Shanxian Chemical
SUPPLEMENTS
Electrophilic Addition of Hydrogen Halides to Alkenes
The electrophilic addition of olefins to hydrogen halides
Principle of electrophilic addition reaction
The electrophilic addition reaction occurs between olefins and hydrogen halides (such as\ (HCl\),\ (HBr\),\ (HI\)). The carbon-carbon double bond in olefins is electron-rich, and the hydrogen atom in the hydrogen halide is partially positively charged. As an electrophilic reagent, it attacks the double bond, so that the\ (\ pi\) bond in the double bond is broken to form a carbon positive ion intermediate. Taking the reaction of propylene with\ (HBr\) as an example, the\ (H\) atom in\ (HBr\) attacks the propylene double bond. One carbon atom in the double bond binds to the\ (H\) atom, and the other carbon atom is positively charged to form a carbon positive ion.
Reactivity
The order of reactivity of different hydrogen halides is\ (HI > HBr > HCl\). This is because\ (H - I\) bond energy in\ (HI\) is the smallest, and\ (H\) atoms are easier to dissociate and participate in the reaction as electrophilic reagents, while\ (H - Cl\) bond energy in\ (HCl\) is relatively large, and\ (H\) atoms are more difficult to dissociate, so the reactivity is low.
Markov rule
In the electrophilic addition reaction of olefins with hydrogen halides, the Markov rule is usually followed. That is, when asymmetric olefins are added to hydrogen halides, hydrogen atoms are mainly added to double-bonded carbon atoms with more hydrogen, and halogen atoms are added to double-bonded carbon atoms with less hydrogen. For example, propylene reacts with\ (HBr\), and the main product is\ (2-bromopropane\), because the formed\ (2 ^ {\ circ}\) carbocation is more stable than\ (1 ^ {\ circ}\) carbocation. The order of stability of carbocation is\ (3 ^ {\ circ} > 2 ^ {\ circ} > 1 ^ {\ circ} > CH_ {3 }^{+}\)。
Anti-Markov rule
In the presence of peroxides, the addition reaction of olefins with\ (HBr\) will produce anti-Markov rule products. This is because the peroxide triggers a free radical reaction, and the reaction mechanism changes. At this time,\ (HBr\) first forms a bromine radical, and the bromine radical is preferentially added to the double-bonded carbon atom containing more hydrogen, resulting in an anti-Markov rule product. However, this phenomenon only applies to\ (HBr\),\ (HCl\) and\ (HI\) in the presence of peroxides still follow the Markov rule, because\ (HCl\) 's\ (H-Cl\) bond energy is large and free radical reactions are not easy to occur;\ (HI\) Although it can produce iodine radicals, the activity of iodine radicals is low and it is difficult to initiate subsequent reactions.
Principle of electrophilic addition reaction
The electrophilic addition reaction occurs between olefins and hydrogen halides (such as\ (HCl\),\ (HBr\),\ (HI\)). The carbon-carbon double bond in olefins is electron-rich, and the hydrogen atom in the hydrogen halide is partially positively charged. As an electrophilic reagent, it attacks the double bond, so that the\ (\ pi\) bond in the double bond is broken to form a carbon positive ion intermediate. Taking the reaction of propylene with\ (HBr\) as an example, the\ (H\) atom in\ (HBr\) attacks the propylene double bond. One carbon atom in the double bond binds to the\ (H\) atom, and the other carbon atom is positively charged to form a carbon positive ion.
Reactivity
The order of reactivity of different hydrogen halides is\ (HI > HBr > HCl\). This is because\ (H - I\) bond energy in\ (HI\) is the smallest, and\ (H\) atoms are easier to dissociate and participate in the reaction as electrophilic reagents, while\ (H - Cl\) bond energy in\ (HCl\) is relatively large, and\ (H\) atoms are more difficult to dissociate, so the reactivity is low.
Markov rule
In the electrophilic addition reaction of olefins with hydrogen halides, the Markov rule is usually followed. That is, when asymmetric olefins are added to hydrogen halides, hydrogen atoms are mainly added to double-bonded carbon atoms with more hydrogen, and halogen atoms are added to double-bonded carbon atoms with less hydrogen. For example, propylene reacts with\ (HBr\), and the main product is\ (2-bromopropane\), because the formed\ (2 ^ {\ circ}\) carbocation is more stable than\ (1 ^ {\ circ}\) carbocation. The order of stability of carbocation is\ (3 ^ {\ circ} > 2 ^ {\ circ} > 1 ^ {\ circ} > CH_ {3 }^{+}\)。
Anti-Markov rule
In the presence of peroxides, the addition reaction of olefins with\ (HBr\) will produce anti-Markov rule products. This is because the peroxide triggers a free radical reaction, and the reaction mechanism changes. At this time,\ (HBr\) first forms a bromine radical, and the bromine radical is preferentially added to the double-bonded carbon atom containing more hydrogen, resulting in an anti-Markov rule product. However, this phenomenon only applies to\ (HBr\),\ (HCl\) and\ (HI\) in the presence of peroxides still follow the Markov rule, because\ (HCl\) 's\ (H-Cl\) bond energy is large and free radical reactions are not easy to occur;\ (HI\) Although it can produce iodine radicals, the activity of iodine radicals is low and it is difficult to initiate subsequent reactions.
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