preparation of alkyl halides from alcohols and … 7... · hydrogen halide reactivity hf hcl hbr hi...
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Preparation of Alkyl Halides fromPreparation of Alkyl Halides fromAlcohols and Hydrogen HalidesAlcohols and Hydrogen Halides
ROH + HX ROH + HX →→ RX + HRX + H22OO
RROOHH + + HHXX →→ RRXX + + HHOOHH
Hydrogen halide reactivityHydrogen halide reactivity
HHFF HHClCl HHBrBr HHII
Reaction of Alcohols with Hydrogen HalidesReaction of Alcohols with Hydrogen Halides
least reactiveleast reactive most reactivemost reactive
Alcohol reactivityAlcohol reactivity
CHCH33OOHH RCHRCH22OOH H RR22CHCHOOH H RR33CCOOHHMethanolMethanol PrimaryPrimary SecondarySecondary TertiaryTertiary
RROOH H + + HHXX →→ RRXX + + HHOOHH
Reaction of Alcohols with Hydrogen HalidesReaction of Alcohols with Hydrogen Halides
least reactiveleast reactive most reactivemost reactive
(CH(CH33))33CCOHOH + + HHClCl (CH(CH33))33CCClCl + H+ H22OO7878--88%88%
2525°°CC
CHCH33(CH(CH22))55CHCH22OHOH + + HHBrBr
CHCH33(CH(CH22))55CHCH22Br Br + H+ H22OO
8787--90%90%
120120°°CC
Preparation of Alkyl HalidesPreparation of Alkyl Halides
73%73%
8080--100100°°CCOHOH ++ HHBrBr BrBr ++ HH22OO
Mechanism of the Reaction of Mechanism of the Reaction of Alcohols with Hydrogen HalidesAlcohols with Hydrogen Halides
the generally accepted mechanism involves the generally accepted mechanism involves three elementary steps. three elementary steps.
Step 1 is a Step 1 is a BrBrøønstednsted acidacid--base reaction.base reaction.
About mechanismsAbout mechanisms
(CH(CH33))33COH + COH + HClHCl (CH(CH33))33CCl + HCCl + H22OO2525°°CC
terttert--Butyl alcoholButyl alcohol terttert--Butyl chlorideButyl chloride
For the reaction:For the reaction:
Step 1: Proton transferStep 1: Proton transfer
....HH ClCl::
....++
fast, bimolecularfast, bimolecular
....HH
OO :: ++(CH(CH33))33CC
HH
ClCl::....
::––++
terttert--ButyloxoniumButyloxonium ionion
HH
OO ::....
(CH(CH33))33CC
Like proton transferLike proton transferfrom a strong acid tofrom a strong acid towater, proton transferwater, proton transferfrom a strong acid tofrom a strong acid toan alcohol is normallyan alcohol is normallyvery very fastfast..
....HH ClCl::
....++
fast, bimolecularfast, bimolecular
....HH
OO :: ++(CH(CH33))33CC
HH
ClCl::....
::––++
terttert--ButyloxoniumButyloxonium ionion
HH
OO ::....
(CH(CH33))33CC
Species formed inSpecies formed inthis step (this step (terttert--butyloxoniumbutyloxonium ion) ision) isan an intermediateintermediate ininthe overall reaction.the overall reaction.
Step 1: Proton transferStep 1: Proton transfer
PotentialPotentialenergyenergy
Reaction coordinateReaction coordinate
Potential energy diagram for Step 1 Potential energy diagram for Step 1
HH ClClδ+δ+ δ−δ−
(CH(CH33))33CCOO
HH
(CH(CH33))33CCOOH + HH + H——ClCl+ + ClCl ––
++(CH(CH33))33CCOO
HH
HH
(CH(CH33))33CC OO
HH
::
HH++
++
slow, slow, unimolecularunimolecular
(CH(CH33))33CC OO
HH
::
HH
::
terttert--Butyl Butyl cationcation
++
Step 2: Step 2: CarbocationCarbocation formationformationDissociation of the Dissociation of the alkyloxoniumalkyloxonium ion involvesion involvesbondbond--breaking, withoutbreaking, withoutany bondany bond--making to making to compensate for it. Itcompensate for it. Ithas a high activationhas a high activationenergy and is energy and is slowslow. .
(CH(CH33))33CC OO
HH
::
HH++
++
slow, slow, unimolecularunimolecular
(CH(CH33))33CC OO
HH
::
HH
::
terttert--Butyl Butyl cationcation
++
A single moleculeA single moleculereacts in this step;reacts in this step;therefore, it istherefore, it isunimolecularunimolecular..
Step 2: Step 2: CarbocationCarbocation formationformation
(CH(CH33))33CC OO
HH
::
HH++
++
slow, slow, unimolecularunimolecular
(CH(CH33))33CC OO
HH
::
HH
::
terttert--Butyl Butyl cationcation
++
The product of thisThe product of thisstep is a step is a carbocationcarbocation..It is an It is an intermediateintermediatein the overall process.in the overall process.
Step 2: Step 2: CarbocationCarbocation formationformation
PotentialPotentialenergyenergy
Reaction coordinateReaction coordinate
Potential energy diagram for Step 2 Potential energy diagram for Step 2
++(CH(CH33))33CCOO
HH
HH
(CH(CH33))33C + HC + H22OO++
δ+δ+(CH(CH33))33CC OO
HH
HHδ+δ+
fast, bimolecularfast, bimolecular
++(CH(CH33))33CC++
terttert--Butyl chlorideButyl chloride
....(CH(CH33))33CC ClCl::
....
....ClCl::::
.... ––
Step 3: Step 3: CarbocationCarbocation capturecaptureBond formation betweenBond formation betweenthe positively chargedthe positively chargedcarbocationcarbocation and the and the negatively charged negatively charged chloride ion is chloride ion is fastfast. .
Two species are Two species are involved in this step.involved in this step.Therefore, this stepTherefore, this stepis is bimolecularbimolecular..
PotentialPotentialenergyenergy
Reaction coordinateReaction coordinate
Potential energy diagram for Step 3Potential energy diagram for Step 3
(CH(CH33))33C + C + ClCl––++
δ+δ+(CH(CH33))33CC ClClδδ−−
(CH(CH33))33CC——ClCl
Potential Energy Diagrams for Potential Energy Diagrams for MultistepMultistep Reactions:Reactions:The SThe SNN1 Mechanism1 Mechanism
The potential energy diagram for a The potential energy diagram for a multistepmultistep mechanism is simply a collection of the mechanism is simply a collection of the potential energy diagrams for the individual potential energy diagrams for the individual steps.steps.
Consider the threeConsider the three--step mechanism for the step mechanism for the reaction of reaction of terttert--butyl alcohol with butyl alcohol with HClHCl..
Potential Energy DiagramPotential Energy Diagram--OverallOverall
(CH(CH33))33COH + COH + HClHCl (CH(CH33))33CCl + HCCl + H22OO2525°°CC
proton transfer
ROHROHROHROH22
++
carbocationformation
RR++
carbocationcapture
RXRX
proton transfer
ROHROHROHROH22
++
carbocationformation
RR++
carbocationcapture
RXRX
(CH3)3C(CH3)3C OO
HH
HH ClClδ+δ+ δ–δ–
proton transfer
ROHROHROHROH22
++
carbocationformation
RR++
carbocationcapture
RXRX
(CH3)3C(CH3)3Cδ+δ+OO
HH
HHδ+δ+
proton transfer
ROHROHROHROH22
++
carbocationformation
RR++
carbocationcapture
RXRX
(CH3)3C(CH3)3Cδ+δ+
ClClδ–δ–