Nomenclature | Alcohols and Phenols | Organic Chemistry

TL;DR

Replace the “e” in the corresponding alkane name with “ol” to form alcohol names.

Briefing Cornell Notes

Briefing

Alcohol and phenol nomenclature hinges on one rule: assign the lowest possible numbers to the carbon(s) attached to the principal functional group, then apply the correct parent-chain name and list substituents with their positions.

For simple alcohols, the “e” in the corresponding alkane is replaced by “ol.” Numbering starts from the end that gives the hydroxyl-bearing carbon the lowest number. A three-carbon alcohol becomes propan-1-ol (often written as propanol with the “1” implied when numbering is unambiguous). With a two-carbon chain (ethanol), both terminal carbons are equivalent, so the hydroxyl position need not be stated.

When multiple ways exist to count the carbon chain, the longest parent chain is chosen according to IUPAC-style logic. Substituents are then numbered so the functional group(s) receive the least possible locants. In one worked case, choosing the longest six-carbon parent chain yields a hexan-ol framework, and the hydroxyl group is placed at the lowest number possible; substituents such as an ethyl group are then reported with their own positions (e.g., “3-ethyl” when it sits on carbon 3 relative to the chosen numbering).

Alkenols and alkynols follow a similar priority approach, but the double bond (or triple bond) must be incorporated into the parent name. The parent chain name includes the unsaturation suffix (for example, “but-…-ene”), while the “ol” suffix is treated as the functional group that must get the lowest locant. In the example with a four-carbon chain, the double bond is placed at carbon 3 (giving but-3-ene), and the hydroxyl is located at carbon 2 (yielding a name of the form but-3-en-2-ol). The key difference emphasized is that the unsaturation becomes part of the complete parent name, while the hydroxyl position is appended as the “-ol” locant.

Halogenated alcohols add another layer: halogens are listed as substituents, and numbering still aims for the least locants for the hydroxyl group. When two halogens are present, the prefix “di-” is used and both halogen positions are reported. Another example includes multiple substituents (bromine, methyl, and hydroxyl), where alphabetical ordering of substituents determines the sequence in the name, but hydroxyl-driven numbering still controls the locants.

The lecture also treats molecules with multiple hydroxyl groups. When two hydroxyls appear on the same carbon chain, the “diol” pattern is used; when three hydroxyls appear, “triol” applies. The locants for each hydroxyl group are written in ascending order (e.g., a three-hydroxyl example is named with “-triol” and locants like 1,2,3).

Finally, phenols are introduced as benzene rings bearing a hydroxyl group. In substituted phenols, the hydroxyl-bearing carbon is assigned position 1 by convention, and other substituents receive their locants relative to it. Common phenol names are contrasted with IUPAC-style systematic names (e.g., “hydroxybenzene” as a common name for phenol). The lecture extends this to polycyclic systems: condensed benzene structures without substituents are named as naphthalene (two fused rings) and anthracene (three fused rings), with hydroxylated versions described as hydroxy-naphthalene or hydroxy-anthracene depending on the number of fused rings and the hydroxyl placement. The overarching takeaway is consistent: choose the correct parent framework, then apply lowest locants and proper suffix/prefix rules for alcohols and phenols.

Cornell Notes

Alcohols and phenols are named by combining a parent carbon framework with functional-group suffixes and substituent locants. For alcohols, the “e” in the alkane name is replaced by “ol,” and numbering starts from the end that gives the hydroxyl group the lowest possible number. For unsaturated alcohols, the double bond/triple bond is built into the parent name (e.g., but-3-ene), while the hydroxyl position is given as the “-ol” locant. Halogen and alkyl substituents are then listed with their positions, and alphabetical order determines substituent listing order. For phenols, the hydroxyl-bearing carbon on the benzene ring is always carbon 1, and additional substituents get locants relative to that point; fused-ring systems follow common names like naphthalene and anthracene (and their hydroxy derivatives).

Why does numbering for an alcohol start from a particular end of the carbon chain?

Numbering is chosen to give the hydroxyl-bearing carbon the lowest possible locant. For a three-carbon alcohol, the hydroxyl ends up on carbon 1, producing propan-1-ol (often written as propanol when the position is obvious). For a two-carbon alcohol, both ends are equivalent, so ethanol doesn’t require an explicit “1” locant.

How are unsaturation and the hydroxyl group combined in names like but-3-en-2-ol?

The double bond (or triple bond) becomes part of the parent name as an unsaturation suffix (e.g., “but-3-ene”). The hydroxyl group is then attached as “-ol” with its own locant. The lecture’s rule of thumb: when a double bond appears, include it in the complete parent name, then append the hydroxyl position as the “ol” locant (e.g., carbon 2 for the hydroxyl in but-3-en-2-ol).

What changes when halogens are present alongside an alcohol group?

Halogens are treated as substituents and listed with their positions. If two identical halogens appear, the name uses “di-” (e.g., a dihalide alcohol). Numbering still prioritizes the hydroxyl group’s lowest locant; once numbering is fixed, halogen positions are reported. Substituent order in the name follows alphabetical listing (so bromine may come before methyl, for example), even though locants are determined by hydroxyl-driven numbering.

How are multiple hydroxyl groups named on the same carbon chain?

Multiple hydroxyls use “diol” or “triol” (and corresponding locants). The lecture emphasizes writing all hydroxyl positions (e.g., 1,2 for a diol; 1,2,3 for a triol) and then using the parent chain name with the appropriate “-diol” or “-triol” suffix. The parent chain is named without replacing the “e” logic for each hydroxyl separately; instead, the functional-group suffix reflects the count.

What special convention applies to phenols on a benzene ring?

The hydroxyl-bearing carbon on the benzene ring is always assigned position 1. Other substituents are then numbered relative to that point. For example, a benzene with hydroxyl at carbon 1 and another substituent at carbon 2 and 3 would be named using locants like 2,3-…-phenol (with “phenol” as the parent functional class).

How do fused-ring phenolic systems get named in this lecture?

Condensed benzene systems without substituents are identified as naphthalene (two fused rings) and anthracene (three fused rings). When hydroxyl is present, the naming becomes “hydroxy-” plus the fused-ring parent (e.g., hydroxy-naphthalene or hydroxy-anthracene), with the hydroxyl placement determined by the convention for the hydroxyl-bearing carbon.

Review Questions

When choosing between two possible numbering directions for an alcohol, what locant is minimized first and why?
In a compound with both a double bond and a hydroxyl group, which part of the name carries the double-bond locant, and which part carries the hydroxyl locant?
For a substituted phenol, what locant is always assigned to the hydroxyl-bearing carbon, and how does that affect numbering of other substituents?

Key Points

1
Replace the “e” in the corresponding alkane name with “ol” to form alcohol names.
2
Number the carbon chain from the end that gives the hydroxyl group the lowest possible locant.
3
For unsaturated alcohols, include the double/triple bond position in the parent name (e.g., but-3-ene) and then attach the hydroxyl position as the “-ol” locant.
4
Halogens are named as substituents with their positions; numbering still follows the hydroxyl-first lowest-locant rule.
5
When multiple hydroxyl groups occur, use “diol” or “triol” and list all hydroxyl locants in order.
6
In phenols, the hydroxyl-bearing benzene carbon is always assigned position 1, and all other substituents are numbered relative to it.
7
For fused-ring systems, use common fused-ring parent names like naphthalene and anthracene, then apply “hydroxy-” for hydroxylated derivatives.

Highlights

Ethanol doesn’t need an explicit hydroxyl locant because both terminal carbons are equivalent.

Unsaturation belongs in the parent name (e.g., but-3-ene), while the hydroxyl position is appended via the “-ol” locant (e.g., but-3-en-2-ol).

In phenols, the hydroxyl-bearing carbon on the benzene ring is always carbon 1, fixing the numbering scheme for every other substituent.

Substituent listing order follows alphabetical order, even though locant selection is driven by the hydroxyl group’s lowest number.

Condensed benzene systems without substituents are named naphthalene (two rings) and anthracene (three rings), with hydroxylated versions described as hydroxy derivatives.

Topics

Alcohol Nomenclature
Phenol Nomenclature
IUPAC Numbering
Unsaturated Alcohols
Halogenated Alcohols
Polyhydric Alcohols
Fused Benzene Rings

Mentioned

Rizwana Mustafa
IUPAC

Nomenclature | Alcohols and Phenols | Organic Chemistry | lec 05 II D Rizwana