Electrophilic Addition to Alkenes: Mechanism Detail

Electrophilic addition is the cornerstone reaction of alkenes, transforming simple unsaturated hydrocarbons into a vast array of functionalized compounds. Understanding its detailed mechanism—the step-by-step dance of electrons—is essential for predicting products, explaining regioselectivity, and rationalizing stereochemical outcomes. This deep dive into the movement of curly arrows, the stability of intermediates, and the three-dimensional fate of molecules will equip you to master this fundamental transformation.

The Curly Arrow Mechanism: A Universal Two-Step Dance

At its heart, electrophilic addition to alkenes follows a consistent two-step pattern involving an electrophile (an electron-loving species) and the alkene’s pi bond ($\pi$ bond), which is a region of high electron density. The mechanism is visualized using curly arrows, which show the movement of electron pairs. The first step is always rate-determining and creates a crucial intermediate: a carbocation.

Consider the addition of hydrogen bromide (HBr) to a symmetrical alkene like ethene. In step one, the electron-rich $\pi$ bond attacks the partially positive hydrogen of HBr. A curly arrow shows this attack, while another arrow shows the breaking of the H-Br bond. This results in a new C-H bond and leaves behind a bromide ion ($B{r}^{-}$) and a carbocation—in this case, a primary carbocation on the original ethene carbon. Step two is a fast nucleophilic attack: the bromide ion donates a pair of electrons to the positively charged carbon, forming the final addition product, bromoethane.

For asymmetrical alkenes like propene, this first step becomes decisive for determining the product's structure, leading us directly to Markovnikov's rule.

Markovnikov's Rule and Carbocation Stability

When HBr adds to an unsymmetrical alkene like propene, two different carbocations could theoretically form. Attack on one end gives a secondary carbocation, while attack on the other gives a less stable primary carbocation. Markovnikov's rule (often stated as "the rich get richer") predicts that the hydrogen atom will add to the carbon of the double bond that already has more hydrogen atoms. The true explanation lies not in the rule itself, but in the stability of the carbocation intermediate.

Carbocation stability increases with the degree of alkyl substitution: tertiary ($3^{\circ }$) > secondary ($2^{\circ }$) > primary ($1^{\circ }$) > methyl. This is due to hyperconjugation, where electrons in adjacent C-H or C-C sigma bonds can delocalize into the empty p-orbital of the carbocation, stabilizing the positive charge. In propene, initial protonation at the less substituted carbon creates a more stable secondary carbocation. Protonation at the more substituted end would create an unstable primary carbocation. The reaction pathway follows the lowest energy route through the most stable intermediate.

Therefore, for HBr and similar acids like $H_{2}S{O}_4$ (which adds water after forming a carbocation), the major product is always the one where the halogen (or other group) ends up on the more substituted carbon. The mechanism for adding $H_{2}S{O}_4$ followed by water is identical in its initial step: the alkene acts as a base and protonates, forming a carbocation. This carbocation is then attacked by the nucleophile, which in dilute acid is water, ultimately yielding an alcohol.

The Bromonium Ion and Stereochemistry of $B{r}_2$ Addition

The addition of bromine ($B{r}_2$) follows the same broad two-step pattern but with a critical stereochemical twist. The $\pi$ bond attacks one bromine atom, polarizing the $Br-Br$ bond. Using curly arrows, we show the $\pi$ bond attacking one $Br$, while the $Br-Br$ bond breaks. This does not create a traditional open-chain carbocation. Instead, the first bromine, now partially positive, can form a bond with both carbon atoms simultaneously, creating a cyclic, three-membered ring intermediate called a bromonium ion.

This cyclic bromonium ion is key. It explains the observed stereospecific anti addition of bromine. Because the bromide ion ($B{r}^{-}$) nucleophile must attack the bromonium ion from the side opposite to the ring, the two bromine atoms add to opposite faces of the original double bond. If you start with a cis- or trans-alkene, this leads to specific stereoisomeric products (racemic mixtures of enantiomers or meso compounds, respectively). The evidence for this mechanism is overwhelming: open carbocations would lead to mixture of stereoisomers and possible rearrangements, which are not observed in simple bromine additions. The isolation of stable bromonium ion salts in some cases provides direct proof.

The Peroxide Effect: Anti-Markovnikov Addition

Markovnikov's rule has a famous exception: the addition of HBr (but not HCl or HI) to alkenes in the presence of organic peroxides or light. This results in anti-Markovnikov addition, where the bromine ends up on the less substituted carbon. This occurs because peroxides initiate a radical chain reaction, completely bypassing the ionic carbocation mechanism.

The peroxide breaks homolytically to form alkoxy radicals, which then abstract a hydrogen from HBr, generating a bromine radical ($Br•$). This bromine radical adds to the alkene. Crucially, the radical adds in a way that forms the more stable radical intermediate: a secondary carbon radical is more stable than a primary one. Therefore, the bromine radical adds to the less substituted end of the alkene, creating a carbon radical on the more substituted carbon. This radical then abstracts a hydrogen from another HBr molecule, generating the anti-Markovnikov product and a new $Br•$ to continue the chain. This mechanism highlights how a change in conditions (peroxides) can switch the mechanism from ionic to radical, altering the regiochemical outcome entirely.

Common Pitfalls

1. Forgetting the Two Steps: A common error is drawing a concerted, one-step mechanism where both bonds form simultaneously. Always remember the carbocation (or bromonium ion) intermediate. The curly arrows for step one must create the intermediate, and step two must consume it.
2. Misapplying Markovnikov's Rule: The rule applies to the addition of the proton (H+) from HX, not the halogen. Students often mis-predict by placing the H on the more substituted carbon. Remember: "H adds to the less substituted carbon" is a correct, modern restatement based on carbocation stability.
3. Incorrect Curly Arrows: Arrows must show the movement of an electron pair. An arrow from the $\pi$ bond to the H of HBr is correct. An arrow from the $\pi$ bond between the two carbons to the Br atom is incorrect—the electrons move from the bond to an atom.
4. Confusing the Peroxide Effect Scope: A major mistake is assuming HCl or HI also give anti-Markovnikov products under peroxide conditions. They do not, because the H-Cl and H-I bonds are too strong to be broken easily in the radical propagation step, halting the chain reaction. The effect is unique to HBr.

Summary

• Electrophilic addition to alkenes is a two-step ionic mechanism involving initial attack by the $\pi$ bond on an electrophile to form a carbocation (or cyclic ion) intermediate, followed by nucleophilic attack.
• Markovnikov's rule is explained by the formation of the most stable carbocation intermediate. Stability follows the order tertiary > secondary > primary due to hyperconjugation.
• Addition of $B{r}_2$ proceeds via a cyclic bromonium ion, which forces anti addition of the two bromine atoms, providing clear stereochemical evidence against a simple open carbocation.
• The peroxide effect is an exception for HBr, causing anti-Markovnikov addition via a radical chain mechanism. The key step is the addition of a bromine radical to form the more stable carbon radical intermediate.
• Mastering curly arrow notation for these mechanisms is non-negotiable for accurately predicting and explaining the products of these reactions.