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Page 1
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+
Page 2
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
Page 3
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.
Page 4
MECHANISM (FYI)
EXAMPLES
Note: aromatics do not react like alkenes (aromatics are unreactive here).
excess H2
Pt cat.
Page 5
APPLICATION: HYDROGENATED VEG OIL
2 moles H2
Pd cat.
Vegetable oil
Page 6
OXIDATION REACTIONS
EPOXIDATIONS
GENERAL PATTERN
meta-chloroperoxybenzoic acid
MECHANISM (FYI)
EXAMPLES
mCPBA
a. mCPBA
b.c. H+
C CNaH3C
mCPBA
Page 7
DIHYDROXYLATIONS
USING MCPBA
PERMANGANATE OXIDATION
Baeyer test Permanganate TLC stain (left) Oxidizer symbol
a. mCPBA
b. NaOH, H2O
Page 8
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 +
Page 9
EXAMPLES
“ASYMMETRIC REACTIONS”
OsO4 (cat.)
NMO, H2O
a. cat. OsO4, NMO, H2O
b. excess NaHc. excess CH3I
Page 10
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
Page 11
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
Page 12
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
+
Page 13
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
Page 14
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
Page 15
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