Addition Reactions of Carbon-Carbon
Pi Bonds - Part 1
H
H
C
C
H
H
H3C
C
δ+
H
CH2
δ−
Br
H3C Br
H
C
H3C
H3C H
+
CH2
C
H3C
Br
CH2
H3C
What Is an Addition Reaction?
Addition reaction: Atoms or groups are added to opposite ends of a pi bond.
C
C
C
C
X
Y
Why should I study this topic?
•Addition reactions common in organic, industrial, and biological chemistry.
•Examples:
H
H
C
H
C
H
H
OH
H
H
H
C
C
H
OH
H
H3PO4
Industrial production of ethanol
H
H
H
H
H
H
H
Pt
H
H
H
H
Partial hydrogenation of
polyunsaturated fat
H
Addition not limited to just alkenes.
Addition Reactions of Carbon-Carbon Pi Bonds Lecture Supplement Part 1 -- Page 1
Alkene Pi Bonds
Details of addition controlled by structure of functional groups
Alkene: Two sp2 carbons
H
H
C
C
H
H
Side view
One pi bond (two lobes)
Top view
End view
Explore alkene pi bond shape with molecular models
How does an alkene appear to the outside world?
Pi electron cloud
How does an alkene react?
Pi electrons
Therefore
Nucleophilic
Therefore
Elec
H
H
Reacts with
electrophiles
C
H
C
Nuc
H
Alkyne Pi Bonds
Details of addition controlled by structure of functional groups
Alkyne: Two sp carbons
pz
H
C
C
H
py
Two perpendicular pi bonds
Side view
Top view
End view
Explore alkyne pi bond shape with molecular models
How does an alkyne appear to the outside world?
Pi electron cloud
How does an alkyne react? Similar to alkene...
Pi electrons
Therefore
Nucleophilic
Therefore
Elec
Reacts with
electrophiles
H
C
C
H
Nuc
Addition Reactions of Carbon-Carbon Pi Bonds Lecture Supplement Part 1 -- Page 2
Generic Pi Bond Addition Mechanism
Elec
C
Elec has +, δ+, or open octet
C
rds?
Lost: Pi bond
Gained: C-Elec bond
Nuc
Elec
C
Carbocation fates!
C
rds?
•Capture a nucleophile → addition
Lost: No bonds
•Similar process for alkynes
Elec Nuc
C
Gained: C-Nuc bond
•Overall process: Electrophilic addition
C
•Not every pi bond addition reaction follows this mechanism
Carbocation Captures Nuc Not Always SN1
An important caveat.
Avoid this common misconception: Carbocation captures a nucleophile is always SN1.
Generic SN1 mechanism:
R
R
R
C
LG
R
R
Nuc
C
R
R
R
C
Nuc
Capture a nuc
leads to substitution.
R
Generic electrophilic addition to an alkene mechanism:
Elec
C
C
Nuc
Elec
C
C
Elec Nuc
C
C
Capture a nuc
leads to addition.
Conclusion: Not all reactions in which a carbocation captures a nucleophile are SN1.
Addition Reactions of Carbon-Carbon Pi Bonds Lecture Supplement Part 1 -- Page 3
Hydrogen Halide Addition to an Alkene
δ+
H
δ−
X
Overall reaction:
X = F, Cl, Br, I
C
H
C
X
H
X
C
C
Why is H-X electrophilic?
Higher
ΔG‡
Mechanism:
Br δ−
H3C H
C
H
δ+
H3C
C
Br-
H
H3C
δ+
CH2
C
CH2
H3C
CH3
Br
C
CH2
CH3
H3C
Br
δ−
H
CH2
H3C
Br δ−
H3C
Lower
ΔG‡
Br-
H
Br
H
C
CH2
H
δ+
C
H3C
C
CH2
H3C
CH2
CH3
CH3
H3C
Markovnikov's Rule
H3C
C
CH2
δ+
H
δ−
Br
H3C
H3C Br
C
H3C
H
CH2
3o 1o
H3C H
+
Major
C
Br
CH2
H3C
Minor
Vladimir Markovnikov
PhD thesis 1870
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.
•"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
H3C
Observation:
C
CH2
δ+
H
δ−
Br
H3C Br
C
H3C
H3C
H
•Alkene has one pi bond
CH2
•Alkene adds one molecule HBr
Questions: Alkyne has two pi bonds. •Does alkyne add two molecules HBr?
H3C
C
C
H
δ+
H
δ−
Br
???
•Does alkyne + HBr follow Markovnikov's
rule?
Explore via mechanism...
Hydrogen Halide Addition to an Alkyne
Mechanism...
H
δ+
H3C
C
C
•Alkyne is nucleophile (just like alkene)
Br
δ−
•Protonate pi bond to form more stable carbocation (just like alkene)
H
•Vinyl carbocation: Has positive charge on C=C carbon
H
Br
H3C
C
CH2
or
C
H3C
CH
•Carbocation fates – be deprotonated?
•Rearrange? Vinyl carbocations do not rearrange
•Capture a nucleophile? Br -
Br
•Are we done yet? Product = alkene – can be protonated...
C
H3C
CH2
Addition Reactions of Carbon-Carbon Pi Bonds Lecture Supplement Part 1 -- Page 5
Hydrogen Halide Addition to an Alkyne
Br
H
δ+
C
H3C
•Alkene is nucleophile
Br
δ−
•Protonate pi bond to form more stable carbocation
CH2
•Which carbocation is favored?
Br
H3C
Br
C
or
CH2
H3C
H
C
CH3
•Carbocation fates – be deprotonated?
•Rearrange?
•Capture a nucleophile? Br -
Br
H
C
C
Br
H
H3C
•Product is a dibromide
H
Hydrogen Halide Addition to an Alkyne
Overall mechanism: Addition follows Markovnikov's rule? Yes No
H
δ+
H3C
C
C
Br
δ−
H
Br
Br
H3C
C
CH2
H3C
Alkyne has two
pi bonds
H
δ+
C
Therefore
Br
δ−
Br
H3C
C
Br
CH3
CH2
H3C
Br
H
C
C
Br
H
H
Markovnikov addition of
two molecules HBr
Addition Reactions of Carbon-Carbon Pi Bonds Lecture Supplement Part 1 -- Page 6
Addition of Water to a Pi Bond (Hydration)
H3C
Observation:
C
CH2
δ+
δ-
δ+
H
δ−
Br
C
H3C
δ+
H
CH2
δ+
δ-
Why? H
Br
H3C
X molecules add to carbon-carbon pi bonds?
Question: Do other H
Example:
H3C Br
δ−
OH
H
H3C
C
CH2
H3C
δ+
H
δ−
OH
H3C OH H
C
CH2
H3C
•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
Addition of Water to a Pi Bond (Hydration)
Does alkene + H3O+ mechanism parallel alkene + HBr mechanism?
H3C
H
OH2
C
H3C
H3C
H3C
OH2
or
C
H3C
H3C
H3C
H
OH2
O
C
H3C
CH3
H3C
OH
C
H
C
CH3
H
H3C
•Protonate pi bond; form more stable carbocation
CH2
+ H3O+
CH2
•Carbocation fates:
Be deprotonated?
Rearrange?
Capture nucleophile? Most prevelant nuc = H2O
•Oxonium ion: Oxygen has three bonds and +1 formal charge
•Deprotonation driven by Le Chatelier's principle
•Use strongest base present = H2O
•The product is an alcohol
CH3
Addition Reactions of Carbon-Carbon Pi Bonds Lecture Supplement Part 1 -- Page 7
Addition of Water to a Pi Bond (Hydration)
Overall mechanism:
H
H3C
H
H3C
OH2
C
H3C
H3C
OH2
CH2
O
H
OH2
H3C
C
C
H3C
H3C
CH3
OH
+ H3O+
C
CH3
H3C
CH3
•Net reaction: Markovnikov addition of water
•Reaction is reversible:
H3C
+ H2O
C
H3C
CH2
H3C
H3O+ Hydration
OH
C
H3O+ E1 dehydration
H3C
CH3
•Equilibrium position controlled by Le Chatelier's principle...
Excess water favors alcohol
Removing water favors alkene
•H3O+ is a _______________________
Alkyne Hydration
H3C
Observation:
C
CH2
δ+
H
OH2
H3C
H3C OH H
C
CH2
H3C
Questions: Alkyne has two pi bonds.
•Alkene has one pi bond.
•Alkene adds one molecule of H2O.
•Addition obeys Markovnikov's rule.
•Does alkyne add H2O?
•Does this addition follow Markovnikov's rule?
•Does alkyne add two molecules H2O?
HO
Ph
C
C
H
δ+
H
OH2
H2O
?
δ+
H H
C
Ph
C
H
OH2
Ph
HO
H
C
C
HO
H
H
H2O
?
Explore via mechanism...
Addition Reactions of Carbon-Carbon Pi Bonds Lecture Supplement Part 1 -- Page 8