Hydrohalogenation Of Alkenes Alkynes — the NEET Chemistry reaction: mechanism, reagents, conditions, structures and exam traps.
Hydrohalogenation of Alkenes/Alkynes Hydrohalogenation is an electrophilic addition reaction where a hydrogen halide (HX, such as HCl, HBr, or HI) adds across the carbon-carbon double bond of an alkene or the carbon-carbon triple bond of an alkyne. This reaction generally follows Markovnikov's Rule, meaning the hydrogen atom adds to the carbon atom of the double/triple bond that already has more hydrogen atoms, and the halogen adds to the carbon with fewer hydrogen atoms. An exception is the addition of HBr to alkenes in the presence of peroxides, which follows Anti-Markovnikov's Rule via a free radical mechanism. Generally, there are no distinct visible observations (like color change or precipitate formation) specifically associated with the hydrohalogenation reaction itself, especially with colorless alkenes/alkynes and hydrogen halides. If using bromine water (red-brown) as a qualitative test for unsaturation, the addition of HX would yield an alkyl halide, and bromine water would no longer decolorize if the double/triple bond is consumed. The products (alkyl halides) are typically colorless liquids or solids. Electrophilic addition of HX to alkenes is exothermic (breaking a π bond and forming two σ bonds). The stability of the carbocation intermediate determines regioselectivity (Markovnikov's rule). ΔH is typically -40 to -60 kJ/mol. Electrophilic Addition (Alkenes with HX, or Alkynes with HX): 1. Protonation of the alkene double bond (or alkyne triple bond) by H+ (from HX) to form a more stable carbocation intermediate (Markovnikov's Rule). 2. Nucleophilic attack by the halide ion (X-) on the carbocation to form the alkyl halide or vinyl halide. Free Radical Addition (Alkenes with HBr/ROOR): 1. Initiation: Peroxide (ROOR) homolytically cleaves to form alkoxy radicals (RO•). RO• abstracts H from HBr to form a bromine radical (Br•). 2. Propagation: Br• adds to the alkene double bond, forming a more stable carbon radical (Anti-Markovnikov's Rule). This new carbon radical then abstracts H from HBr to form the alkyl bromide and regenerate Br•. 3. Termination: Combination of any two radicals to form a stable molecule. Confusing Markovnikov vs. Anti-Markovnikov: Remember peroxides (ROOR) are ONLY for HBr for Anti-Markovnikov addition via a radical mechanism. HCl and HI do not exhibit anti-Markovnikov addition with peroxides. Carbocation Rearrangements: For electrophilic addition (Markovnikov), if a more stable carbocation can be formed via a hydride or alkyl shift, rearrangement will occur, leading to rearranged (and often major) products. Always check for this possibility. Stereochemistry: For electrophilic addition, expect a racemic mixture if a new chiral center is formed due to the planar carbocation intermediate. For radical addition (HBr/ROOR), anti-addition is generally preferred. Alkynes: Adding one equivalent of HX to a terminal alkyne gives a vinyl halide. Adding two equivalents gives a geminal dihalide (both halogens on the same carbon), always following Markovnikov's rule twice. Reagent Reactivity: HI is generally more reactive than HBr, which is more reactive than HCl due to decreasing bond strength (H-I < H-Br < H-Cl) and increasing nucleophilicity of the halide ion.