Last time we saw how a nucleophilic addition of a carbon atom to the carbonyl carbon could be carried out through the use of a Grignard reagent. Oxidation is usually used to describe a process in which electrons are removed from a molecule or atom. Using these symbols we can keep track of the oxidation levels of atoms as electrons are added or removed.
Reduction means the addition of hydrogen to a molecule or the removal of oxygen from a molecule.
Addition or removal of water does not involve, by itself, an oxidation or a reduction reaction. The addition of water to an aldehyde to form a hydrate does not involve oxidation or reduction. You may wish to look at the formation of an acetal or hemiacetal in this way. If the net change in the number of hydrogens and oxygens comes out to be a ratio of two hydrogens to one oxygen, neither oxidation nor reduction is involved. Now, let's turn to what we have to do to make an oxidation reaction go.