The three families at a glance
Alcohols, phenols and ethers are the three classes of organic compounds in which carbon is joined to oxygen by a single bond. The distinction between them is purely structural. An alcohol contains one or more hydroxyl ($\ce{-OH}$) groups attached directly to a carbon atom of an aliphatic system, as in methanol $\ce{CH3OH}$. A phenol carries the $\ce{-OH}$ group attached directly to a carbon atom of an aromatic ring, as in $\ce{C6H5OH}$. An ether arises when a hydrogen atom of a hydrocarbon is replaced by an alkoxy ($\ce{-OR}$) or aryloxy ($\ce{-OAr}$) group; equivalently, it is what you get on replacing the hydrogen of the $\ce{-OH}$ in an alcohol or phenol by an alkyl or aryl group, as in dimethyl ether $\ce{CH3OCH3}$.
The single thread connecting all three is the divalent oxygen atom. In alcohols and phenols one of its bonds goes to hydrogen; in ethers both go to carbon. Everything that follows in this unit — acidity, hydrogen bonding, esterification, cleavage by $\ce{HI}$ — is downstream of which carbon and how many carbons the oxygen sits next to. Classification, therefore, is not a clerical exercise; it is the first prediction you make about behaviour.
The same divalent oxygen, three different carbon environments. Each branch then sub-divides on a second criterion — number of hydroxyls, or the nature of the carbon bearing oxygen.
Classifying alcohols
NCERT classifies alcohols along two independent axes. The first counts hydroxyl groups; the second examines the carbon to which the $\ce{-OH}$ is attached.
By number of hydroxyl groups
Depending on whether a molecule carries one, two, three or many $\ce{-OH}$ groups, it is called monohydric, dihydric, trihydric or polyhydric. Ethanol $\ce{C2H5OH}$ is monohydric; ethylene glycol $\ce{HOCH2CH2OH}$ is dihydric; glycerol, $\ce{HOCH2CH(OH)CH2OH}$, is trihydric. NIOS notes that a 1,2-diol — two hydroxyls on adjacent carbons — is termed a vicinal diol or glycol.
By the carbon bearing –OH (sp³ C–OH)
When the $\ce{-OH}$ sits on an $sp^3$ carbon, alcohols are graded by how many other carbon (alkyl) groups are attached to that carbon. One alkyl group makes it primary (1°), two make it secondary (2°) and three make it tertiary (3°). This single count governs Lucas-test behaviour, ease of dehydration and the oxidation product, so it is worth fixing firmly.
Top row: the 1°/2°/3° series differs only in the number of carbons on the hydroxyl-bearing carbon. Bottom row: allylic and benzylic alcohols keep an $sp^3$ C–OH but lie next to unsaturation, whereas the vinylic alcohol places –OH on an $sp^2$ carbon.
Allylic, benzylic and vinylic alcohols
Two further sub-types still belong to the $sp^3$ C–OH family. In an allylic alcohol the $\ce{-OH}$ sits on an $sp^3$ carbon adjacent to a carbon–carbon double bond (the allylic position), as in prop-2-en-1-ol $\ce{CH2=CH-CH2OH}$. In a benzylic alcohol the $\ce{-OH}$ sits on an $sp^3$ carbon next to an aromatic ring, as in benzyl alcohol $\ce{C6H5CH2OH}$. NCERT stresses that allylic and benzylic alcohols may themselves be primary, secondary or tertiary, so the two classification axes overlap rather than compete.
A separate category contains compounds with an $sp^2$ C–OH. Here the $\ce{-OH}$ is bonded directly to a carbon that is part of a carbon–carbon double bond — a vinylic carbon — giving a vinylic alcohol such as $\ce{CH2=CH-OH}$. The distinction between allylic and vinylic is a perennial trap: allylic keeps the hydroxyl one carbon away from the double bond on an $sp^3$ centre, vinylic puts it directly on the $sp^2$ double-bond carbon.
Allylic vs vinylic — count the bond between –OH and the C=C
Students routinely mislabel $\ce{CH2=CH-CH2OH}$ as vinylic. It is allylic: the hydroxyl carbon is $sp^3$ and lies one bond away from the double bond. Only when the $\ce{-OH}$ is on the doubly bonded ($sp^2$) carbon itself — as in $\ce{CH2=CH-OH}$ — is the alcohol vinylic.
Rule: $sp^3$ carbon adjacent to C=C → allylic · $sp^3$ carbon adjacent to ring → benzylic · $sp^2$ carbon of C=C → vinylic.
Classifying phenols and ethers
Phenols are graded only on the number of hydroxyl groups they bear on the ring. A single $\ce{-OH}$ gives a monohydric phenol such as phenol itself, $\ce{C6H5OH}$. Two $\ce{-OH}$ groups give a dihydric phenol (the benzenediols) and three give a trihydric phenol. The same mono-, di-, tri-, poly-hydric vocabulary used for alcohols carries over directly, the only difference being that the hydroxyls now sit on an aromatic skeleton.
Ethers follow a different logic because they have no $\ce{-OH}$ to count. They are classified by the two groups flanking the oxygen. An ether is simple or symmetrical when the two alkyl or aryl groups are identical, and mixed or unsymmetrical when they differ. NCERT gives diethyl ether $\ce{C2H5OC2H5}$ as a symmetrical ether, whereas $\ce{C2H5OCH3}$ and $\ce{C2H5OC6H5}$ are unsymmetrical.
| Class | Sub-type | Defining feature | Example |
|---|---|---|---|
| Alcohol | Monohydric | one –OH | $\ce{C2H5OH}$ |
| Alcohol | Dihydric (glycol) | two –OH (vicinal in glycols) | $\ce{HOCH2CH2OH}$ |
| Alcohol | Trihydric | three –OH | glycerol |
| Alcohol | 1° / 2° / 3° | 1 / 2 / 3 carbons on C–OH | $\ce{(CH3)3COH}$ is 3° |
| Phenol | Monohydric | one ring –OH | $\ce{C6H5OH}$ |
| Phenol | Dihydric | two ring –OH | catechol |
| Ether | Symmetrical | two identical groups on O | $\ce{C2H5OC2H5}$ |
| Ether | Unsymmetrical | two different groups on O | $\ce{C2H5OCH3}$ |
IUPAC nomenclature of alcohols
In the common system an alcohol takes the name of its alkyl group followed by the word "alcohol": $\ce{CH3OH}$ is methyl alcohol, $\ce{(CH3)2CHOH}$ is isopropyl alcohol. The IUPAC system instead treats the alcohol as a derivative of the parent alkane, so alcohols are named as alkanols. The procedure, as set out in both NCERT §7.2 and NIOS §26.1.1, follows four steps.
First, select the longest carbon chain that contains the carbon bearing the $\ce{-OH}$; this is the parent chain. Replace the terminal "e" of the corresponding alkane with the suffix "ol". Second, number the chain from the end nearer to the hydroxyl group so that the $\ce{-OH}$ receives the lowest possible locant, and place that locant before "ol". Third, number and locate any other substituents. Fourth, write the substituents in alphabetical order with their locants. Crucially, the hydroxyl group takes precedence over double and triple bonds when the chain is numbered.
Polyhydric alcohols keep the alkane's terminal "e" intact and add the multiplying prefix di, tri and so on before "ol", with a locant for every hydroxyl. Thus $\ce{HOCH2CH2OH}$ is ethane-1,2-diol and glycerol is propane-1,2,3-triol. Cyclic alcohols use the prefix "cyclo" and treat the $\ce{-OH}$ carbon as C-1, giving names such as cyclohexanol and 2-methylcyclopentanol.
| Structure | Common name | IUPAC name |
|---|---|---|
| $\ce{CH3OH}$ | Methyl alcohol | Methanol |
| $\ce{CH3CH2CH2OH}$ | n-Propyl alcohol | Propan-1-ol |
| $\ce{(CH3)2CHOH}$ | Isopropyl alcohol | Propan-2-ol |
| $\ce{CH3CH2CH2CH2OH}$ | n-Butyl alcohol | Butan-1-ol |
| $\ce{(CH3)2CHCH2OH}$ | Isobutyl alcohol | 2-Methylpropan-1-ol |
| $\ce{(CH3)3COH}$ | tert-Butyl alcohol | 2-Methylpropan-2-ol |
| $\ce{HOCH2CH2OH}$ | Ethylene glycol | Ethane-1,2-diol |
| $\ce{HOCH2CH(OH)CH2OH}$ | Glycerol | Propane-1,2,3-triol |
Once you can name them, see how they are made in Preparation of Alcohols.
Naming phenols
The simplest hydroxy derivative of benzene is phenol; "phenol" serves as both its common name and an accepted IUPAC name. For substituted phenols the common system uses the positional terms ortho (1,2-disubstituted), meta (1,3-) and para (1,4-). The three methylphenols are the cresols: o-cresol is 2-methylphenol, m-cresol is 3-methylphenol and p-cresol is 4-methylphenol.
The dihydroxybenzenes have entrenched common names that NEET expects you to recognise on sight. The 1,2-isomer is catechol (benzene-1,2-diol), the 1,3-isomer is resorcinol (benzene-1,3-diol) and the 1,4-isomer is hydroquinone or quinol (benzene-1,4-diol). Notice that the IUPAC names follow the same diol logic used for aliphatic alcohols, simply applied to the benzene ring.
| Compound | Common name | IUPAC name |
|---|---|---|
| $\ce{C6H5OH}$ | Phenol | Phenol |
| 2-methyl $\ce{C6H4OH}$ | o-Cresol | 2-Methylphenol |
| 3-methyl $\ce{C6H4OH}$ | m-Cresol | 3-Methylphenol |
| 4-methyl $\ce{C6H4OH}$ | p-Cresol | 4-Methylphenol |
| 1,2-dihydroxybenzene | Catechol | Benzene-1,2-diol |
| 1,3-dihydroxybenzene | Resorcinol | Benzene-1,3-diol |
| 1,4-dihydroxybenzene | Hydroquinone / quinol | Benzene-1,4-diol |
Naming ethers
Common names of ethers list the two alkyl or aryl groups as separate words in alphabetical order, followed by "ether": $\ce{CH3OC2H5}$ is ethyl methyl ether. When both groups are identical the prefix "di" is used, so $\ce{C2H5OC2H5}$ is diethyl ether.
The IUPAC system regards an ether as a hydrocarbon in which a hydrogen atom has been replaced by an $\ce{-OR}$ or $\ce{-OAr}$ group. The larger group is chosen as the parent hydrocarbon, and the smaller group becomes an alkoxy substituent. Ethers are thus named as alkoxyalkanes. By this rule $\ce{CH3OCH3}$ is methoxymethane, $\ce{C2H5OC2H5}$ is ethoxyethane and $\ce{CH3OCH2CH2CH3}$ is 1-methoxypropane. For aromatic ethers, $\ce{C6H5OCH3}$ is methoxybenzene (anisole) and $\ce{C6H5OC2H5}$ is ethoxybenzene (phenetole).
| Structure | Common name | IUPAC name |
|---|---|---|
| $\ce{CH3OCH3}$ | Dimethyl ether | Methoxymethane |
| $\ce{C2H5OC2H5}$ | Diethyl ether | Ethoxyethane |
| $\ce{CH3OCH2CH2CH3}$ | Methyl n-propyl ether | 1-Methoxypropane |
| $\ce{C6H5OCH3}$ | Methyl phenyl ether (anisole) | Methoxybenzene |
| $\ce{C6H5OC2H5}$ | Ethyl phenyl ether (phenetole) | Ethoxybenzene |
| $\ce{CH3OCH2CH2OCH3}$ | — | 1,2-Dimethoxyethane |
The parent in an ether is the bigger group
When naming $\ce{CH3OCH2CH2CH3}$ as an alkoxyalkane, the propyl chain — not the methyl — is the parent, giving 1-methoxypropane, never "propoxymethane". The smaller group always becomes the alkoxy prefix.
Rule: larger group = parent alkane · smaller group = alkoxy substituent.
Worked naming examples
The examples below are taken directly from NCERT Unit 7. Work through each by first identifying the class, then applying the relevant naming rule.
Name the alcohol $\ce{CH3CH(CH3)CH(Cl)CH(CH3)CH2OH}$ (the 4-chloro-2,3-dimethylpentanol skeleton).
The longest chain bearing the $\ce{-OH}$ has five carbons (pentane → pentanol). Numbering from the hydroxyl end gives $\ce{-OH}$ at C-1, methyl groups at C-2 and C-3, and chlorine at C-4. Alphabetising the substituents (chloro before dimethyl) yields 4-chloro-2,3-dimethylpentan-1-ol.
Give the IUPAC name of $\ce{CH3CH(OH)CH2CH(OH)CH(CH3)CH3}$ — the 2,5-dimethylhexanediol skeleton.
Two hydroxyls on a six-carbon chain make it a hexanediol; the "e" of hexane is retained. With $\ce{-OH}$ at C-1 and C-3 and methyl groups at C-2 and C-5, the name is 2,5-dimethylhexane-1,3-diol (NCERT Intext 7.3).
Name the phenol carrying methyl groups at positions 2 and 6.
Phenol is the parent; the ring carbon bearing $\ce{-OH}$ is C-1, and the two methyls flank it at C-2 and C-6. The name is 2,6-dimethylphenol.
Provide the IUPAC name of $\ce{CH3CH(OC2H5)CH3}$ (the ethoxypropane skeleton).
The parent is propane (the larger group), and the ethoxy group sits on C-2, giving 2-ethoxypropane. Its common name is ethyl isopropyl ether.
These four cover the recurring exam patterns: locant assignment in a branched alcohol, the retained-"e" rule for diols, ring numbering in phenols, and parent selection in ethers. NCERT's Exercise 7.1 and 7.23 supply many more of the same type, and structures such as cyclohex-3-en-1-ol or 3-cyclohexylpentan-3-ol simply combine these rules with the cyclo prefix and the precedence of $\ce{-OH}$ over the double bond.
Lock these in before you move on
- Alcohol = $\ce{-OH}$ on aliphatic ($sp^3$ usually) carbon; phenol = $\ce{-OH}$ on aromatic ($sp^2$) ring carbon; ether = two carbons on oxygen.
- Alcohols classify by count of $\ce{-OH}$ (mono/di/tri/polyhydric) and by carbons on the C–OH (1°/2°/3°); allylic and benzylic keep $sp^3$ C–OH, vinylic puts $\ce{-OH}$ on $sp^2$ C=C.
- IUPAC alcohols = alkanols: drop "e", add "ol"; number for lowest $\ce{-OH}$ locant; $\ce{-OH}$ outranks C=C and C≡C. Polyhydric: keep "e", add di/tri before "ol".
- Phenols: cresols are methylphenols; catechol = benzene-1,2-diol, resorcinol = 1,3-diol, quinol/hydroquinone = 1,4-diol.
- Ethers: name as alkoxyalkanes with the larger group as parent; anisole = methoxybenzene, phenetole = ethoxybenzene; symmetrical vs unsymmetrical depends on whether the two O-bound groups match.