# Lecture 13: Alkene Reactions: Addition Mechanisms ## Organic Chemistry I --- ## Learning Objectives By the end of this lecture, students will be able to: 1. Explain why alkenes undergo addition reactions as their characteristic reactivity 2. Describe the general mechanism of electrophilic addition to alkenes 3. Apply Markovnikov's rule to predict the regiochemistry of addition 4. Explain the mechanistic basis for Markovnikov's rule (carbocation stability) 5. Describe the mechanism of HX addition to alkenes 6. Predict products of acid-catalyzed hydration of alkenes 7. Identify carbocation rearrangements in electrophilic addition reactions 8. Describe anti-Markovnikov additions and their mechanisms --- ## Lecture Content ### I. Reactivity of Alkenes Alkenes are electron-rich species because the pi bond creates a region of high electron density above and below the molecular plane. Pi electrons are more loosely held and higher in energy than sigma electrons, making them more accessible to electrophiles. As a result, alkenes function as nucleophiles (Lewis bases) and characteristically undergo electrophilic addition reactions, in which an electrophile attacks the pi bond, the pi bond breaks, and two new sigma bonds form. The general equation for electrophilic addition is C=C + E-Nu yielding E-C-C-Nu, and the degree of unsaturation decreases by one. This reaction is essentially the reverse of elimination: while elimination breaks two sigma bonds to form one pi bond, addition breaks one pi bond to form two sigma bonds. The thermodynamics generally favor addition because two sigma bonds are stronger than the combination of one pi bond and one E-Nu bond that are broken. ### II. Mechanism of Electrophilic Addition: HX Addition The addition of hydrogen halides (HCl, HBr, HI) to alkenes proceeds through a two-step mechanism with a carbocation intermediate. In the first step, which is rate-determining, the pi electrons attack the proton of HX. One carbon gains the hydrogen while the other becomes a carbocation. A curved arrow is drawn from the pi bond to the hydrogen to show this electron flow. In the second step, the halide ion (X-) rapidly attacks the highly electrophilic carbocation to form the alkyl halide product. The addition of HBr to propene, for example, gives 2-bromopropane as the Markovnikov product. ### III. Markovnikov's Rule Markovnikov's rule, first articulated in 1869, states that when HX adds to an unsymmetrical alkene, the hydrogen adds to the carbon bearing more hydrogens and the halide adds to the carbon bearing fewer hydrogens. A modern and more mechanistically informative restatement is that the electrophile (H+) adds in the way that produces the more stable carbocation intermediate. The mechanistic basis is straightforward. In the rate-determining protonation step, the proton can add to either carbon of the double bond. Adding it to the less substituted carbon generates the more substituted, more stable carbocation, which forms faster according to Hammond's postulate. The alternative pathway, adding the proton to the more substituted carbon to generate a less substituted and less stable carbocation, is disfavored and does not significantly compete. For example, when HCl reacts with 2-methylpropene, the proton adds to C1 to form a tertiary carbocation at C2, leading to the Markovnikov product 2-chloro-2-methylpropane. The alternative would require forming a primary carbocation at C1, which is far too unstable to be a viable intermediate. Markovnikov's rule applies to all electrophilic additions that proceed through carbocation intermediates. Panel A: The two-step mechanism of HBr addition to propene. Step 1: pi electrons of propene attack H of HBr (curved arrow from C=C to H), forming a secondary carbocation at C2 (Markovnikov pathway, favored) vs. a primary carbocation at C1 (anti-Markovnikov, disfavored). Step 2: Br- attacks the secondary carbocation to give 2-bromopropane. The primary pathway is crossed out with an X. Panel B: An energy diagram comparing the two pathways: the Markovnikov path has a lower activation energy (more stable transition state leading to secondary carbocation) than the anti-Markovnikov path (less stable transition state leading to primary carbocation). ### IV. Carbocation Rearrangements in Addition Reactions Because electrophilic addition proceeds through carbocation intermediates, the same rearrangements observed in SN1 and E1 reactions can occur here. If a more stable carbocation is accessible via a 1,2-hydride shift or 1,2-methyl shift, the rearrangement will take place, often faster than the nucleophile can capture the initial carbocation. A telling example is the addition of HCl to 3,3-dimethyl-1-butene. Initial protonation at C1 generates a secondary carbocation at C2, which then undergoes a 1,2-methyl shift from C3 to form a more stable tertiary carbocation at C3. Chloride attacks this rearranged cation to give 2-chloro-2,3-dimethylbutane as the major product, while the unrearranged Markovnikov product (3-chloro-2,2-dimethylbutane) is only a minor product. The appearance of rearranged products is strong evidence that a carbocation intermediate is involved, and one should always consider rearrangement when predicting the products of electrophilic additions. ### V. Acid-Catalyzed Hydration Acid-catalyzed hydration adds water across a double bond in the presence of an acid catalyst to produce an alcohol. The overall transformation is C=C + H2O (with H+ catalyst) yielding an alcohol, and the reaction follows Markovnikov's rule, placing the hydroxyl group on the more substituted carbon. The mechanism has three steps. First, the alkene is protonated by H3O+ or H2SO4 to form a carbocation. Second, water, acting as a nucleophile, attacks the carbocation to produce an oxonium ion (a protonated alcohol). Third, deprotonation of the oxonium ion yields the alcohol product. The reaction is an equilibrium process, and Le Chatelier's principle governs which direction is favored: excess water drives hydration forward, while excess acid and heat drive the reverse reaction (dehydration). A limitation of acid-catalyzed hydration is that carbocation rearrangements can divert the reaction to give unexpected products. For substrates where clean Markovnikov hydration without rearrangement is needed, oxymercuration-demercuration is the preferred alternative. ### VI. Oxymercuration-Demercuration Oxymercuration-demercuration achieves Markovnikov hydration of alkenes without the risk of carbocation rearrangement. In the first step, the alkene reacts with mercuric acetate (Hg(OAc)2) in aqueous THF to form a mercurinium ion, a bridged three-membered ring intermediate in which mercury bridges both carbons. Water then attacks the more substituted carbon of this bridged intermediate from the opposite face (anti addition). In the second step, treatment with sodium borohydride (NaBH4) replaces the mercury with hydrogen, yielding the Markovnikov alcohol. The key advantage over acid-catalyzed hydration is the nature of the intermediate. The bridged mercurinium ion is not an open carbocation, so rearrangements cannot occur. The reaction produces exclusively the Markovnikov product under mild conditions, making it the method of choice for reliable Markovnikov hydration. Panel A: Comparison of acid-catalyzed hydration vs. oxymercuration-demercuration of 3,3-dimethyl-1-butene. Acid-catalyzed: gives rearranged product (2-methyl-2-pentanol via carbocation rearrangement). Oxymercuration-demercuration: gives the Markovnikov product without rearrangement (3,3-dimethyl-2-butanol). Both reactions are shown with their mechanisms and key intermediates. Panel B: The mercurinium ion intermediate drawn as a three-membered ring with Hg bridging the two carbons, showing that the positive charge is on Hg (not carbon), which prevents rearrangement. Water attacks at the more substituted carbon in an anti fashion. ### VII. Regioselectivity Summary Markovnikov addition reactions, in which the electrophile adds to the less substituted carbon and the nucleophilic component ends up on the more substituted carbon, include HX addition (HCl, HBr, HI), acid-catalyzed hydration (H3O+), and oxymercuration-demercuration. Anti-Markovnikov additions, covered in detail in Lecture 14, include hydroboration-oxidation (where OH ends up on the less substituted carbon) and radical HBr addition with peroxides (where Br ends up on the less substituted carbon). The regiochemistry is fundamentally dictated by the mechanism: reactions that proceed through carbocation intermediates give Markovnikov products, while concerted and radical mechanisms give anti-Markovnikov products. ### VIII. Introduction to Stereochemistry of Addition Addition reactions can be classified by the facial selectivity of group delivery. Syn addition places both new groups on the same face of the double bond and is characteristic of concerted mechanisms such as hydroboration, catalytic hydrogenation, and epoxidation. Anti addition places the groups on opposite faces and occurs in reactions proceeding through bridged intermediates, such as halogenation (via halonium ions) and oxymercuration. Reactions through open carbocation intermediates generally produce mixtures because the planar cation can be attacked from either face. The detailed stereochemistry of each reaction type is the subject of Lecture 14. Diagrams illustrating syn vs. anti addition to an alkene. Panel A (Syn): Both H and OH add to the same face of the cyclohexene ring (hydroboration-oxidation), yielding the cis product. Arrows show both groups approaching from the same side. Panel B (Anti): Br2 adds to cyclohexene through a bromonium ion intermediate, with the second Br- attacking from the opposite face, yielding the trans-1,2-dibromocyclohexane. Panel C (No selectivity): HBr addition through an open carbocation gives a mixture of cis and trans products. ---