Question

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

Answer 1

The `"-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 `"C-1"` in ethanal.

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

Since `"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 `"C-1"` in ethanol.

Here, `"C-1"` "owns" one of the electrons in the `"C-C"` bond, both of the electrons in the `"C-H"` bonds, and none of the electrons in the `"C=O"` bond.

Since `"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 `"CHO"` group is more highly oxidized than a `"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 `"CHO"` group is more highly oxidized than the `"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.

`"C-1"` in the alcohol group has two H atoms attached, while `"C-1"` in the aldehyde group has one H atom attached.

Therefore, the `"CHO"` group is more highly oxidized than the `"CH"_2"OH"` group.