Alkene Reactions Addition and Polymerization

For a pre-med student, mastering alkene reactions is not just an organic chemistry requirement; it’s foundational to understanding biochemistry, pharmacology, and metabolic pathways. These reactions, centered on the reactive carbon-carbon double bond, showcase predictable patterns of reactivity and regioselectivity that are consistently tested on the MCAT. A deep grasp of the mechanisms—especially the pivotal role of carbocation intermediates—will allow you to predict products accurately, a critical skill for the Chemical and Physical Foundations section.

The Framework: Electrophilic Addition

The pi bond of an alkene is a region of high electron density, making it attractive to electron-deficient species called electrophiles. This interaction initiates the most characteristic pathway for alkenes: electrophilic addition. The general mechanism follows two key steps. First, the alkene’s pi electrons attack the electrophile, forming a new bond and creating a carbocation intermediate. Second, a nucleophile (often the conjugate base of the electrophile or a solvent molecule) rapidly attacks the positively charged carbocation. Understanding this two-step process is crucial because it explains why certain products dominate and why rearrangements can occur.

Key Addition Reactions and Their Rules

Different electrophile-nucleophile pairs lead to distinct, high-yield transformations. The regiochemistry—where the new atoms add—is governed by specific rules you must know.

Hydrohalogenation: This is the addition of H-X (e.g., HCl, HBr). The hydrogen acts as the electrophile. The reaction follows Markovnikov's rule, which states that the hydrogen atom adds to the carbon of the double bond that already has more hydrogen atoms. The halide adds to the more substituted carbon. The reason is stability: the initial protonation forms the most stable carbocation possible (tertiary > secondary > primary). For example, adding HBr to propene gives 2-bromopropane, not 1-bromopropane.

Hydration: The addition of water ($H_{2}O$) in the presence of an acid catalyst (like $H_{2}S{O}_4$) follows an identical Markovnikov pathway. The electrophile is $H^{+}$ from the acid, forming a carbocation. Water then acts as the nucleophile, and a final deprotonation yields the alcohol. This is a direct route from an alkene to an alcohol.

Halogenation: Here, a halogen like $B{r}_2$ or $C{l}_2$ is the electrophile. This reaction proceeds via a three-membered cyclic halonium ion intermediate instead of a free carbocation, which leads to anti addition of the two halogen atoms across the double bond. No Markovnikov considerations apply because the two atoms added are identical.

Hydroboration-Oxidation: This two-step sequence is the premier method for anti-Markovnikov addition of water. First, borane ($B{H}_3$) adds across the double bond. The boron atom, being electrophilic, adds to the less substituted carbon due to steric factors and partial positive charge development in the transition state. Subsequent oxidation with hydrogen peroxide ($H_2{O}_2$) and a base replaces the boron with a hydroxyl group ($-OH$). The net result is an alcohol with the $-OH$ on the less substituted carbon, opposite to the product of acid-catalyzed hydration.

Catalytic Hydrogenation: This is a reduction of the double bond. In the presence of a metal catalyst (like Pd, Pt, or Ni), hydrogen gas ($H_2$) adds in a syn fashion (both atoms add to the same face of the alkene). This reaction is not electrophilic addition; it occurs via adsorption onto the metal surface. It’s a critical method for saturating double bonds and is common in biochemistry for fatty acid hydrogenation.

Carbocation Rearrangements: A Critical MCAT Twist

When an electrophilic addition proceeds through a carbocation intermediate, that carbocation can sometimes become more stable by rearranging. This is a major source of unexpected products and a favorite MCAT trap. The two main rearrangements are hydride shift (movement of a hydrogen with its two electrons) and alkyl shift (movement of a carbon with its bonding electrons).

Consider the addition of HCl to 3-methylbut-1-ene. Initial protonation could yield a primary carbocation. However, a nearby methyl group with its attached hydrogen can perform a hydride shift, moving the hydride to the positive center. This transforms the unstable primary carbocation into a more stable tertiary one. The chloride ion then attacks, yielding a product that appears to violate Markovnikov’s rule at first glance. The MCAT will test your ability to recognize when a rearrangement is possible (look for a secondary or primary carbocation adjacent to a tertiary carbon or a branch) and to draw the correct, more stable carbocation before the nucleophile attacks.

From Monomers to Polymers: Addition Polymerization

Alkenes serve as the monomers for some of the world’s most important plastics via addition polymerization. In this chain-growth process, the double bond of each alkene monomer (like ethylene, propylene, or styrene) is opened by an initiator (often a radical). This creates a reactive species that adds to another monomer, propagating a long chain. The key mechanistic steps—initiation, propagation, and termination—mirror radical reaction mechanisms. For the MCAT, you should recognize common polymers from their monomers: polyethylene from ethylene, polypropylene from propylene, and polystyrene from styrene. Understanding that the polymer’s properties (flexibility, strength) are determined by the monomer structure and the polymerization conditions is often the conceptual link tested.

Common Pitfalls and MCAT Strategy

1. Misapplying Markovnikov's Rule: The most common error is applying Markovnikov's rule to reactions where it does not apply, such as halogenation or hydroboration-oxidation. Correction: Markovnikov’s rule applies only to the addition of unsymmetrical reagents (like H-X or $H_{2}O$) to unsymmetrical alkenes in reactions that proceed via carbocation intermediates.
2. Ignoring Stereochemistry: Many students forget that additions can be syn or anti. Halogenation gives anti addition due to the halonium ion, while hydrogenation gives syn addition. Correction: Always consider the mechanism. If a cyclic intermediate is formed (halonium ion), addition must be anti. If the mechanism involves a metal surface (hydrogenation), addition is syn.
3. Overlooking Rearrangements: Failing to check for possible carbocation rearrangements will lead you to the wrong product choice on the exam. Correction: Any time you draw a carbocation, immediately ask: "Can a hydride or alkyl shift create a more stable carbocation (3° > 2° > 1°)?" If yes, you must draw the rearranged intermediate.
4. Confusing Reaction Conditions: The MCAT will test if you know the specific reagents for a desired transformation. For example, mixing up the reagents for Markovnikov hydration ($H_3{O}^{+}$) vs. anti-Markovnikov hydration ($1.B{H}_3,2.H_2{O}_2,O{H}^{-}$). Correction: Create a mental flowchart linking the desired product (e.g., alcohol on more-substituted carbon) directly to its required reagent set.

Summary

• Alkenes primarily undergo electrophilic addition initiated by the attack of their electron-rich pi bond on an electrophile, often forming a carbocation intermediate.
• Markovnikov's rule predicts the regiochemistry for additions of unsymmetrical reagents like H-X and $H_{2}O$: the hydrogen adds to the less substituted carbon, placing the positive charge (and subsequent nucleophile) on the more substituted, more stable carbon.
• Hydroboration-oxidation is the key exception, providing anti-Markovnikov addition of water through a concerted, sterically controlled mechanism.
• Carbocation rearrangements (hydride and alkyl shifts) are a critical complication in Markovnikov additions, allowing the reaction pathway to funnel through the most stable intermediate, which can change the expected product.
• In addition polymerization, alkene monomers (like ethylene) use their double bonds to link into long-chain polymers (like polyethylene), a process with vast industrial and biological implications.