哥伦比亚大学有机化学经典课件.ppt

4.8Preparation of Alkyl Halides fromAlcohols and Hydrogen Halides ROH + HX ? RX + H2O Hydrogen halide reactivity HI HBr HCl HFmost reactive least reactive Alcohol reactivityR3COH R2CHOH RCH2OH CH3OHTertiary Secondary Primary Methanolmost reactive least reactive (CH3)3COH + HCl The key intermediate in reaction of secondary and tertiary alcohols with hydrogen halides is a carbocation. A carbocation is a cation in which carbon has6 valence electrons and a positive charge. The key intermediate in reaction of secondary and tertiary alcohols with hydrogen halides is a carbocation. The overall reaction mechanism involves threeelementary steps; the first two steps lead to thecarbocation intermediate, the third step is the conversion of this carbocation to the alkyl halide. (CH3)3COH + HCl (CH3)3C Figure 4.8 Structure of methyl cation. Carbon is sp2 hybridized.All four atoms lie in same plane. Figure 4.8 Structure of methyl cation. Empty 2p orbital. Axis of 2p orbital is perpendicular to plane of atoms. Most carbocations are too unstable to beisolated. When R is an alkyl group, the carbocation isstabilized compared to R = H. Electronic effects transmitted through ??bonds are called inductive effects. The more stable a carbocation is, the faster it isformed. Reactions involving tertiary carbocations occurat faster rates than those proceeding via secondarycarbocations. Reactions involving primary carbocations or CH3+ are rare. Carbocations are Lewis acids (electron-pairacceptors). Carbocations are electrophiles (electron-seekers). Lewis bases (electron-pair donors) exhibit just theopposite behavior. Lewis bases are nucleophiles(nucleus-seekers). The last step in the mechanism of the reaction oftert-butyl alcohol with hydrogen chloride is the reaction between an electrophile and a nucleophile. tert-Butyl cation is the electrophile. Chloride ionis the nucleophile. Fig. 4.11 Combination of tert-butyl ca