4.8Preparation of Alkyl Halides fromAlcohols and Hydrogen Halides,ROH + HX RX + H2O,Reaction of Alcohols with Hydrogen Halides,ROH + HX RX + HOH,Hydrogen halide reactivityHI HBr HCl HFmost reactiveleast reactive,Reaction of Alcohols with Hydrogen Halides,ROH + HX RX + HOH,Alcohol reactivityR3COH R2CHOH RCH2OH CH3OHTertiarySecondaryPrimaryMethanolmost reactiveleast reactive,Preparation of Alkyl Halides,(CH3)3COH + HCl,(CH3)3CCl + H2O,78-88%,+ H2O,73%,CH3(CH2)5CH2OH + HBr,CH3(CH2)5CH2Br + H2O,87-90%,25C,80-100C,120C,Preparation of Alkyl Halides,CH3CH2CH2CH2OH,CH3CH2CH2CH2Br,NaBrH2SO4,70-83%,heat,A mixture of sodium bromide and sulfuric acid may be used in place of HBr.,4.9Mechanism of the Reaction of Alcohols with Hydrogen Halides,Carbocation,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.,Carbocation,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.,Example,(CH3)3COH + HCl,(CH3)3CCl + H2O,25C,Carbocation intermediate is:,tert-Butyl alcohol,tert-Butyl chloride,tert-Butyl cation,Mechanism,Step 1: Proton transfer from HCl to tert-butyl alcohol,(CH3)3C,O,H,.,:,fast, bimolecular,tert-Butyloxonium ion,Mechanism,Step 2: Dissociation of tert-butyloxonium ion,+,slow, unimolecular,(CH3)3C,tert-Butyl cation,+,Mechanism,Step 3: Capture of tert-butyl cation by chloride ion.,fast, bimolecular,+,(CH3)3C,+,tert-Butyl chloride,4.10Structure, Bonding, andStability of Carbocations,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.,Carbocations,Most carbocations are too unstable to beisolated. When R is an alkyl group, the carbocation isstabilized compared to R = H.,Carbocations,Methyl cationleast stable,Carbocations,C,H3C,H,H,+,Ethyl cation(a primary carbocation) is more stable than CH3+,Carbocations,C,H3C,CH3,H,+,Isopropyl cation(a secondary carbocation) is more stable than CH3CH2+,Carbocations,C,H3C,CH3,CH3,+,tert-Butyl cation(a tertiary carbocation) is more stable than (CH3)2CH+,Figure 4.9 Stabilization of carbocationsvia the inductive effect,positively chargedcarbon pullselectrons in bondscloser to itself,+,Figure 4.9 Stabilization of carbocationsvia the inductive effect,positive charge isdispersed , i.e., sharedby carbon and thethree atoms attachedto it,Figure 4.9 Stabilization of carbocationsvia the inductive effect,electrons in CCbonds are more polarizable than thosein CH bonds; therefore, alkyl groupsstabilize carbocationsbetter than H.,Electronic effects transmitted through bonds are called inductive effects.,Figure 4.10 Stabilization of carbocationsvia hyperconjugation,electrons in this bond can be sharedby positively chargedcarbon because thes orbital can overlap with the empty 2porbital of positivelycharged carbon,+,Figure 4.10 Stabilization of carbocationsvia hyperconjugation,electrons in this bond can be sharedby positively chargedcarbon because thes orbital can overlap with the empty 2porbital of positivelycharged carbon,Figure 4.10 Stabilization of carbocationsvia hyperconjugation,Notice that an occupiedorbital of this type isavailable when sp3hybridized carbon is attached to C+, but is not availabe when His attached to C+. Therefore,alkyl groupsstabilize carbocationsbetter than H does.,Carbocations,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,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).,Mechanism,Step 3: Capture of tert-butyl cation by chloride ion.,fast, bimolecular,+,(CH3)3C,+,tert-Butyl chloride,Carbocations,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.,(CH3)3C,+,+,nucleophile (Lewis base),electrophile (Lewis acid),Fig. 4.11 Combination of tert-butyl cation andchloride ion to give tert-butyl chloride,4.11Potential Energy Diagrams forMultistep Reactions:The SN1 Mechanism,Potentialenergy,Reaction coordinate,Recall.,H2O + HBr,the potential energy diagram for proton transfer from HBr to water,The potential energy diagram for a multistep mechanism is simply a co