Oxidation-Reduction in Organic Chemistry

Oxidation of a carbon atom corresponds to an increase in the number of oxygen atoms, and/or bonds to the

carbon atom; and/or a decrease in the total number of hydrogen atoms bonded to the carbon atom.

increasing oxidation state of carbon

-4 -2 0 +2 +4

H

H

H

C H

H

H

H

C OH

O

CHH

O

COHH

O

COHHO

reducing oxidation state of carbon

increasing oxidation state of carbon

-3 -2 -1

HC CH

C C

H

H H

H

C C H

HH

H H

Hreducing oxidation state of carbon

Calculating the oxidation stateof each carbon in a molecule that containscarbons in different oxidation states.

CH3CH2OH C2H6O

Calculating Oxidation Numbers

• 1. Write the Lewis structure and include unshared electron pairs.

H

C

H

H

H

O

H

C H••

••

Calculating Oxidation Numbers

• 2. Assign the electrons in a covalent bond between two atoms to the more electronegative partner.

H

O

H

C

H

H

H

C H••••

••••

••

••

••

••••

• 3. For a bond between two atoms of the same element, assign the electrons in the bond equally.

H

O

H

C

H

H

H

C H••••

••••

••

••

••

••••

Calculating Oxidation Numbers

• 3. For a bond between two atoms of the same element, assign the electrons in the bond equally.

H

O

H

C

H

H

H

C H••••

••••

••

••

••

•••• ••

Calculating Oxidation Numbers

• 4. Count the number of electrons assigned to each atom and subtract that number from the number of valence electrons in the neutral atom; the result is the oxidation number.

H

O

H

C

H

H

H

C H••••

••••

••

••

••

•••• ••

Each H = +1C of CH3 = -3

C of CH2O = -1O = -2

Calculating Oxidation Numbers

• Each of the carbons below have zero formal charge, but they have different oxidation states.

• Calculate the oxidation number for each.

Oxidation Numbers

-4 -2 0 +2 +4

Recognizing oxidation and reduction.1) Is the conversion from formic acid carbon dioxide an

oxidation or a reduction?2) Formaldehyde methanol?3) Acetic acid acetone

A. 1) Oxidation 2) Oxidation 3) OxidationB. 1) Oxidation 2) Reduction 3) OxidationC. 1) Reduction 2) Oxidation 3) ReductionD. 1) Oxidation 2) Reduction 3) Reduction

Question

X Y

X less electronegative than carbonY more electronegative than carbon

oxidation

reductionC C

GeneralizationOxidation of carbon occurs when a bond between carbon and an atom which is less electronegative than carbon is replaced by a bond to an atom that is more electronegative than carbon. The reverse process is reduction.

CH3Cl HClCH4 Cl2+ +

Oxidation

+ 2Li LiClCH3Cl CH3Li +Reduction

Examples

Question

• To carry out the reaction shown below we need:

•• CH3OH ® H2C=O•• A) an oxidizing agent• B) a reducing agent

Fig. 1 Synthesis of alcohols from olefins.

G Dong et al. Science 2011;333:1609-1612

Published by AAAS

Fig. 2 Proposed cooperative catalytic system for alcohol synthesis from olefins and water.

G Dong et al. Science 2011;333:1609-1612

Published by AAAS

Fig. 3 General reaction scheme for styrene hydration to produce 2-phenylethanol (2a), ethylbenzene (3a), 1-phenylethanol (4a), phenylacetaldehyde (5a), and acetophenone (6a).

G Dong et al. Science 2011;333:1609-1612

Published by AAAS

Fig. 4 (A and B) Proposed mechanism and initial mechanistic studies.

G Dong et al. Science 2011;333:1609-1612

Published by AAAS