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Addition Reactions of Carbon-CarbonPi Bonds - Part 1
C CH
H H
H
C
H3C
H3C
CH2H Brδ+ δ−
C
H3C
H3C
CH2
Br H
+ C
H3C
H3C
CH2
H Br
What Is an Addition Reaction?Addition reaction: Atoms or groups are added to opposite ends of a pi bond.
Industrial production of ethanolC
H
H
C
H
H
H3PO4
H OHC C
H
H
H
H
H
OH
Partial hydrogenation ofpolyunsaturated fatH
H
H H
H HPt H
H
HH H
H
Why should I study this topic?•Addition reactions common in organic, industrial, and biological chemistry.•Examples:
Addition not limited to just alkenes.
C C C C
X Y
Addition Reactions of Carbon-Carbon Pi Bonds Lecture Supplement Part 1 -- Page 1
Alkene Pi BondsDetails of addition controlled by structure of functional groups
How does an alkene appear to the outside world?
Alkene: Two sp2 carbons
C CH
H H
H
Explore alkene pi bond shape with molecular models
Side view Top view End viewOne pi bond (two lobes)
How does an alkene react?
Pi electron cloud
Pi electrons NucleophilicReacts withelectrophiles
Therefore ThereforeCC
H H
HH
Nuc
Elec
C CH
H H
H
Alkyne Pi BondsDetails of addition controlled by structure of functional groups
How does an alkyne appear to the outside world?
Alkyne: Two sp carbons
Explore alkyne pi bond shape with molecular models
Two perpendicular pi bonds
How does an alkyne react? Similar to alkene...
Pi electron cloud
Pi electrons NucleophilicReacts withelectrophiles
Therefore Therefore
Side view Top view
CCNuc
Elec
HH
C CH H
pz
pyC CH H
pz
pyC CH H
pz
py
End view
Addition Reactions of Carbon-Carbon Pi Bonds Lecture Supplement Part 1 -- Page 2
Generic Pi Bond Addition Mechanism
•Similar process for alkynes
•Overall process: Electrophilic addition
•Not every pi bond addition reaction follows this mechanism
Elec has +, δ+, or open octet
Lost: Pi bond Gained: C-Elec bond
Lost: No bonds Gained: C-Nuc bond
rds?
Carbocation fates!
•Capture a nucleophile → addition
C C
rds?
Elec
C
Elec
C
Nuc
C
Elec
C
Nuc
Carbocation Captures Nuc Not Always SN1An important caveat.
Conclusion: Not all reactions in which a carbocation captures a nucleophile are SN1.
Avoid this common misconception: Carbocation captures a nucleophile is always SN1.
Generic electrophilic addition to an alkene mechanism:
C C
Elec
C
Elec
C
Nuc
C
Elec
C
Nuc Capture a nucleads to addition.
Capture a nucleads to substitution.
R C
R
R
LG R C
R
R
Nuc R C
R
R
Nuc
Generic SN1 mechanism:
Addition Reactions of Carbon-Carbon Pi Bonds Lecture Supplement Part 1 -- Page 3
Hydrogen Halide Addition to an Alkene
X = F, Cl, Br, I
Why is H-X electrophilic?
H Xδ+ δ−
Mechanism:C CH2
H
H3C
CH3
Br-
C CH2H3C
CH3
HBr-
C CH2
H
H3C
CH3
Br
C CH2
Br
H3C
CH3
H
C
H3C
H3C
CH2
δ+ δ−H Br
Overall reaction: C CH X
C
H
C
X
HigherΔG‡
LowerΔG‡
C
H3C
H3C
CH2
H
Br δ−
δ+
C
H3C
H3C
CH2
H
Brδ−
δ+
C
H3C
H3C
CH2H Brδ+ δ−
C
H3C
H3C
CH2
Br H
+ C
H3C
H3C
CH2
H Br
Markovnikov's Rule
Major Minor
3o 1o
Markovnikov's Rule: In the addition of a protic acid HX to an alkene, the acid hydrogen(H) becomes attached to the carbon with fewer alkyl substituents, and the halide (X)group becomes attached to the carbon with more alkyl substituents.
Vladimir MarkovnikovPhD thesis 1870
•"Them that has, gets" "The (hydrogen) rich get (hydrogen) richer"
•Orientation due to stability of carbocation intermediate
•Similar for HF, HCl, HBr, HI, H-OH2+, etc.
Addition Reactions of Carbon-Carbon Pi Bonds Lecture Supplement Part 1 -- Page 4
Hydrogen Halide Addition to an Alkyne
Observation:•Alkene has one pi bond
•Alkene adds one molecule HBr
Questions: Alkyne has two pi bonds.
C
H3C
H3C
CH2H Brδ+ δ−
C
H3C
H3C
CH2
Br H
C CH3C HH Brδ+ δ−
???
Explore via mechanism...
•Does alkyne add two molecules HBr?
•Does alkyne + HBr follow Markovnikov's
rule?
Hydrogen Halide Addition to an AlkyneMechanism...
•Alkyne is nucleophile (just like alkene)
•Protonate pi bond to form more stable carbocation (just like alkene)H3C C C H
H Brδ+ δ−
or C
H3C
CH
H
H3C C CH2
Br
CH3C CH2
Br
•Vinyl carbocation: Has positive charge on C=C carbon
•Carbocation fates – be deprotonated?
•Rearrange? Vinyl carbocations do not rearrange
•Capture a nucleophile? Br -
•Are we done yet? Product = alkene – can be protonated...
Addition Reactions of Carbon-Carbon Pi Bonds Lecture Supplement Part 1 -- Page 5
H3C C
Br
H
CH2
Hydrogen Halide Addition to an Alkyne
•Alkene is nucleophile
•Protonate pi bond to form more stable carbocation
or
•Carbocation fates – be deprotonated?
•Rearrange?
•Capture a nucleophile? Br -
CH3C CH2
Br
CH3C CH2
Br H Brδ+ δ−
H3C C
Br
CH3
H3C C
Br
Br
C
H
H
H
•Which carbocation is favored?
•Product is a dibromide
Hydrogen Halide Addition to an Alkyne
Overall mechanism: Addition follows Markovnikov's rule? Yes No
H3C C C H
H Brδ+ δ−
H3C C CH2
Br
CH3C CH2
Br H Brδ+ δ−
Br
H3C C
Br
CH3 H3C C
Br
Br
C
H
H
H
Markovnikov addition oftwo molecules HBr
Alkyne has twopi bonds
Therefore
Addition Reactions of Carbon-Carbon Pi Bonds Lecture Supplement Part 1 -- Page 6
C
H3C
H3C
CH2H OHδ+
C
H3C
H3C
CH2
OH Hδ−
Addition of Water to a Pi Bond (Hydration)
Observation: H Brδ+ δ-
Question: Do other molecules add to carbon-carbon pi bonds?
Example:
H Xδ+ δ-
H OHδ+
C
H3C
H3C
CH2H Brδ+ δ−
C
H3C
H3C
CH2
Br H
•Prediction: Markovnikov addition of water
•But... H2O (pKa 15.7) is not sufficiently acidic to protonate pi bond
•Strong acid + H2O → H3O+ (pKa -1.8) Strong acid = H2SO4, HI, HBr, HCl, H3PO4
Why?
δ−
Addition of Water to a Pi Bond (Hydration)Does alkene + H3O+ mechanism parallel alkene + HBr mechanism?
•Protonate pi bond; form more stable carbocation
•Carbocation fates: Be deprotonated?
CH3C CH3
H3C
or CH3C CH2
HH3COH2
•Oxonium ion: Oxygen has three bonds and +1 formal charge
CH3C CH3
OH3C H
H
OH2
CH3C CH3
OHH3C
+ H3O+ •The product is an alcohol
Rearrange? Capture nucleophile? Most prevelant nuc = H2O
CH3C CH2
H3C H OH2
•Deprotonation driven by Le Chatelier's principle•Use strongest base present = H2O
Addition Reactions of Carbon-Carbon Pi Bonds Lecture Supplement Part 1 -- Page 7
Addition of Water to a Pi Bond (Hydration)
Overall mechanism:
•Net reaction: Markovnikov addition of water
•Reaction is reversible:
CH3C CH2
H3CH OH2
CH3C CH3
H3C OH2
CH3C CH3
OH3C H
H
CH3C CH3
OHH3COH2
+ H3O+
CH3C CH2
H3C
+ H2OH3O+ Hydration
H3O+ E1 dehydrationC
H3C CH3
OHH3C
•Equilibrium position controlled by Le Chatelier's principle...
Excess water favors alcohol
Removing water favors alkene
•H3O+ is a _______________________
Ph C C HH OH2δ+
H2O
CPh
HO
CH
H
H OH2δ+
H2OPh C
HO
HO
C
H
H
H
Alkyne Hydration
Observation:
•Alkene has one pi bond.
•Alkene adds one molecule of H2O.
•Addition obeys Markovnikov's rule.
Questions: Alkyne has two pi bonds.
Explore via mechanism...
•Does alkyne add H2O?
•Does this addition follow Markovnikov's rule?
•Does alkyne add two molecules H2O?
C
H3C
H3C
CH2H OH2δ+
C
H3C
H3C
CH2
OH H
? ?
Addition Reactions of Carbon-Carbon Pi Bonds Lecture Supplement Part 1 -- Page 8
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