Club Chemistry: Naming Alkane Alcohols

Nomenclature of Hydroxylated Compounds With Carbon Single Bonds

Alcohols are a diverse class of organic compounds. All alcohols contain one or more hydroxyl (OH) groups. Ethanol (ethyl alcohol) is the best-known member of the family.

The alcohols hold a special place in the annals of chemistry and human history. Ethyl alcohol is the most popular drug of all time, and causative agent of much enjoyment as well as tragedy. From the perspective of the chemist, alcohols provide a myriad of opportunities for the study and synthesis of new and valuable compounds.

The Hydroxyl Group

The defining feature of an alcohol is the presence of they hydroxyl group (OH). Hydroxyl groups are polarizing, since oxygen is an electronegative element. This means that when an OH is bonded to a carbon atom, the bonding electrons will tend to spend more time closer to the oxygen atom than the carbon atom. This gives an imbalance of electric charge across the molecule, making the region surrounding the OH group slightly more negative that the rest of the molecule.

The polarity of alcohols has important chemical implications. Alcohols tend to be soluble to varying degrees in both polar and non-polar solvents. This also makes alcohols good solvents themselves, capable of dissolving a wide range of compounds. The OH group provides opportunities for many synthetic reactions.

Straight Chain Alkane Alcohols

The names of straight chain alcohols are based on the names of analogous carbon compounds. For alkanes, remove the "-e" from the end of the name and replace it with an "-ol". Examples are methanol and ethanol. This rule applies if there is no ambiguity about where the OH group is located.

For alcohols on a chain of more than two carbons, a number indicates the carbon holding the OH group. For example, 1-propanol, is not the same as 2-propanol.

If more that one OH group is present, the usual "lowest possible number" system is used. Molecules with two OH groups are diols, three OHs are triols, etc. For example, a hexane with hydroxyl groups at C-1 and C-3 is 1,3-hexanediol. Another example from biochemistry is 1,2,3-propanetriol — also called glycerol (Figure 1).

Cyclic Alkane Alcohols

An OH group can be substituted for hydrogen on cyclic hydrocarbons. To name a cyclic alkanol, begin with the name of the cyclic compound. Consider cyclopentane (C5H10). To name a monosubstituted alcohol of cyclopentane, remove the "-ane" and replace it with "-anol" — cyclopentanol.

If there is more than one hydroxyl group (diols, triols, etc.), use a numbering system to produce the lowest numbers for the locations of the OH groups, similarly to what is done when naming substituted cyclic alkanes. However, the name of the cyclic compound comes first wth the hydroxyl group locations appended to the end. Examples are cyclopentane-1,2-diol, and cyclohexane-1,2,3-triol.

Biomolecular Alcohols

Although they seldom contain only single-bonded carbons atoms, large biomolecules often contain hydroxyl groups, and by definition these are also alcohols. However, they are not easily named. Compounds such cholesterol are identified as alcohols by the "-ol" at the end of the name, but one would seldom choose to use its full name ((3S,8S,9S,10R,13R,14S,17R)-10,13-dimethyl-17-[(2R)-6-methylheptan-2-yl]-2,3,4,7,8,9,11,12,14,15,16,17-dodecahydro-1H-cyclopenta[a]phenanthren-3-ol)!

Another example is the sterols, which are steroids with an OH group attached to C-3 of the steroid. It is best to learn the short names of such compounds as needed, and not to worry about the IUPAC rules for naming them.