LON-CAPA OxiationLevels

Oxidation Levels

Introduction

Most atoms have one or two stable oxidation states. Carbon has 9!! Many of the reactions that organic molecules undergo involve changes in the oxidation level of one or more carbon atoms within the compound. For example, during the combustion of methane, which produces carbon dioxide, the oxidation level of the carbon atom changes from -4 to +4:

The procedure for calculating the oxidation level of an atom is similar to that for determining its formal charge.

Rule 3: Calculating Oxidation Levels

To determine the oxidation level of an atom within a molecule, separate the atom from its bonding partner(s), assigning all bonding electrons to the more electronegative of the bonded atoms. Then compare the number of electrons that "belong" to each atom to the atomic number of that atom. Figure 1uses color coding to illustrate the procedure for methane, CH₄.

Figure 1

Assigning Electrons I

Since carbon is more electronegative than hydrogen, both electrons from each C-H bond are assigned to the carbon. Counting its two inner shell electrons, the carbon has 10 electrons assigned to it. Its oxidation level is the sum of its nuclear charge (atomic number) and the its electronic charge; 6+ (-10) = -4. The oxidation level of each hydrogen atom is 1 + (0) = +1. Note that the sum of the oxidation levels of all of the atoms in the molecule equals zero. This is always true of neutral molecules and provides a convenient way for you to check your calculations.

Figure 2 illustrates the assignment of electrons in carbon dioxide. No color coding is used.

Figure 2

AssigningCl Electrons II

All of the bonding electrons are assigned to the oxygen atoms. So are the lone pairs. Counting its two inner shell electrons, each oxygen has 10 electrons assigned to it. The oxidation level of each oxygen is 8 + (-10) = -2. The oxidation level of the carbon is 6 + (-2) = +4. Again, note that the sum of the oxidation levels of all the atoms in CO₂ equals zero.

Exercise 1 Calculate the oxidation level of each atom the following molecules. Check your work.

a. methyl lithium, CH₃Li

1. The oxidation level of C is

2. The oxidation level of H is

3. The oxidation level of Li is

b. methanol, CH₃OH

1. The oxidation level of C is

2. The oxidation level of H is

3. The oxidation level of O is

c. formic acid, HCO₂H

1. The oxidation level of C is

2. The oxidation level of H is

3. The oxidation level of the doubly bonded O is

4. The oxidation level of the singly bonded O is

d. methylene chloride, CH₂Cl₂

1. The oxidation level of C is

2. The oxidation level of H is

3. The oxidation level of Cl is

If two bonded atoms have the same electronegativity, the electrons they share are divided equally between the two atoms. Figure 3 shows how the electrons are assigned for acetic acid.

Figure 3

Assigning Electrons III

The oxidation level of each hydrogen atom is +1. The oxidation levels of the methyl and carboxyl carbons are -3 and +3, respectively. Right? The oxidation level of each oxygen is -2. To check: 4 x (+1) + (-3) + (+3) + 2 x (-2) = 0!

Exercise 2 Calculate the oxidation level of each atom in the following molecules. Check your work.

a. ethane, CH₃CH₃

1. The oxidation level of C is

2. The oxidation level of H is

b. ethanol, CH₃CH₂OH

1. The oxidation level of C₁ is

2. The oxidation level of C₂ is

3. The oxidation level of H is

4. The oxidation level of O is

c. glycine, NH₂CH₂CO₂H

1. The oxidation level of C₁ is

2. The oxidation level of C₂ is

3. The oxidation level of H is

4. The oxidation level of the doubly bonded O is

5. The oxidation level of the singly bonded O is

6. The oxidation level of the N is

An equivalent method of calculating oxidation levels ignores the inner shell electrons; the oxidation level of an atom is the difference between its group number and the number of valence electrons assigned to it . Prove to yourself that this method works by using it to calculate the oxidation levels of all the atoms in acetic acid.

In the same way that chemists calculate the index of hydrogen deficiency of an empirical formula almost without thinking, they also perform subconscious calculations of the oxidation level of each atom within a structure. So, if you want to think like an organic chemist (which is advisable if you want to get a good grade), you should practice calculating oxidation levels until you can do it in your head.

Additional Exercises