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Addition Reactions• Addition Reactions to Alkenes (Section 8.1)• Markovnikov’s Rule (Section 8.2)• Stereochemistry of Ionic Addition to Alkenes (Section 8.3)• H2SO4 Additions to Alkenes (Section 8.4)• H2O Additions to Alkenes (Section 8.5)• Oxymercuration/Demurcuration (Section 8.6)• Hydroboration/Oxidation (Section 8.7)• Addition of Br2 and Cl2 to Alkenes (Section 8.12)• Stereochemistry of Dihalide Additions (Section 8.13)• Halohydrin Formation [Net Addition of X-OH] (Section 8.14)• Divalent Carbon Compounds: Carbenes (Section 8.15)• Oxidations of Alkenes (Sections 8.16-8.17)• Additions to Alkynes (Sections 8.18-8.19)• Oxidative Cleavage of Alkynes (Section 8.20)• Applications in Synthesis (Section 8.21)
Addition Reactions: Addition to Alkenes
C C A B C C BAAddition
• Have Already Looked at Addition of H2 (Hydrogenation)
• Will Now Add Additional Reagents to Our Arsenal
HX (I, Br, Cl)
H2SO4
H2O
Br2
Cl2
I2
Why Do Additions to Alkenes Work?
• Conversion of Bond to 2 Bonds Typically Energy Favored
• Two Bonds Higher Energy than One + One
• Overall Process is thus Typically Exothermic
• Electrons are Exposed (ABOVE and BELOW sp2 Plane)
• Bonds Good at Capturing Electrophiles (H+, Lewis Acids, X2)
• Metal Ions With Vacant Orbitals Also Good Electrophiles
• Let’s Look at the Addition Reaction of a Hydrogen Halide
Addition Reactions: HX to Alkenes
C C
H Br
H Br H Br
• General Order of HX Reactivity:
HI > HBr > HCl > HF
• Usually Dissolved in Solvent (CH3CO2H, CH2Cl2)
• Can be Bubbled Through Solution as a Gas
• Addition of HCl not Generally Useful (Works w/ Silica Gel)
Addition Reactions: HBr to Alkenes
C C
H Br
H Br H Br
• Bond (Nucleophile) Protonate Carbocation Intermediate
• Carbocation Captured by Br¯ (Nucleophile) HBr Added
• HBr (or other HX) Addition in Two Overall Steps
• H+ and Carbocation are the Respective Electrophiles
• This is a SYMMETRIC Alkene ASYMMETRIC ALKENES?
Markovnikov’s Rule: HBr to Alkenes
HBrCH2Cl2, 0 oC
Br
Br
MAJOR MINOR (TRACE)
• 2-Bromopropane is Major Product
• Only Very Small Amount of 1-Bromopropane Observed
• True With Other Alkenes
HBrCH2Cl2, 0 oC
Br
Br
MAJOR MINOR (TRACE)
Markovnikov’s Rule: Why?
HBrCH2Cl2, 0 oC
Br
Br
MAJOR MINOR (TRACE)
• Product Distribution Explained When Looking at Intermediates
• Recall Discussion of Carbocation Stability (2° > 1°)
• Major Product Formed From More Stable C+ Intermediate
H Br
H
H
Less StableCarbocation
More StableCarbocation
Markovnikov’s Rule: C+ Stability
• We Know 2° Carbocations More Stable Than 1°
• Major Product Formed From More Stable C+ Intermediate
• Means TS in 2° Carbocation Pathway Lower in Energy
• Lower Energy of Activation
• Activation Energies in 1° Carbocation Pathways Much Larger
H Br
H
H
Less StableCarbocation
More StableCarbocation
Markovnikov’s Rule: Summary
In the ionic additions of an unsymmetrical reagent to a double bond, the positive portion of the adding reagent attaches itself to a carbon atom of the double bond so as to yield the MORE STABLE CARBOCATION as an INTERMEDIATE
MARKOVNIKOV’S RULE:
I ClI
Cl
I
Cl
I Cl
Recall Bond Polarization:
This Addition “Preference” is Called REGIOSELECTIVITY
Stereochemistry in Ionic Additions
H Br
H
CH3
Br
Br
CH3
H
Br
+CH3
H
Br
Top Capture
Bottom Capture
• Just as We Saw in SN1: C+ Has TWO FACES
• Top and Bottom Attack Give Two Stereochemical Products
• R and S Enantiomers Formed as a Racemic Mixture (50:50)
H2SO4 Addition to Alkenes
• Must Add COLD Sulfuric Acid; Form Alkyl Hydrogen Sulfates
• Carbenes are Highly Reactive Species; Short-Lived
• Excellent Utility is in the Synthesis of Cyclopropanes
• Let’s Look at Some Reactions Making Use of Carbenes
Divalent Carbon Compounds: Carbenes
CH2C C + C C
CH2
KOC(CH3)3
CHCl3
Cl
Cl
CH2I2, Zn(Cu)
• Halogen Substituted Carbenes from Haloforms (CHCl3, etc.)
• Last Reaction is Called the “Simmons-Smith” Reaction
Oxidation: Syn Dihydroxylation
• C=C is Oxidized by OsO4
• Addition of Hydroxyl Groups Proceeds w/ SYN Stereochemistry
• Can Also use KMNO4 (More Powerful, May Cleave Diol)
• If Using KMNO4, Want COLD Reaction Temperatures
• OsO4 is Expensive; Can Use Catalytically if NMO is Added
1. OsO4, pyridine
2. Na2SO3/H2O
OH
OH
Propene 1,2-propanediol(propylene glycol)
Oxidation: Syn Dihydroxylation (2)
• Syn Addition Due to 5-Centered Transition State
• Transition State Same for KMNO4 Oxidations
• Cleavage of Osmate Ester Does Not Change C-O Stereochem
OOs
O
O O
OsO4, 25 oC
Pyridine
Osmate Ester
Oxidative Cleavage of Alkenes
• Diol Believed to be Intermediate in Cleavage Reaction
• Unsubstituted Alkene Carbons Oxidized to Carbon Dioxide
• Monosubstituted Alkene Carbons Oxidized to Carboxylates
• Disubstituted Alkene Carbons Oxidized to Ketones
KMnO4, NaOHH2O, O
O
1. KMnO4, NaOH
2. H3O+
O+ O C O
2
How You May See Oxidative CleavageAn Unknown Alkene (C8H16) Gives TwoProducts When Treated w/ Hot KMnO4:
1. KMnO4, H2O NaOH, 2. H3O+
OO
OH+C8H16
The Products are a Carboxylic Acid and a Ketone, So Our Alkene Must BeTrisubstituted. We Don't Know if it is CIS or TRANS, but we Can Put the Rest of the Structure Together:
or
Ozonolysis of Alkenes
Et
Me Me1. O3, CH2Cl2, -78 oC
2. Zn/HOAcO
Et
Me
Me H
O
+
Me
1. O3, CH2Cl2, -78 oC
2. Zn/HOAcMe
O
O
H
• Milder Conditions than Treating w/ KMnO4
• “Workup” w/ Zn/HOAc Oxidative Cleavage (Ald and Ket)
• Go Through Exceptionally Unstable Intermediate (Ozonide)
Dihalide Addition To Alkynes
• Addition Reactions, Just as in Alkenes (adds Once or Twice)
• Anti Additions, First Product Usually a Trans Dihaloalkene
• Can Get Relatively Good Trans Dihaloalkene Yields (1 eq X2)
Me MeBr2
CCl4
Br
Br
Me
Me
Br2
CCl4
Br
Br
Me
Br
Br
Me
Me MeCl2
CCl4
Cl
Cl
Me
Me
Cl2CCl4
Cl
Cl
Me
Cl
Cl
Me
Addition of HX to Alkynes
• Addition Reactions, Just as in Alkenes (adds Once or Twice)
• Final Product Typically Geminal Dihaloalkene
• Both Additions Follow Markovnikov’s Rule (explains gem.)
• Alumina Accelerates Reaction Rate (as seen w/ Alkenes)
Me Me HBr
Br
H
Me
Me
HBr
H
H
Br
Br
MeMe
geminal dihaloalkane
Oxidative Cleavage of Alkynes
• Can Use Either Ozonolysis or KMnO4 as with Alkenes
• Products of the Oxidative Cleavage are Carboxylic Acids
Me Et1. O3, CH2Cl2,
2. Zn/HOAcMe
O
OH Et
O
OH
+
Me Et1. KMnO4, NaOH
2. HOAcMe
O
OH Et
O
OH
+
iPr Ph1. O3, CH2Cl2,
2. Zn/HOAciPr
O
OH Ph
O
OH
+
Anti-Markovnikov HBr Addition
• Addition of Peroxides (ROOR) ANTI-MARKOVNIKOV
• Goes Through a Radical Mechanism (Chapter 10)
• Right Now Focus on Regiochemistry (Know the Reaction)