Jumat, 09 Oktober 2009

Oxidation Levels

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.

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!

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.

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