马尔科夫尼科夫规则Markovnikov's rule

丙烯与 HBr 加成，可能生成 1-溴丙烷或 2-溴丙烷。H⁺ 加到端位 CH₂ 上得到仲碳正离子 CH₃CH⁺CH₃，加到中间碳上得到伯碳正离子 CH₃CH₂CH₂⁺。仲碳正离子更稳定、生成更快，所以主产物是 2-溴丙烷。

Propene adding HBr could give 1-bromopropane or 2-bromopropane. Adding H⁺ to the terminal CH₂ gives the secondary carbocation CH₃CH⁺CH₃; adding it to the middle carbon gives the primary carbocation CH₃CH₂CH₂⁺. The secondary cation is more stable and forms faster, so the major product is 2-bromopropane.

加成的第一步（H⁺ 加到双键上生成碳正离子）是决速步。碳正离子的稳定性为叔 > 仲 > 伯，因为烷基的推电子诱导效应和超共轭能分散正电荷。生成更稳定碳正离子的过渡态能量更低，这条路线更快，对应的产物占多数。

The first step of the addition, H⁺ adding to the double bond to form a carbocation, is rate-determining. Carbocation stability is tertiary > secondary > primary because the electron-donating inductive effect of alkyl groups and hyperconjugation spread the positive charge. The transition state leading to the more stable carbocation is lower in energy, so that route is faster and its product predominates.

在过氧化物存在下，HBr 与烯烃按自由基机理加成，Br• 先加到双键上生成更稳定的碳自由基，结果 Br 连在含氢较多的碳上，称为反马氏加成或过氧化物效应；HCl 和 HI 不发生这一反应。硼氢化–氧化反应也得到反马氏取向的醇。

With peroxides present, HBr adds to alkenes by a radical mechanism: Br• adds first to give the more stable carbon radical, so Br ends up on the carbon with more hydrogens. This is anti-Markovnikov addition, the peroxide effect; HCl and HI do not show it. Hydroboration–oxidation also gives alcohols with anti-Markovnikov orientation.