An alcohol is defined in organic chemistry as a compound containing a hydroxyl functional group (-OH) directly bonded to a saturated carbon atom. This saturated carbon is part of a larger hydrocarbon structure, and the hydroxyl group is the primary site of chemical reactivity within the molecule.
The International Union of Pure and Applied Chemistry (IUPAC) developed a comprehensive system of nomenclature to ensure every chemical structure has a unique, unambiguous name. This standardized approach provides clarity for scientists globally. The IUPAC rules for alcohols modify the naming conventions for simple alkanes to indicate the presence and position of the hydroxyl group.
Establishing the Parent Name
The first step in naming an alcohol is to identify the longest continuous carbon chain that incorporates the hydroxyl group. This chain is designated as the parent chain, even if a longer carbon chain exists elsewhere that does not contain the -OH group. The parent name is derived from the corresponding alkane with the same number of carbons.
To transform the alkane name into the alcohol name, the final letter ‘e’ is removed and the suffix ‘-ol’ is added. For example, a two-carbon alkane, ethane, becomes ethanol, and a three-carbon alkane, propane, becomes propanol. This suffix change signifies the presence of the alcohol functional group.
The parent chain must be numbered to indicate the precise location of the hydroxyl group. Numbering must begin from the end closest to the -OH group, ensuring the carbon bearing the hydroxyl group receives the lowest possible number. This numbering priority is given to the functional group over any other substituents.
The position number of the -OH group is then placed just before the ‘-ol’ suffix. For instance, a three-carbon chain with the -OH group on the first carbon is named propan-1-ol, distinguishing it from propan-2-ol. Using the number immediately before the suffix is the current preferred IUPAC format.
Naming Chains with Alkyl Substituents
When the parent chain includes alkyl groups, these are treated as substituents and must be correctly located and named. The rule that the hydroxyl group must receive the lowest possible number takes precedence over all other numbering considerations. This means the numbering direction is fixed by the -OH group, even if it results in higher numbers for the side chains.
Once the parent chain is numbered, the locations of all alkyl substituents are determined. The substituent names, along with their position numbers, are placed as prefixes to the parent name. Multiple identical substituents are indicated using prefixes like ‘di-‘ or ‘tri-‘.
The final name is assembled by listing the substituents alphabetically, ignoring the numerical prefixes like ‘di-‘ or ‘tri-,’ and then preceding the parent chain name. For example, a molecule with a methyl group on the second carbon of a butan-1-ol chain is named 2-methylbutan-1-ol.
Naming Diols, Triols, and Cyclic Alcohols
Specialized rules apply when a molecule contains multiple hydroxyl groups, known as polyhydroxy alcohols, or when the parent chain forms a ring structure. For molecules with two hydroxyl groups, called diols, or three, called triols, the naming convention changes slightly. In these cases, the final ‘e’ from the parent alkane name is retained, and a numerical prefix like ‘di-‘ or ‘tri-‘ is placed immediately before the ‘-ol’ suffix.
The positions of all hydroxyl groups must be indicated using separate numbers placed before the modified suffix. For example, a two-carbon chain with hydroxyl groups on both carbons is named ethane-1,2-diol. This retention of the ‘e’ signals the presence of multiple functional groups.
For cyclic alcohols, the parent name begins with the prefix ‘cyclo-‘. The carbon atom directly bonded to the hydroxyl group is automatically assigned position number 1. This implied position number 1 is not included in the final name, simplifying the nomenclature.
Numbering then proceeds around the ring in the direction that gives the lowest possible numbers to any other substituents present. A six-carbon ring with a single hydroxyl group is simply named cyclohexanol. If a methyl group were also present, the full name might be 2-methylcyclohexanol, with the hydroxyl group implicitly at position 1.
Common and Non-Systematic Names
While the IUPAC system provides the standard method for naming alcohols, many simple alcohols are known by common or trivial names used widely in industry. These non-systematic names are often shorter and clearer for everyday use, especially for smaller molecules. One common naming method involves designating the alkyl group attached to the hydroxyl group, followed by the word “alcohol.”
For example, the IUPAC name propan-2-ol is frequently referred to by the common name isopropyl alcohol. Similarly, the IUPAC name ethanol is commonly known as ethyl alcohol. Many of these common names, such as tert-butyl alcohol or ethylene glycol, are accepted by IUPAC for use in non-formal contexts.