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Transcript
4.8 Preparation of Alkyl Halides from Alcohols and Hydrogen Halides ROH + HX → RX + H2O Reaction Reactionof ofAlcohols Alcoholswith withHydrogen HydrogenHalides Halides ROH + HX → RX + HOH Hydrogen halide reactivity HI most reactive HBr HCl HF least reactive Reaction Reactionof ofAlcohols Alcoholswith withHydrogen HydrogenHalides Halides ROH + HX → RX + HOH Alcohol reactivity R3COH Tertiary R2CHOH Secondary most reactive RCH2OH CH3OH Primary Methanol least reactive Preparation Preparationof ofAlkyl AlkylHalides Halides 25°C (CH3)3COH + HCl (CH3)3CCl + H 2O 78-88% OH + HBr 80-100°C Br + H2O 73% CH3(CH2)5CH 2OH + HBr 120°C CH3(CH2)5CH 2Br + H2O 87-90% Preparation Preparationof ofAlkyl AlkylHalides Halides A mixture of sodium bromide and sulfuric acid may be used in place of HBr. CH3CH2CH2CH2OH NaBr H2SO4 heat CH3CH2CH2CH2Br 70-83% 4.9 Mechanism of the Reaction of Alcohols with Hydrogen Halides Carbocation Carbocation R + C R R The key intermediate in reaction of secondary and tertiary alcohols with hydrogen halides is a carbocation. A carbocation is a cation in which carbon has 6 valence electrons and a positive charge. Carbocation Carbocation R + C R R The key intermediate in reaction of secondary and tertiary alcohols with hydrogen halides is a carbocation. The overall reaction mechanism involves three elementary steps; the first two steps lead to the carbocation intermediate, the third step is the conversion of this carbocation to the alkyl halide. Example Example (CH3)3COH + HCl 25°C tert-Butyl alcohol (CH3)3CCl + H 2O tert-Butyl chloride Carbocation intermediate is: H 3C + C CH3 CH3 tert-Butyl cation Mechanism Step 1: Proton transfer from HCl to tert-butyl alcohol (CH3)3C .. O: + H .. : Cl .. H fast, bimolecular H + (CH3)3C O : + H tert-Butyloxonium ion .. – : Cl: .. Mechanism Step 2: Dissociation of tert-butyloxonium ion H + (CH3)3C O : H slow, unimolecular H + (CH3)3C + tert-Butyl cation :O: H Mechanism Step 3: Capture of tert-butyl cation by chloride ion. + (CH3)3C + .. – : Cl: : Cl .. fast, bimolecular (CH3)3C .. Cl .. : tert-Butyl chloride 4.10 Structure, Bonding, and Stability of Carbocations Figure Figure4.8 4.8 Structure Structureof ofmethyl methylcation. cation. Carbon is sp2 hybridized. All four atoms lie in same plane. Figure Figure4.8 4.8 Structure Structureof ofmethyl methylcation. cation. Empty 2p orbital. Axis of 2p orbital is perpendicular to plane of atoms. Carbocations Carbocations R + C R R Most carbocations are too unstable to be isolated. When R is an alkyl group, the carbocation is stabilized compared to R = H. Carbocations Carbocations H + C H H Methyl cation least stable Carbocations Carbocations H 3C + C H H Ethyl cation (a primary carbocation) is more stable than CH3+ Carbocations Carbocations H 3C + C CH3 H Isopropyl cation (a secondary carbocation) is more stable than CH3CH2+ Carbocations Carbocations H 3C + C CH3 CH3 tert-Butyl cation (a tertiary carbocation) is more stable than (CH3)2CH+ Figure Figure4.9 4.9 Stabilization Stabilizationof ofcarbocations carbocations via viathe theinductive inductiveeffect effect + positively charged carbon pulls electrons in σ bonds closer to itself Figure Figure4.9 4.9 Stabilization Stabilizationof ofcarbocations carbocations via viathe theinductive inductiveeffect effect δ+ δ+ δ+ δ+ positive charge is "dispersed ", i.e., shared by carbon and the three atoms attached to it Figure Figure4.9 4.9 Stabilization Stabilizationof ofcarbocations carbocations via viathe theinductive inductiveeffect effect δ+ δ+ δ+ δ+ electrons in C—C bonds are more polarizable than those in C —H bonds; therefore, alkyl groups stabilize carbocations better than H. Electronic effects transmitted through σ bonds are called "inductive effects." Figure Figure4.10 4.10 Stabilization Stabilizationof ofcarbocations carbocations via viahyperconjugation hyperconjugation + electrons in this σ bond can be shared by positively charged carbon because the s orbital can overlap with the empty 2p orbital of positively charged carbon Figure Figure4.10 4.10 Stabilization Stabilizationof ofcarbocations carbocations via viahyperconjugation hyperconjugation δ+ δ+ electrons in this σ bond can be shared by positively charged carbon because the s orbital can overlap with the empty 2p orbital of positively charged carbon Figure Figure4.10 4.10 Stabilization Stabilizationof ofcarbocations carbocations via viahyperconjugation hyperconjugation δ+ δ+ Notice that an occupied orbital of this type is available when sp3 hybridized carbon is attached to C+, but is not availabe when H is attached to C+. Therefore,alkyl groups stabilize carbocations better than H does. Carbocations Carbocations R + C R R The more stable a carbocation is, the faster it is formed. Reactions involving tertiary carbocations occur at faster rates than those proceeding via secondary carbocations. Reactions involving primary carbocations or CH3+ are rare. Carbocations Carbocations R + C R R Carbocations are Lewis acids (electron-pair acceptors). Carbocations are electrophiles (electron-seekers). Lewis bases (electron-pair donors) exhibit just the opposite behavior. Lewis bases are nucleophiles (nucleus-seekers). Mechanism Step 3: Capture of tert-butyl cation by chloride ion. + (CH3)3C + .. – : Cl: : Cl .. fast, bimolecular (CH3)3C .. Cl .. : tert-Butyl chloride Carbocations Carbocations + (CH3)3C + .. – : Cl: .. (CH3)3C .. Cl .. : The last step in the mechanism of the reaction of tert-butyl alcohol with hydrogen chloride is the reaction between an electrophile and a nucleophile. tert-Butyl cation is the electrophile. Chloride ion is the nucleophile. Fig. -butyl cation Fig.4.11 4.11 Combination Combinationof oftert tert-butyl cationand and chloride -butyl chloride chlorideion ionto to give givetert tert-butyl chloride nucleophile (Lewis base) + electrophile (Lewis acid) – 4.11 Potential Energy Diagrams for Multistep Reactions: The SN1 Mechanism Recall... Recall... the potential energy diagram for proton transfer from HBr to water δ+ H 2O δ− H Br Potential energy H2O + H —Br Reaction coordinate + H2O—H + Br – Extension Extension The potential energy diagram for a multistep mechanism is simply a collection of the potential energy diagrams for the individual steps. Consider the mechanism for the reaction of tert-butyl alcohol with HCl. (CH3)3COH + HCl 25°C (CH3)3CCl + H 2O Mechanism Step 1: Proton transfer from HCl to tert-butyl alcohol (CH3)3C .. O: + H .. : Cl .. H fast, bimolecular H + (CH3)3C O : + H tert-butyloxonium ion .. – : Cl: .. Mechanism Step 2: Dissociation of tert-butyloxonium ion H + (CH3)3C O : H slow, unimolecular H + (CH3)3C + tert-Butyl cation :O: H Mechanism Step 3: Capture of tert-butyl cation by chloride ion. + (CH3)3C + .. – : Cl: : Cl .. fast, bimolecular (CH3)3C .. Cl .. : tert-Butyl chloride carbocation formation carbocation capture R+ proton transfer ROH + ROH2 RX carbocation formation (CH (CH33))33C C δ+ δ+ O O H H carbocation capture δ– δ– Cl Cl R+ H H ROH + ROH2 RX δ+ δ+ (CH (CH33))33C C H H O O carbocation capture δ+ δ+ H H R+ proton transfer ROH + ROH2 RX δ+ δ+ carbocation formation (CH (CH33))33C C R+ proton transfer ROH + ROH2 RX δ– δ– Cl Cl Mechanistic Mechanisticnotation notation The mechanism just described is an example of an SN1 process. SN1 stands for substitution-nucleophilic unimolecular. The molecularity of the rate-determining step defines the molecularity of the overall reaction. Mechanistic Mechanisticnotation notation The molecularity of the rate-determining step defines the molecularity of the overall reaction. δ+ δ+ (CH (CH33))33C C H H O O δ+ δ+ H H Rate-determining step is unimolecular dissociation of alkyloxonium ion. 4.12 Effect of Alcohol Structure on Reaction Rate slow step is: ROH2+ → R+ + H 2O The more stable the carbocation, the faster it is formed. Tertiary carbocations are more stable than secondary, which are more stable than primary, which are more stable than methyl. Tertiary alcohols react faster than secondary, which react faster than primary, which react faster than methanol. Hammond's Hammond'sPostulate Postulate If two succeeding states (such as a transition state and an unstable intermediate) are similar in energy, they are similar in structure. Hammond's postulate permits us to infer the structure of something we can't study (transition state) from something we can study (reactive intermediate). carbocation formation carbocation capture R+ proton transfer ROH + ROH2 RX carbocation formation R+ proton transfer ROH + ROH2 Rate is carbocation governed by capture energy of this transition state. Infer structure of this transition state from structure of state of closest energy; in this case the nearest state is the RXcarbocation. 4.13 Reaction of Primary Alcohols with Hydrogen Halides. The SN2 Mechanism Preparation Preparationof ofAlkyl AlkylHalides Halides 25°C (CH3)3COH + HCl (CH3)3CCl + H 2O 78-88% OH + HBr 80-100°C Br + H2O 73% CH3(CH2)5CH 2OH + HBr 120°C CH3(CH2)5CH 2Br + H2O 87-90% Preparation Preparationof ofAlkyl AlkylHalides Halides Primary carbocations are too high in energy to allow SN1 mechanism. Yet, primary alcohols are converted to alkyl halides. Primary alcohols react by a mechanism called SN2 (substitution-nucleophilic -bimolecular). CH3(CH2)5CH 2OH + HBr 120°C CH3(CH2)5CH 2Br + H2O 87-90% The TheSSNN22Mechanism Mechanism Two-step mechanism for conversion of alcohols to alkyl halides: (1) proton transfer to alcohol to form alkyloxonium ion (2) bimolecular displacement of water from alkyloxonium ion by halide Example Example CH3(CH2)5CH 2OH + HBr 120°C CH3(CH2)5CH 2Br + H2O Mechanism Step 1: Proton transfer from HBr to 1 -heptanol CH3(CH2)5CH 2 .. O: + H .. : Br .. H fast, bimolecular H + CH3(CH2)5CH 2 O : H Heptyloxonium ion + .. – : Br: .. Mechanism Step 2: Reaction of alkyloxonium ion with bromide ion. H .. – + : Br: + CH3(CH2)5CH 2 O : .. H slow, bimolecular H CH3(CH2)5CH 2 .. Br .. : 1-Bromoheptane + :O: H δ+ δ+ δ– δ– Br Br CH CH22 OH OH22 CH CH33(CH (CH22))44 CH CH22 proton transfer ROH + ROH2 RX 4.14 Other Methods for Converting Alcohols to Alkyl Halides Reagents Reagentsfor forROH ROHto toRX RX Thionyl chloride SOCl2 + ROH → RCl + HCl + SO2 Phosphorus tribromide PBr3 + 3ROH → 3RBr + H3PO3 Examples Examples CH3CH(CH2)5CH 3 SOCl2 K2CO3 CH3CH(CH2)5CH 3 Cl OH (81%) (pyridine often used instead of K 2CO 3) (CH3)2CHCH2OH PBr3 (CH3)2CHCH2Br (55-60%)