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4.8 Mechanism of the Reaction of Alcohols with Hydrogen Halides

4.8 Mechanism of the Reaction of Alcohols with Hydrogen Halides. About mechanisms. A mechanism describes how reactants are converted to products. Mechanisms are often written as a series of chemical equations showing the elementary steps.

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4.8 Mechanism of the Reaction of Alcohols with Hydrogen Halides

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  1. 4.8Mechanism of the Reaction of Alcohols with Hydrogen Halides

  2. About mechanisms • A mechanism describes how reactants areconverted to products. • Mechanisms are often written as a series ofchemical equations showing the elementary steps. • An elementary step is a reaction that proceedsby way of a single transition state. • Mechanisms can be shown likely to be correct,but cannot be proven correct.

  3. 25°C (CH3)3CCl + H2O (CH3)3COH + HCl tert-Butyl alcohol tert-Butyl chloride About mechanisms For the reaction: • the generally accepted mechanism involves three elementary steps. • Step 1 is a Brønsted acid-base reaction.

  4. .. + : .. .. : O (CH3)3C H Cl H fast, bimolecular H .. – + + : : : O Cl (CH3)3C .. H tert-Butyloxonium ion Like proton transferfrom a strong acid towater, proton transferfrom a strong acid toan alcohol is normallyvery fast. Step 1: Proton transfer

  5. .. + : .. .. : O (CH3)3C H Cl H fast, bimolecular H .. – + + : : : O Cl (CH3)3C .. H tert-Butyloxonium ion Step 1: Proton transfer Two molecules reactin this elementarystep; therefore it is bimolecular.

  6. .. + : .. .. : O (CH3)3C H Cl H fast, bimolecular H .. – + + : : : O Cl (CH3)3C .. H tert-Butyloxonium ion Step 1: Proton transfer Species formed inthis step (tert-butyloxonium ion) isan intermediate inthe overall reaction.

  7.   (CH3)3CO (CH3)3CO H Cl H H Potentialenergy + + Cl– H Reaction coordinate Potential energy diagram for Step 1 (CH3)3COH + H—Cl

  8. H + : O (CH3)3C H slow, unimolecular H + + : : O (CH3)3C H tert-Butyl cation Step 2: Carbocation formation Dissociation of the alkyloxonium ion involvesbond-breaking, withoutany bond-making to compensate for it. Ithas a high activationenergy and is slow.

  9. H + : O (CH3)3C H slow, unimolecular H + + : : O (CH3)3C H tert-Butyl cation Step 2: Carbocation formation A single moleculereacts in this step;therefore, it isunimolecular.

  10. H + : O (CH3)3C H slow, unimolecular H + + : : O (CH3)3C H tert-Butyl cation Step 2: Carbocation formation The product of thisstep is a carbocation.It is an intermediatein the overall process.

  11. H   (CH3)3CO (CH3)3C O H H Potentialenergy + + (CH3)3C + H2O H Reaction coordinate Potential energy diagram for Step 2

  12. + R R C R 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.

  13. CH3 + C H3C CH3 Figure 4.9 Structure of tert-Butyl Cation. • Positively charged carbon is sp2 hybridized. • All four carbons lie in same plane. • Unhybridized p orbital is perpendicular to plane of four carbons.

  14. .. – : : Cl .. : Cl (CH3)3C .. Step 3: Carbocation capture Bond formation betweenthe positively chargedcarbocation and the negatively charged chloride ion is fast. + + (CH3)3C fast, bimolecular Two species are involved in this step.Therefore, this stepis bimolecular. .. tert-Butyl chloride

  15. .. – : : Cl .. : Cl (CH3)3C .. Step 3: Carbocation capture This is a Lewis acid-Lewis base reaction.The carbocation is theLewis acid; chlorideion is the Lewis base. + + (CH3)3C fast, bimolecular The carbocation is an electrophile. Chloride ion is anucleophile. .. tert-Butyl chloride

  16. – + Cl (CH3)3C (CH3)3CCl Lewis acid Lewis base Electrophile Nucleophile Step 3: Carbocation capture +

  17.  - (CH3)3C Cl Potentialenergy + (CH3)3C + Cl– Reaction coordinate Potential energy diagram for Step 3 (CH3)3C—Cl

  18. 4.9Potential Energy Diagrams for Multistep Reactions:The SN1 Mechanism

  19. 25°C (CH3)3CCl + H2O (CH3)3COH + HCl Potential Energy Diagram-Overall • 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 alcohol with HCl.

  20. carbocation formation carbocation capture R+ proton transfer + ROH ROH2 RX

  21. carbocation formation carbocation capture – + R+ O H Cl (CH3)3C H proton transfer + ROH ROH2 RX

  22. carbocation formation carbocation capture H + + O (CH3)3C R+ H proton transfer + ROH ROH2 RX

  23. carbocation formation carbocation capture + – Cl (CH3)3C R+ proton transfer + ROH ROH2 RX

  24. Mechanistic 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 theoverall reaction.

  25. H + + O (CH3)3C H Mechanistic notation • The molecularity of the rate-determining step defines the molecularity of theoverall reaction. Rate-determining step is unimoleculardissociation of alkyloxonium ion.

  26. 4.10Structure, Bonding, and Stabilityof Carbocations

  27. + R R C R Carbocations • Most carbocations are too unstable to beisolated, but occur as reactive intermediates ina number of reactions. • When R is an alkyl group, the carbocation isstabilized compared to R = H.

  28. + H H C H Carbocations Methyl cationleast stable

  29. Carbocations + H H3C C H Ethyl cation(a primary carbocation) is more stable than CH3+

  30. Carbocations + CH3 H3C C H Isopropyl cation(a secondary carbocation) is more stable than CH3CH2+

  31. Carbocations + CH3 H3C C CH3 tert-Butyl cation(a tertiary carbocation) is more stable than (CH3)2CH+

  32. Figure 4.15 Stabilization of carbocations via the inductive effect positively chargedcarbon pullselectrons in  bondscloser to itself +

  33.   Figure 4.15 Stabilization of carbocations via the inductive effect positive charge is"dispersed ", i.e., sharedby carbon and thethree atoms attachedto it  

  34.   Figure 4.15 Stabilization of carbocations via the inductive effect electrons in C—Cbonds are more polarizable than thosein C—H bonds; therefore, alkyl groupsstabilize carbocationsbetter than H. • Electronic effects transmitted through bonds are called "inductive effects."  

  35. Figure 4.16 Stabilization of carbocations via hyperconjugation electrons in this bond can be sharedby positively chargedcarbon because the orbital can overlap with the empty 2porbital of positivelycharged carbon +

  36.   Figure 4.16 Stabilization of carbocations via hyperconjugation electrons in this bond can be sharedby positively chargedcarbon because the orbital can overlap with the empty 2porbital of positivelycharged carbon

  37.   Figure 4.16 Stabilization of carbocations via hyperconjugation Notice that an occupiedorbital of this type isavailable when sp3hybridized carbon is attached to C+, but is not available when His attached to C+. Therefore,alkyl groupsstabilize carbocationsbetter than H does.

  38. 4.11Effect of Alcohol Structure on Reaction Rate

  39. H H + R O •• •• •• O H H + Slow step is: R + • The more stable the carbocation, the fasterit is formed. • Tertiary carbocations are more stable thansecondary, 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 fasterthan methanol.

  40. H d+ d+ O CH3 •• H H + Eact O CH3 + •• •• H H + O CH3 •• H High activation energy for formation of methyl cation.

  41. H d+ d+ O RCH2 •• H H + O Eact RCH2 + •• •• H H + O RCH2 •• H Smaller activation energy for formation ofprimary carbocation.

  42. H d+ d+ O R2CH •• H H + O R2CH + •• •• H H + O R2CH •• H Activation energy for formation of secondarycarbocation is less than that for formation ofprimary carbocation. Eact

  43. H d+ d+ R3C O •• H H + O R3C + •• •• H Eact H + O R3C •• H Activation energy for formation of tertiarycarbocation is less than that for formation ofsecondary carbocation.

  44. 4.12Reaction of Primary Alcohols with Hydrogen Halides.The SN2 Mechanism

  45. Br OH Preparation of Alkyl Halides 25°C (CH3)3CCl + H2O 78-88% • (CH3)3COH + HCl 80-100°C + + HBr H2O 73% 120°C CH3(CH2)5CH2OH + HBr CH3(CH2)5CH2Br + H2O 87-90%

  46. Preparation of Alkyl 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). 120°C CH3(CH2)5CH2OH + HBr CH3(CH2)5CH2Br + H2O 87-90%

  47. 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 from alkyloxonium ion by halide

  48. Example 120°C CH3(CH2)5CH2OH + HBr CH3(CH2)5CH2Br + H2O

  49. CH3(CH2)5CH2 O H Br fast, bimolecular H .. – + + : : : O CH3(CH2)5CH2 Br .. H Heptyloxonium ion Mechanism Step 1: Proton transfer from HBr to 1-heptanol .. .. + : : .. H

  50. H .. + – : : O : Br CH3(CH2)5CH2 .. H slow, bimolecular H .. + : : : CH3(CH2)5CH2 Br O .. H 1-Bromoheptane Mechanism Step 2: Reaction of alkyloxonium ion with bromide ion. +

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