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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