucleophile can attack from either face. This leads to a mixture of syn and anti addition, and if a new stereocenter forms, you typically get a racemic mixture.
• Cyclic halonium ion intermediate (e.g., $Br_2$ addition): The bridged ring blocks one face of the molecule, forcing the nucleophile to attack from the opposite side. This gives exclusively anti addition. If the alkene is cyclic, the two halogen atoms end up in a trans (diaxial) arrangement.

When predicting products, always ask two questions: (1) Where does the electrophile add? (regioselectivity, governed by Markovnikov's rule and carbocation stability), and (2) From which face does the nucleophile attack? (stereochemistry, governed by the type of intermediate).