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4.7 Preparation of Alkyl Halides from Alcohols and Hydrogen Halides

4.7 Preparation of Alkyl Halides from Alcohols and Hydrogen Halides. ROH + HX  RX + H 2 O. least reactive. most reactive. Reaction of Alcohols with Hydrogen Halides. R O H + H X  R X + H O H. Hydrogen halide reactivity H F H Cl H Br H I. least reactive.

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4.7 Preparation of Alkyl Halides from Alcohols and Hydrogen Halides

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  1. 4.7Preparation of Alkyl Halides fromAlcohols and Hydrogen Halides • ROH + HX  RX + H2O

  2. least reactive most reactive Reaction of Alcohols with Hydrogen Halides ROH + HX RX + HOH • Hydrogen halide reactivity HF HCl HBr HI

  3. least reactive most reactive Reaction of Alcohols with Hydrogen Halides ROH + HX RX + HOH • Alcohol reactivityCH3OH RCH2OH R2CHOH R3COHMethanol Primary Secondary Tertiary

  4. 80-100°C + + HBr Br H2O OH 73% 120°C CH3(CH2)5CH2OH + HBr CH3(CH2)5CH2Br + H2O 87-90% Preparation of Alkyl Halides 25°C (CH3)3CCl + H2O 78-88% • (CH3)3COH + HCl

  5. NaBrH2SO4 CH3CH2CH2CH2Br CH3CH2CH2CH2OH heat 70-83% Preparation of Alkyl Halides A mixture of sodium bromide and sulfuric acid may be used in place of HBr.

  6. 4.8Mechanism of the Reaction of Alcohols with Hydrogen Halides

  7. 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.

  8. 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.

  9. .. + : .. .. : 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

  10. .. + : .. .. : 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.

  11. .. + : .. .. : 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.

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

  13. 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.

  14. 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.

  15. 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.

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

  17. + 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.

  18. 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.

  19. .. – : : 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

  20. .. – : : 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

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

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

  23. 4.9Potential Energy Diagrams for Multistep Reactions:The SN1 Mechanism

  24. 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.

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

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

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

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

  29. 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.

  30. 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.

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