# Which group is more oxidized, -CHO or -CH_2OH, and why?

Dec 14, 2015

The $\text{-CHO}$ group is more oxidized.

#### Explanation:

There are three methods we could use to determine the relative levels of oxidation.

1. By using the oxidation number of the carbon atom

One definition of oxidation is: an increase in the oxidation number.

Let's calculate the oxidation number of $\text{C-1}$ in ethanal.

According to the rules for calculating oxidation numbers, $\text{C-1}$ "owns" one of the electrons in the $\text{C-C}$ bond, both of the electrons in the $\text{C-H}$ bond, and none of the electrons in the $\text{C=O}$ bond.

Since $\text{C-1}$ "owns" only three valence electrons, it has effectively "lost" an electron, so it has an oxidation number of +1.

Now, let's repeat the process for $\text{C-1}$ in ethanol.

Here, $\text{C-1}$ "owns" one of the electrons in the $\text{C-C}$ bond, both of the electrons in the $\text{C-H}$ bonds, and none of the electrons in the $\text{C=O}$ bond.

Since $\text{C-1}$ now "owns" five valence electrons, it has effectively "gained" an electron, so it has an oxidation number of -1.

The aldehyde carbon has a higher oxidation number than the alcohol carbon, so a $\text{CHO}$ group is more highly oxidized than a $\text{CH"_2"OH}$ group.

2. By counting the number of oxygen atoms

A second definition of oxidation is: an increase in the number of oxygen atoms.

Both groups contain one O atom, but the O in the aldehyde is double-bonded, so we can count it twice (as we do when determining $R , S$ configurations).

Thus, the $\text{CHO}$ group is more highly oxidized than the $\text{CH"_2"OH}$ group.

3. By counting the number of hydrogen atoms

A third definition of oxidation is: a decrease in the number of hydrogen atoms.

$\text{C-1}$ in the alcohol group has two H atoms attached, while $\text{C-1}$ in the aldehyde group has one H atom attached.

Therefore, the $\text{CHO}$ group is more highly oxidized than the $\text{CH"_2"OH}$ group.