All functional groups are different, but some are more different than others. For example, the structures of a carboxylic acid, an ester, and an amide are all very similar: in each case the carbon atom carrying the functional group is bonded to two heteroatoms, one of the bonds being a double bond. You will see in Chapter 10 that this similarity in structure is mirrored in the reactions of these three types of compounds and in the ways in which they can be inter converted. Carboxylic acids, esters, and amides can be changed one into another by reaction with simple reagents such as water, alcohols, or amines plus appropriate catalysts. To change them into aldehydes or alcohols requires a different type or reagent, a reducing agent (a rea gent which adds hydrogen atoms). We say that the carbon atoms carrying functional groups that can be interconverted without the need for reducing agents (or oxidizing agents) have the same oxidation level—in this case, we call it the ‘carboxylic acid oxidation level’.

In fact, amides can quite easily be converted into nitriles just by dehydration (removal of water), so we must give nitrile carbon atoms the same oxidation level as carboxylic acids, esters, and amides. Maybe you’re beginning to see the structural similarity between these four functional groups that you could have used to assign their oxidation level? In all four cases, the carbon atom has three bonds to heteroatoms, and only one to C or H. It doesn’t matter how many het eroatoms there are, just how many bonds to them. Having noticed this, we can also assign both carbon atoms in ‘CFC-113’, one of the environmentally unfriendly aerosol propellants/refriger ants that have caused damage to the earth’s ozone layer, to the carboxylic acid oxidation level.

Aldehydes and ketones contain a carbon atom with two bonds to heteroatoms; they are at the ‘aldehyde oxidation level’. The common laboratory solvent dichloromethane CH₂Cl₂ also has two bonds to heteroatoms, so it too contains a carbon atom at the aldehyde oxidation level, as do acetals.

Alcohols, ethers, and alkyl halides have a carbon atom with only one single bond to a heteroatom. We assign these the ‘alcohol oxidation level’, and they are all easily made from alcohols without oxidation or reduction.

We must include simple alkanes, which have no bonds to heteroatoms, as an ‘alkane oxidation level’.

The small class of compounds that have a carbon atom with four bonds to heteroatoms is related to CO2 and best described as at the carbon dioxide oxidation level. Alkenes and alkynes obviously don’t fi t easily into these categories as they have no bonds to heteroatoms. Alkenes can be made from alcohols by dehydration without any oxidation or reduction so it seems sensible to put them in the alcohol column. Similarly, alkynes and aldehydes are related by hydration/dehydration without oxidation or reduction.