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CHAPTER 12: REDOX REACTIONS
REDOX REACTION OVERVIEW
EXAMPLES
à CO2
Cut potatoes and peeled bananas turn brown in the air.
Bleach reacts with stains to turn them colorless and/or water soluble.
Energy can be derived from certain organic substances.
OXIDATION STATES
SAMPLE REDOX REACTION
C2H6O + 2 CrO3 + H2O à C2H4O2 + 2 H2CrO3
Half Reactions:
H3C C
O
H
HO
CrO
OH++ 2
H
OCr
O+ 2
OH H
H3CC
O
O
H+
HO
H
C C
O
H
H
HH
H
H + HO
H CC
O
O
H
HH
H+ 4H+
OCr
O
O
OCr
O
O
+4H+ +O
Cr
O
OH H
OCr
O
OH H+
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FORMAL CHARGE VS. OXIDATION STATES Formal Charge: theoretical charge if bonds are purely covalent (split each bonding pair in half). Oxidation State:
Formal Charge Oxidation States Oxidation States
SIMPLISTIC METHOD FOR IDENTIFYING REDOX
OXIDATION AND REDUCTION
1. Oxidation (loss of electrons):
2. Reduction (gain of electrons):
OXIDATION STATES OF CARBON
Structure
Functional Gp Alkane Alcohol Aldehyde Carboxylic acid CO2
Ox State of C
H3C C
O
H
H
H
H3C C
O
H
H
H
H3CC
O
O
H
H C
H
H
H
+ 2 O2spark
O C O + 2 H2O
6 CO2 + 6 H2Osunlight
enzyme HC
C
O
CC
CH2OH
OH
OH
OH
+ 6 O2
H
H
Hglucose
H C
H
H
H
H C
H
OH
H HC
H
O
HC
OH
OC
O
O
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EXAMPLES Is the organic compound in each reaction oxidized, reduced, or neither?
Ox, Red, or neither
REDUCTION REACTIONS
HYDROGENATION
GENERAL PATTERN
OS
R
O O
Br
C C
H
H H
H
C C
ClH
H H
HH
D2
Pd cat.
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MECHANISM (FYI)
EXAMPLES
Note: aromatics do not react like alkenes (aromatics are unreactive here).
excess H2
Pt cat.
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APPLICATION: HYDROGENATED VEG OIL
2 moles H2
Pd cat.
Vegetable oil
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OXIDATION REACTIONS
EPOXIDATIONS
GENERAL PATTERN
meta-chloroperoxybenzoic acid
MECHANISM (FYI)
EXAMPLES
mCPBA
a. mCPBA
b.c. H+
C CNaH3C
mCPBA
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DIHYDROXYLATIONS
USING MCPBA
PERMANGANATE OXIDATION
Baeyer test Permanganate TLC stain (left) Oxidizer symbol
a. mCPBA
b. NaOH, H2O
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OSMIUM TETROXIDE OXIDATION
MECHANISM (MOSTLY FYI)
To prevent using a stoichiometric amount of osmium (which is expensive and toxic), a modification is often used:
OsO4 (cat.)
NMO, H2O
NMO, H2O +
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EXAMPLES
“ASYMMETRIC REACTIONS”
OsO4 (cat.)
NMO, H2O
a. cat. OsO4, NMO, H2O
b. excess NaHc. excess CH3I
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OZONOLYSIS
GENERAL PATTERN
PARTIAL MECHANISM (FYI)
EXAMPLES
a. O3
b. Zn, H2O
a. O3
b. Zn, H2O
a. O3
b. Zn, H2O
a. O3
b. S(CH3)2
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THINKING BACKWARDS Propose a structure that would yield only the products shown.
CHROMIUM OXIDATIONS
GENERAL PATTERNS
Type Example Product FG
Primary ROH
Secondary ROH
Tertiary ROH
Aldehyde
O
+O
a. O3
b. Zn, H2O
H
O
O
O
H H
O
H+
a. O3
b. S(CH3)2
C8H12
OH H2SO4 (aq)
CrO3
OH
H2SO4 (aq)
Na2Cr2O7
OH
H2SO4 (aq)
K2Cr2O7
H
O
H2SO4 (aq)
CrO3
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Julia Roberts
Erin Brokovich
Erin Brokovich was instrumental in a litigation against PG+E in 1993 ($333 million), for contaminating groundwater with “hexavalent chromium.” The story was made into a movie in 2000.
MECHANISM (FYI)
CHROMIC ACID (JONES) TEST
H2SO4 (aq)acetone
OH CrO3 or Na2Cr2O7
+
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EXAMPLES Give the major organic product. Assume excess inorganic reagent.
APPLICATION: BIOLOGICAL OXIDATIONS
OH
H2SO4 (aq)
CrO3
H
O O
H2SO4 (aq)
Na2Cr2O7
HOOH
H2SO4 (aq)
K2Cr2O7
HO
OH
H2SO4 (aq)
CrO3
1
2
3
4
5
6
7
8
SCoA
O
O2C CO2HO CO2
O2C CO2
CO2
OH
O2C CO2
O
O2C SCoA
O
O2CCO2
O2CCO2
O2CCO2
OH
O2CCO2
O
Citric Acid Cycle NAD+
NADH + H+ + CO2
+ H2O HSCoA
GDPGTP + HSCoA
NAD+ + HSCoA
NADH + H+ + CO2
FAD
FADH2
H2O
NAD+
NADH + H+
citrate
isocitrate
a-ketoglutarate
succinyl-SCoAsuccinate
fumarate
malate
oxaloacetate
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PCC VARIATION
Type Example Product FG
Secondary ROH
Primary ROH
Pyridinium chlorochromate (PCC, oxidizing agent)
H OH
OCH2Cl2
PCC
H OH
OH2SO4 (aq)
CrO3
OH
HOCH2Cl2
PCC
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REVIEW PROBLEMS
1. Give a structure that matches this criteria: • Has a molecular formula of C6H12O.
• Reacts with CrO3 / H2SO4 and produces a blue-green precipitate.
• Undergoes a dehydration reaction with concentrated H3PO4.
2. Give the major organic product(s), paying attention to stereochemistry. Indicate if a racemic mixture
is formed.
CrO3OOH
H2SO4 (aq)a. mCPBA
b. NaOH, H2O
OsO4 (cat.)
NMO, H2O
OH
PCC
CH2Cl2
O
D2, Pt a. O3
b. Zn, H2O