Classification & Nomenclature

The Oxygen Family: Alcohols, Phenols and Ethers

When a hydrogen atom of a hydrocarbon is replaced by an -OH (hydroxyl) group, or two carbon groups are joined through an oxygen atom, we get the three families of this chapter:

• Alcohols (R-OH): the -OH group is attached to an sp$^3$ carbon of an alkyl chain (e.g. ethanol, CH$_3$CH$_2$OH).
• Phenols (Ar-OH): the -OH group is attached directly to an sp$^2$ carbon of a benzene ring (e.g. phenol, C$_6$H$_5$OH).
• Ethers (R-O-R'): two alkyl or aryl groups are joined by an oxygen atom (e.g. diethyl ether, CH$_3$CH$_2$-O-CH$_2$CH$_3$).

The difference between an alcohol's sp$^3$ C-OH and a phenol's sp$^2$ (aromatic) C-OH controls nearly all their differences in acidity and reactivity.

Classifying Alcohols and Phenols

Alcohols are classified two ways:

(a) By the number of -OH groups:

• Monohydric (one -OH, e.g. ethanol), dihydric (two, e.g. ethane-1,2-diol/glycol), trihydric (three, e.g. propane-1,2,3-triol/glycerol).

(b) By the carbon bearing the -OH (for monohydric alcohols):

• Primary (1°): -OH carbon attached to one other carbon (e.g. CH$_3$CH$_2$OH, ethanol).
• Secondary (2°): attached to two carbons (e.g. (CH$_3$)$_2$CHOH, propan-2-ol).
• Tertiary (3°): attached to three carbons (e.g. (CH$_3$)$_3$COH, 2-methylpropan-2-ol).

Special types by the adjacent carbon: allylic (-OH on an sp$^3$ C next to C=C), benzylic (-OH on an sp$^3$ C attached to a ring), and vinylic (-OH on an sp$^2$ C=C carbon).

Phenols are similarly classed as monohydric, dihydric (e.g. catechol, resorcinol, quinol) or trihydric by the number of -OH groups on the ring.

Classifying Ethers and IUPAC Nomenclature

Ethers are classified by whether the two groups are the same:

• Simple (symmetrical): both groups identical, e.g. diethyl ether (C$_2$H$_5$-O-C$_2$H$_5$).
• Mixed (unsymmetrical): different groups, e.g. ethyl methyl ether (CH$_3$-O-C$_2$H$_5$).

IUPAC nomenclature:

• Alcohols are named as alkanols: the -e of the alkane is replaced by -ol, and the position of -OH gets the lowest locant. CH$_3$OH = methanol; CH$_3$CH$_2$CH$_2$OH = propan-1-ol; (CH$_3$)$_2$CHOH = propan-2-ol.
• Phenols are named as substituted phenols (phenol is the parent); the ring carbons are numbered to give -OH position 1. e.g. 2-methylphenol, 4-nitrophenol.
• Ethers are named as alkoxyalkanes: the smaller group + oxygen is the alkoxy substituent on the larger (parent) chain. CH$_3$-O-CH$_2$CH$_3$ = methoxyethane; CH$_3$-O-C$_6$H$_5$ = methoxybenzene (anisole).

Key Point: Alcohol = alkanol; ether = alkoxyalkane (smaller group as the alkoxy prefix). For phenols, "phenol" is the parent and substituents are numbered to give -OH the lowest locant.

Solved Examples

Example 1: Classify the alcohol

Classify (CH$_3$)$_3$COH and (CH$_3$)$_2$CHOH as 1°, 2° or 3°.

Solution: In (CH$_3$)$_3$COH the C-OH carbon is attached to three carbons → tertiary (3°). In (CH$_3$)$_2$CHOH the C-OH carbon is attached to two carbons → secondary (2°).

Example 2: IUPAC name of an alcohol

Give the IUPAC name of CH$_3$CH$_2$CH$_2$CH$_2$OH.

Solution: A four-carbon chain with -OH on C1 → butan-1-ol.

Example 3: Name an ether

Give the IUPAC name of CH$_3$-O-CH$_2$CH$_3$.

Solution: The smaller group (methyl + O = methoxy) is the substituent on the larger chain (ethane): methoxyethane (common name: ethyl methyl ether).

Example 4: Dihydric alcohol

What is the common name and IUPAC name of CH$_2$(OH)-CH$_2$(OH)?

Solution: It is a dihydric alcohol: common name ethylene glycol; IUPAC name ethane-1,2-diol.

Example 5: Phenol vs alcohol

Is C$_6$H$_5$-CH$_2$-OH a phenol? Explain.

Solution: No. In C$_6$H$_5$-CH$_2$-OH (benzyl alcohol) the -OH is on an sp$^3$ carbon attached to the ring, not directly on the ring. It is a benzylic alcohol. A phenol has -OH directly on an sp$^2$ ring carbon (e.g. C$_6$H$_5$-OH).

Example 6: Classify the ether

Classify CH$_3$CH$_2$-O-CH$_2$CH$_3$ and CH$_3$-O-C$_6$H$_5$.

Solution: CH$_3$CH$_2$-O-CH$_2$CH$_3$ (diethyl ether) has two identical groups → simple/symmetrical ether. CH$_3$-O-C$_6$H$_5$ (anisole) has different groups → mixed/unsymmetrical ether.

Example 7: Allylic alcohol

Why is CH$_2$=CH-CH$_2$-OH called an allylic alcohol?

Solution: Because the -OH is on an sp$^3$ carbon that is directly attached to a carbon-carbon double bond (the allylic position).

Example 8: IUPAC name of a substituted phenol

Give the IUPAC name of the phenol with a methyl group at the ortho position.

Solution: Numbering -OH as C1 and methyl as C2: 2-methylphenol (common name: o-cresol).

Example 9: Trihydric alcohol

Name the trihydric alcohol present in fats and oils.

Solution: Glycerol (propane-1,2,3-triol), CH$_2$(OH)-CH(OH)-CH$_2$(OH).

Example 10: Name an alkoxybenzene

Give the IUPAC and common names of CH$_3$-O-C$_6$H$_5$.

Solution: IUPAC: methoxybenzene; common name: anisole.