Halogenation

"When the halogenation reaction is carried out on a cycloalkene, such as cyclopentene, only the trans stereoisomer of the dihalide addition product is formed rather than the mixture of cis and trans isomers that might have been expected if a planar carbocation intermediate were involved. We say that the reaction occurs with anti stereochemistry ..."

"Hydrohalogenation is the addition of hydrogen halides HX (X = Cl, Br, and I) to alkenes to form alkyl halides. Two bonds are broken in this reaction—the weak π bond of the alkene and the HX bond—bond two new σ bonds are formed—one to H and one to X. Because X is more electronegative than H, the H – X bond is polarized, with a partial positive charge on H. Because the electrophilic (H) end of HX is attracted to the electron-rich double bond, these reactions are called electrophilic additions."

"An example of electrophilic aromatic substitution is halogenation. Benzene is normally unreactive in the presence of halogens, because halogens are not electrophilic enough to disrupt its aromaticity. However, the halogen may be activated by Lewis acidic catalysts, such as ferric halides (FeX3) or aluminum halides (AlX3), to become a much more powerful electrophile. (...) The halogenation of benzene becomes more exothermic as we proceed from I2(endothermic) to F2 (exothermic and explosive). Chlorinations and brominations are achieved with the help of Lewis acid catalysts that polarize the X – X bond and activate the halogen by increasing its electrophilic power."