OpenOChem Learn First Semester Topics Alkenes and Addition Reactions Calculating Oxidation States of Carbon

Calculating Oxidation States of Carbon

Understanding oxidation states of carbon atoms is essential for tracking electron flow in organic reactions and evaluating redox processes. Oxidation states help us assign a numerical value to the "electron density" around a carbon atom relative to its bonding partners. By following a few straightforward rules, we can systematically calculate the oxidation state of any carbon atom in a molecule.

Rules for Determining the Oxidation State of Carbon Atoms

C–C Bonds Do Not Affect the Oxidation State
Bonds between two carbon atoms are treated as neutral because the electrons are shared equally. Therefore, the oxidation state of a carbon atom is unaffected by the number of carbon atoms it is bonded to.
C–H Bonds Decrease the Oxidation State by 1 per Bond
Hydrogen is less electronegative than carbon, meaning that carbon "owns" the bonding electrons. Each C–H bond reduces the oxidation state of the carbon atom by 1.
C–X Bonds Increase the Oxidation State by 1 per Bond
When carbon is bonded to a more electronegative atom (X), such as oxygen, nitrogen, sulfur, or halogens, the bonding electrons are counted toward the electronegative atom. Each such bond increases the oxidation state of the carbon atom by 1.

Procedure for Determining the Oxidation State of Carbon Atoms

Identify the bonds around the carbon atom of interest.
For each C–H bond, subtract 1 from the oxidation state.
For each C–X bond (X = electronegative atom), add 1 to the oxidation state.
Ignore C–C bonds when calculating the oxidation state.
Sum the contributions from all bonds to determine the oxidation state of the carbon atom.

Examples

1. Methane (CH₄)

Bonds: 4 C–H bonds
Contribution: Each C–H bond decreases the oxidation state by 1. There are no O atoms.
$Oxidation state=0−4=−4\text{Oxidation state} = 0 - 4 = -4$
Oxidation state of carbon: -4

2. Methanol (CH₃OH)

Bonds: 3 C–H bonds, 1 C–O bond
Contribution:
- C–H bonds: $−1×3=−3-1 \times 3 = -3$
- C–O bond: $+1+1$
  $Oxidation state=−3+1=−2\text{Oxidation state} = -3 + 1 = -2$
Oxidation state of carbon: -2

3. Formaldehyde (CH₂O)

Bonds: 2 C–H bonds, 1 C=O bond
Contribution:
- C–H bonds: $−1×2=−2-1 \times 2 = -2$
- C=O bond: $+2+2$ (double bond counts as two C–O bonds)
  $Oxidation state=−2+2=0\text{Oxidation state} = -2 + 2 = 0$
Oxidation state of carbon: 0

4. Formic Acid (HCOOH)

Bonds: 1 C–H bond, 1 C=O bond, 1 C–O bond
Contribution:
- C–H bond: $-1-1$
- C=O bond: $+2+2$
- C–O bond: $+1+1$
  $Oxidation state=−1+2+1=+2\text{Oxidation state} = -1 + 2 + 1 = +2$
Oxidation state of carbon: +2

5. Carbon Dioxide (CO₂)

Bonds: 2 C=O bonds
Contribution:
- Each C=O bond contributes $+2×2=+4+2 \times 2 = +4$
  $Oxidation state=+4\text{Oxidation state} = +4$
Oxidation state of carbon: +4

This systematic approach illustrates how carbon's oxidation state changes as it forms different types of bonds, ranging from highly reduced in methane to highly oxidized in carbon dioxide. These trends align with carbon's role as a central player in redox reactions and energy metabolism, making this concept essential for understanding organic chemistry.

Here are a few questions and example reactions for your students:

Questions to Determine the Oxidation State of Carbon Atoms

Ethane (C₂H₆):
What is the oxidation state of the carbon atoms in ethane?
Acetic Acid (CH₃COOH):
Determine the oxidation state of both carbon atoms in acetic acid. Is one carbon more oxidized than the other?
Chloroform (CHCl₃):
Calculate the oxidation state of the carbon atom in chloroform.
Carbon Monoxide (CO):
What is the oxidation state of the carbon atom in carbon monoxide?
Ethanol (CH₃CH₂OH):
Calculate the oxidation state of both carbon atoms in ethanol.

Example Reactions: Oxidation, Reduction, or Neither?

For each reaction, determine if it is an oxidation, reduction, or neither based on the oxidation state changes of the carbon atoms.

1. Combustion of Methane

$CH₄ + 2O 2 ⟶ CO 2 +2H 2 O$

Hint: Determine the oxidation state of the carbon in $CH₄$ and $CO₂$ .

2. Oxidation of Methanol to Formaldehyde

$CH₃OH ⟶ CH₂O+H₂$

Hint: Compare the oxidation states of the carbon atom in $CH₃OH$ and $CH₂O$ .

3. Reduction of Formaldehyde to Methanol

$CH₂O+H₂ ⟶ CH₃OH$

Hint: Is the carbon gaining or losing electron density?

4. Oxidation of Ethanol to Acetaldehyde

$CH₃CH₂OH ⟶ CH₃CHO + H₂$

Hint: Determine the oxidation state of the terminal carbon before and after the reaction.

5. Decarboxylation of Formic Acid

$HCOOH ⟶ CO₂ + H₂$

Hint: What happens to the oxidation state of carbon in formic acid and carbon dioxide?

6. Conversion of Carbon Monoxide to Methane

$CO + 3 H₂ ⟶ CH₄ + H₂O$

Hint: Track the oxidation state of the carbon atom from $CO$ to $CH₄$ .

Answer Key for Reaction Types

Combustion of Methane: Oxidation (Carbon goes from -4 in $CH4CH₄$ to +4 in $CO2CO₂$ ).
Oxidation of Methanol to Formaldehyde: Oxidation (Carbon goes from -2 in $CH3OHCH₃OH$ to 0 in $CH2OCH₂O$ ).
Reduction of Formaldehyde to Methanol: Reduction (Carbon goes from 0 in $CH2OCH₂O$ to -2 in $CH3OHCH₃OH$ ).
Oxidation of Ethanol to Acetaldehyde: Oxidation (Terminal carbon goes from -1 in $CH3CH2OHCH₃CH₂OH$ to 0 in $CH3CHOCH₃CHO$ ).
Decarboxylation of Formic Acid: Oxidation (Carbon goes from +2 in $HCOOHHCOOH$ to +4 in $CO2CO₂$ ).
Conversion of Carbon Monoxide to Methane: Reduction (Carbon goes from +2 in $COCO$ to -4 in $CH4CH₄$ ).