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Reaction of Alcohols with Hydrogen Halides 4.7 Preparation of Alkyl Halides from Alcohols and Hydrogen Halides ROH + HX → RX RX + HOH Hydrogen halide reactivity HF HCl HBr HI ROH + HX → RX + H2O least reactive Preparation of Alkyl Halides Reaction of Alcohols with Hydrogen Halides ROH + HX → RX RX + HOH most reactive 25° 25°C (CH3)3COH + HCl HCl 78-88% Alcohol reactivity CH3OH Methanol RCH2OH Primary R2CHO CHOH Secondary OH + HBr R3COH Tertiary 80-100° 80-100°C Br + H2O 73% CH3(CH2)5CH2OH + HBr HBr least reactive (CH3)3CCl + H2O 120° 120°C CH3(CH2)5CH2Br + H2O most reactive 87-90% Preparation of Alkyl Halides Question 5 A mixture of sodium bromide and sulfuric acid may be used in place of HBr. CH3CH2CH2CH2OH NaBr NaBr H2SO4 heat CH3CH2CH2CH2Br What is the product of the reaction of 2methylcyclohexanol with HBr? HBr? A) B) C) D) 70-83% Question 6 What product will be isolated from the reaction of 2-pentanol with HBr? HBr? A) B) C) D) 4.8 Mechanism of the Reaction of Alcohols with Hydrogen Halides About mechanisms A mechanism describes how reactants are converted to products. About mechanisms For the reaction: (CH3)3COH + HCl 25° 25°C (CH3)3CCl + H2O Mechanisms are often written as a series of chemical equations showing the elementary steps. tert-Butyl tert-Butyl alcohol An elementary step is a reaction that proceeds by way of a single transition state. the generally accepted mechanism involves three elementary steps. Mechanisms can be shown likely to be correct, but cannot be proven correct. Step 1 is a Brø Brønsted acid-base reaction. Like proton transfer from a strong acid to water, proton transfer .. .. : from a strong acid to O : + H Cl: Cl .. an alcohol is normally H very fast. fast. fast, bimolecular Step 1: Proton Transfer (CH3)3C H + (CH3)3C O : + H terttert-Butyloxonium ion .. – : Cl: .. tert-Butyl tert-Butyl chloride Step 1: Proton Transfer (CH3)3C .. O: + H .. : Cl: Cl .. H Two molecules react in this elementary step; therefore it is bimolecular. bimolecular. fast, bimolecular H + (CH3)3C O : + H terttert-Butyloxonium ion .. – : Cl: .. Potential Energy Diagram for Step 1 Step 1: Proton Transfer (CH3)3C .. O: + H .. : Cl: Cl .. δ+ (CH3)3CO H Potential energy fast, bimolecular H + (CH3)3C O : + .. – : Cl: .. H terttert-Butyloxonium ion Species formed in this step (tert (tert-butyloxonium ion) is an intermediate in the overall reaction. δ− H Cl H (CH3)3COH + H— H—Cl + (CH3)3CO H Reaction coordinate Step 2: Carbocation Formation Dissociation of the alkyloxonium ion involves H bond-breaking, without + any bond-making to (CH3)3C O : compensate for it. It H has a high activation energy and is slow. slow. slow, unimolecular Step 2: Carbocation Formation A single molecule reacts in this step; therefore, it is unimolecular. unimolecular. H + (CH3)3C O : H slow, unimolecular H + (CH3)3C + H + (CH3)3C : O: tert-Butyl tert-Butyl cation H Step 2: Carbocation Formation + (CH3)3C (CH3)3C Potential energy + tert-Butyl tert-Butyl cation : O: H H + (CH3)3CO H O δ+ H + (CH3)3C + H2O slow, unimolecular H : O: Potential Energy Diagram for Step 2 δ+ H + tert-Butyl tert-Butyl cation H + (CH3)3C O : The product of this step is a carbocation. It is an intermediate in the overall process. H + Cl – H H Reaction coordinate Carbocation R + C Figure 4.8 Structure of tert-Butyl tert-Butyl Cation R CH3 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. Step 3: Carbocation Capture + (CH3)3C (CH3)3C tert-Butyl tert-Butyl chloride + C CH3 Positively charged carbon is sp2 hybridized. All four carbons lie in same plane. Unhybridized p orbital is perpendicular to plane of four carbons. Step 3: Carbocation Capture Bond formation between the positively charged .. – : carbocation and the + : Cl: Cl .. negatively charged chloride ion is fast. fast. fast, bimolecular .. Cl: Cl .. : H3C Two species are involved in this step. Therefore, this step is bimolecular. bimolecular. + (CH3)3C .. – : Cl: : Cl .. + fast, bimolecular (CH3)3C tert-Butyl tert-Butyl chloride δ+ (CH3)3C + – C+ Cl CH3 Lewis acid Lewis base Electrophile Nucleophile The carbocation is an electrophile. electrophile. Chloride ion is a nucleophile. nucleophile. .. Cl: Cl .. : Potential Energy Diagram for Step 3 Step 3: Carbocation Capture H3C CH3 This is a Lewis acidLewis base reaction. The carbocation is the Lewis acid; chloride ion is the Lewis base. H3C CH3 C Cl Cl δ− Potential energy + (CH3)3C + Cl– H 3C (CH3)3C—Cl Reaction coordinate Question 7 Question 8 What is the rate-determining step in the reaction of cyclobutanol with HCl? HCl? The species shown below represents _____ of the reaction between isopropyl alcohol and hydrogen bromide. A) protonation of the OH group A) the alkyloxonium ion intermediate B) the transition step of the bimolecular proton transfer C) the transition state of the attack of the nucleophile on the carbocation D) the transition state of the unimolecular dissociation B) attack of the bromide on the carbocation C) simultaneous formation of the C-Br bond and the breaking of the C-OH bond D) carbocation formation Potential Energy Diagram - Overall 4.9 Potential Energy Diagrams for Multistep Reactions: The SN1 Mechanism The potential energy diagram for a multistep mechanism is simply a collection of the potential energy diagrams for the individual steps. Consider the three-step mechanism for the reaction of tert-butyl tert-butyl alcohol with HCl. (CH3)3COH + HCl carbocation formation (CH3)3CCl + H2O carbocation formation carbocation capture (CH 33)33C R+ 25° 25°C δ+ O H δ– Cl carbocation capture R+ H proton transfer ROH proton transfer + ROH2 ROH RX + ROH2 RX carbocation formation H δ+ (CH 33)33C O carbocation formation carbocation capture carbocation capture δ+ δ+ R+ H (CH 33)33C R+ proton transfer proton transfer ROH + ROH2 ROH RX Mechanistic Notation + ROH2 RX Mechanistic Notation The mechanism just described is an example of an SN1 process. The molecularity of the rate-determining step defines the molecularity of the overall reaction. SN1 stands for substitution-nucleophilicunimolecular. unimolecular. δδ++ (CH (CH33))33C C The molecularity of the rate-determining step defines the molecularity of the overall reaction. H H O O δδ++ H H Rate-determining step is unimolecular dissociation of alkyloxonium ion. Activation energy for formation of tertiary carbocation is less than that for formation of secondary carbocation. Question 9 Select the most stable carbocation. carbocation. δ+ R3C δ+ H O •• A) B) C) D) H R3C H + • O• H Eact + R3C + •• O •• H H δ– Cl Preparation of Alkyl Halides 25° 25°C (CH3)3COH + HCl 4.12 Reaction of Primary Alcohols with Hydrogen Halides. The SN2 Mechanism (CH3)3CCl + H2O 78-88% OH + HBr 80-100° 80-100°C Br + H2O 73% 120° 120°C CH3(CH2)5CH2OH + HBr CH3(CH2)5CH2Br + H2O 87-90% Preparation of Alkyl Halides Primary carbocations are too high in energy to allow S N1 mechanism. Yet, primary alcohols are converted to alkyl halides. Primary alcohols react by a mechanism called SN2 (substitution-nucleophilic-bimolecular). CH3(CH2)5CH2OH + HBr 120° 120°C The SN2 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 (water is the “leaving group” group”) by the halide which is the “nucleophile” nucleophile” CH3(CH2)5CH2Br + H2O 87-90% Example CH3(CH2)5CH2OH + HBr 120° 120°C CH3(CH2)5CH2Br + H2O Mechanism Step 1: Proton transfer from HBr to 1-heptanol CH3(CH2)5CH2 Mechanism Step 2: Reaction of alkyloxonium ion with bromide ion. H .. – + + CH3(CH2)5CH2 O : : Br: Br .. : .. : Br .. .. O: + H H H fast, bimolecular H + CH3(CH2)5CH2 O : slow, bimolecular + .. – : Br: .. H H CH3(CH2)5CH2 Heptyloxonium ion + 1-Bromoheptane :O: H δ+ δ– Br .. Br: Br .. : CH2 OH2 Question 11 CH3(CH2)4CH2 Which of the following is the least able to serve as a nucleophile in a chemical reaction? A) BrB) OH- proton transfer C) NH3 ROH + ROH2 D) CH3+ RX Reagents for ROH to RX Thionyl chloride SOCl2 + ROH → RCl + HCl + SO 2 Examples CH3CH(CH2)5CH3 SOCl SOCl2 K2CO3 Cl OH (81%) Phosphorus tribromide PBr 3 + 3ROH → 3RBr + H 3PO 3 Also: Phosphorus trichloride CH3CH(CH2)5CH3 (pyridine often used instead of K2CO3) (CH3)2CHCH 2OH PBr3 (CH3)2CHCH 2Br (55-60%)