the characteristic reaction of alkenes is addition to the double bond. + a—b c c a c c b reactions...
TRANSCRIPT
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The characteristic reaction of alkenes is addition to the double bond.
+ A—BC C A C C B
Reactions of Alkenes
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Reactions involving carbocations:
1. Substitutions via Sn1.
2. Eliminations via the E1.
3. Additions to alkenes and alkynes.
Reminder:
Reactions of Alkenes
CH3
+
2° carbocation
CH3
+
3° carbocation
1,2-alkyl shift for stability
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Terms: Driven by weaker pi bond to 2, new, stronger sigma bonds.
Overall Transformation: C=C to a new functional group.
Regioselectivity: Which side a specific group is added to.
Stereoselectivity: Whether the addition is cis or trans.
Nucleophile: pi bond has high electron density; reacts with electrophiles.
Electrophile: An electron poor species, which reacts with electron rich
species.
Electrophilic Addition: An electrophile adds to a nucleophile, where two
systems combine to form one product. In actuality, the nucleophile
attacks the electrophile
Alkene Addition Terms
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syn addition anti addition
syn Addition versus anti Addition
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General Format:
Depending on timing, two different mechanisms are possible:
Two Step: Full Carbocation Mechanism
1: Reaction of the C=C with E+ Carbocation
2: Nucleophilic attack.
Electrophilic Addition
NuE
NuE +
:Nu-Nu E +
E
Nu
+ - E +
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One Step: Partial Carbocation Mechanism
Simultaneous formation of the two sigma bonds.
Tips:
1. Find the electrophile.
2. The alkene attacks the electrophile
- Regioselectivity.
3. Figure out if carbocations are formed.
- 2 step reaction is not stereoselective.
Electrophilic Addition
E Nu E Nu
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One Step: Partial Carbocation Mechanism
Simultaneous formation of the two sigma bonds.
Tips:
4. Determine if the new sigma bonds are from the same species
- Syn addition.
5. Determine if the two new sigma bonds are from different species
- Anti addition.
Electrophilic Addition
E Nu E Nu
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CH3CH2 CH2CH3
H H
CH3CH2CH2CHCH2CH3
Br
(76%)
CHCl3, -30°CC C
HBr
Example
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Electrophilic addition of hydrogen halides to alkenes proceeds by rate-determining formation of a carbocation intermediate.
Mechanism
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Electrons flow fromthe system of thealkene (electron rich) toward the positivelypolarized proton of the hydrogen halide.
Mechanism
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Free Radical Mechanism:
Free Radical Mechanism
Br Br
H
BrBr
H
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Which mechanism is used for the following reactions?
Carbocation
Radical
Learning Check
C C H X C C XHX= I, Br, Cl
+
C C H X C C XHX= I, Br, Cl
+
hv
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Electrophilic Addition:
Hydrogenation:
Hydrohalogenation:
Hydration:
Hydroboration:
Alkene Reactions
Nu E
E
Nu
+
C C H H C C
H H
catalyst+
C C H X C C XHX= I, Br, Cl
+
C C H OH C C OHH+H+
C C H OH C C OHH+1. BH3
2. H2O2/NaOH
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Halogenation:
Halohydration:
Hydrohalogenation:
Epoxidation:
Ozonolysis:
Alkene Reactions
C C H X C C XHX= I, Br, Cl
+
C C X X C C
X
X
+X= I, Br, Cl
C C X XH2O
C C
OH
X
HX+X= I, Br, Cl
+
C C RCO3H C C
O
RCO2H+ +
C C O3C O CO+
+
work-up
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Reaction Type: Electrophilic Addition (Reduction). Used for heats of hydrogenation stability.
Overall Reaction: H2 + Alkene Alkane
Requires: Metal catalyst (Pt, Pd, Ni, Rh).
Regioselectivity: None.
Stereospecificity: Only syn addition products.
Hydrogenation
C C H H C C
H H
catalyst+
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H HC C
A
B
X
YH H
Mechanism of Catalytic Hydrogenation
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H
H H H
Mechanism of Catalytic Hydrogenation
C CA
B
X
YAbsorption of H
by catalyst
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H
H H HCC
AB
XY
Mechanism of Catalytic Hydrogenation
Addition of alkene to catalyst.
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H
H H HCC
AB
XY
Mechanism of Catalytic Hydrogenation
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H H
H
CC
AB
XY
H
Mechanism of Catalytic Hydrogenation
Addition of H to alkene.
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H HH
CC
AB
XY
H
Mechanism of Catalytic Hydrogenation
Alkane leaves catalyst with H in
syn position.
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CO2CH3
CO2CH3
(100%)
H2, Pt
Example of syn-Addition
CO2CH3
CO2CH3
H
H
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Example 1:
Example 2:
Learning Check
H2
or
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When an unsymmetrically substituted alkene reacts with a hydrogen halide, the hydrogen adds to the carbon that has the greater number of hydrogen substituents, and the halogen adds to the carbon that has the fewer hydrogen substituents.
Markovnikov's Rule
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Occurs due to the formation of the most stable carbocation:
Markovnikov rules extend to other additions which don’t involve halides:
Markovnikov Example
C C
CH3
H X C C
CH3
Br
X= I, Br, Cl+ C C
CH3
Br
+
Major product
CH3 CH3
HOH
H3O+
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Br
CH3CH2CHCH3CH2CH3CH2CH
acetic acid
HBr
Mechanistic Basis for Markovnikov's Rule:
Example 1
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Br
CH3CH2CHCH3CH2CH3CH2CH
HBr
CH3CH2CH—CH3 + Br – +
CH3CH2CH2—CH2
+
primary carbocation is less stable: not formed
Mechanistic Basis for Markovnikov's Rule:
Example 1
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Mechanistic Basis for Markovnikov's Rule:
Example 2
H
CH3
CH3
Cl
HCl
0°C
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HCl
H
CH3
CH3
Cl
H H CH3
+
secondary carbocation is less stable: not formed
CH3
H
+
H
Cl–
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Reactions that are Anti Markovnikov add the hydrogen to the side with the least number of hydrogens. The ‘group’ is attached to the less substituted carbon.
Reaction does not go through a carbocation intermediate.
Anti Markovnikov
H
H
CH3
H
1. BH3
2. H2O2/ NaOH
H
H
CH3
HH OH
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Reaction of Alkenes with HBr (no radical formation).
Reaction Type:
Overall Reaction: HX + Alkenes Alkyl Halide.
Hydrogen Halide Reactivity Order:
HI > HBr > HCl > HF (same as acidity order)
Regioselectivity: Markovnikov Rule.
“For addition of hydrogen halides to alkenes, the H atom adds to the C with the most H atoms already present” meaning it goes through the more stable carbocation intermediate.”
Hydrohalogenation
Electrophilic Addition; Reduction (H) and Oxidation (X)
C C H X C C XHX= I, Br, Cl
+
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Reaction of Alkenes with HBr (no radical formation).
Stereoselectivity: None, during mechanism, a planar carbocation is formed.
Requirements: HBr, No light (hv), no peroxides, no radicals created.
Hydrohalogenation
C C H X C C XHX= I, Br, Cl
+
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Mechanism:
Hydrohalogenation
H
H
H
CH3
H+
H
C
CH3CH3
+
Br-H
C
CH3CH3
BrProtonate alkene.
Can be viewed as an acid base reaction (Lewis).
Pi electrons act as a Lewis Base.
Bromide ion is nucleophile. Carbocation is electrophile.
Forms most stable carbocation.
Nucleophilic attack.Forms alkyl bromide.