Nomenclature of alcohols, phenols and ethers

How are alcohols, phenols and ethers named?

In plain words: methane becomes methanol, propane becomes propan-2-ol with the –OH on C2, and a diol keeps the “e” (ethane-1,2-diol). Phenols are named as derivatives of phenol, and an ether is named as an alkoxy group on the larger hydrocarbon: CH₃OC₂H₅ is methoxyethane.

Common and IUPAC names of alcohols

The IUPAC rules (NCERT page 196), which apply the general naming of compounds with functional groups from Class 11:

1. Take the alkane name and replace its final “e” by “ol”.
2. Number the longest carbon chain (the parent chain) from the end nearest the –OH group; give the positions of –OH and of other substituents by the numbers of the carbons they are on. The parent chain is the longest chain that contains the –OH carbon: NCERT’s Class 11 rule numbers “the longest chain of carbon atoms containing the functional group” (NCERT Class 11 Chemistry, Unit 8, Section 8.5.3, page 266). So CH₃CH₂CH₂CH(CH₂OH)CH₂CH₃ is 2-ethylpentan-1-ol, not a hexane, although its longest chain has six carbons.
3. Alcohols with two or more –OH groups (NCERT’s “polyhydric alcohols” on page 196) keep the “e” of the alkane and add “ol” with a multiplying prefix: HO–CH₂–CH₂–OH is ethane-1,2-diol.
4. Cyclic alcohols take the prefix cyclo, with the –OH carbon as C1: cyclohexanol, 2-methylcyclopentanol (NCERT page 196).

NCERT Table 7.1 (page 196), compound: common name, IUPAC name:

• CH₃OH: methyl alcohol, methanol
• CH₃CH₂CH₂OH: n-propyl alcohol, propan-1-ol
• CH₃CH(OH)CH₃: isopropyl alcohol, propan-2-ol
• CH₃CH₂CH₂CH₂OH: n-butyl alcohol, butan-1-ol
• CH₃CH(OH)CH₂CH₃: sec-butyl alcohol, butan-2-ol
• (CH₃)₂CHCH₂OH: isobutyl alcohol, 2-methylpropan-1-ol
• (CH₃)₃COH: tert-butyl alcohol, 2-methylpropan-2-ol
• HOCH₂CH₂OH: ethylene glycol, ethane-1,2-diol
• HOCH₂CH(OH)CH₂OH: glycerol, propane-1,2,3-triol

NCERT Example 7.1 (page 198) names CH₃CHClCH(CH₃)CH(CH₃)CH₂OH as 4-chloro-2,3-dimethylpentan-1-ol: the –OH end is C1, and the substituents follow in alphabetical order.

Names of phenols and ethers

Phenols (NCERT pages 196–197). The simplest, C₆H₅OH, is phenol, its common name and an accepted IUPAC name. Substituted phenols are named as derivatives of phenol, with the –OH carbon as C1 and the other groups numbered round the ring, much as other substituted benzene compounds are named; NCERT Example 7.1 (iii) (page 198) is 2,6-dimethylphenol (reasoned from NCERT pages 197–198). Common names of substituted phenols use ortho (1,2-), meta (1,3-) and para (1,4-):

• o-, m- and p-cresol are 2-, 3- and 4-methylphenol.
• The benzenediols: catechol is benzene-1,2-diol, resorcinol benzene-1,3-diol, and hydroquinone or quinol benzene-1,4-diol.

Ethers (NCERT pages 197–198).

• Common names: the two alkyl or aryl groups as separate words in alphabetical order, then “ether”: CH₃OC₂H₅ is ethyl methyl ether. With two identical groups the prefix “di” is used: C₂H₅OC₂H₅ is diethyl ether.
• IUPAC names: an ether is a hydrocarbon in which a hydrogen is replaced by –OR or –OAr; the larger group is the parent hydrocarbon, and the smaller becomes an alkoxy (or aryloxy) prefix.

NCERT Table 7.2 (page 197), compound: common name, IUPAC name:

• CH₃OCH₃: dimethyl ether, methoxymethane

• C₂H₅OC₂H₅: diethyl ether, ethoxyethane

• CH₃OCH₂CH₂CH₃: methyl n-propyl ether, 1-methoxypropane

• C₆H₅OCH₃: methyl phenyl ether (anisole), methoxybenzene

• C₆H₅OCH₂CH₃: ethyl phenyl ether (phenetole), ethoxybenzene

• C₆H₅O(CH₂)₆CH₃: heptyl phenyl ether, 1-phenoxyheptane

• CH₃OCH(CH₃)₂: methyl isopropyl ether, 2-methoxypropane

• C₆H₅OCH₂CH₂CH(CH₃)₂: phenyl isopentyl ether, 3-methylbutoxybenzene

• CH₃OCH₂CH₂OCH₃: 1,2-dimethoxyethane

• A cyclohexane ring with two CH₃ groups on one carbon and –OC₂H₅ on the next: 2-ethoxy-1,1-dimethylcyclohexane (no common name). Starting at the C(CH₃)₂ carbon gives the locants 1,1,2, lower than 1,2,2 from the ethoxy carbon, and ethoxy is written before methyl, in alphabetical order (reasoned).

• NCERT slip (page 197), common names: NCERT’s rule writes the groups “as separate words in alphabetical order”, but its example prints “ethylmethyl ether” as one word, and two rows of Table 7.2 break the order: NCERT’s Class 11 book counts iso- as part of a group’s name when alphabetising, while sec- and tert- are not counted (NCERT Class 11 Chemistry, Unit 8, page 265). So CH₃OCH(CH₃)₂ is isopropyl methyl ether, not “methyl isopropyl ether”, and C₆H₅OCH₂CH₂CH(CH₃)₂ is isopentyl phenyl ether, not “phenyl isopentyl ether”. The structures and the IUPAC names are right. In an exam, recognise NCERT’s forms; when you write a common name yourself, use separate words in alphabetical order.

• NCERT slip (page 197), IUPAC name: Table 7.2 prints the IUPAC name of C₆H₅OCH₂CH₂CH(CH₃)₂ as “3- Methylbutoxybenzene”. The 3-methyl belongs to the butoxy group, not to the ring, and NCERT’s Class 11 rule places the name of such a branched group in parentheses (NCERT Class 11 Chemistry, Unit 8, page 265): (3-methylbutoxy)benzene. Without them the name can be read as a methyl on ring carbon 3. In an exam, write (3-methylbutoxy)benzene; an option printed as NCERT prints it means the same compound.

Solved examples

Common mistakes to avoid

Quick revision summary

• Alcohol (IUPAC): alkane name, “e” replaced by “ol”, chain numbered from the end nearer –OH (propan-2-ol); diols and triols keep the “e” (ethane-1,2-diol, propane-1,2,3-triol); cyclic alcohols take cyclo with –OH on C1.
• Phenols: phenol; cresols are 2-, 3- and 4-methylphenol; catechol, resorcinol and hydroquinone are benzene-1,2-, 1,3- and 1,4-diol.
• Ethers: common name, groups in alphabetical order plus “ether”; IUPAC name, alkoxy (or aryloxy) prefix on the larger hydrocarbon (methoxyethane, methoxybenzene).

Questions students ask

Is the IUPAC name of phenol hydroxybenzene, benzenol or phenol?

Phenol is itself the IUPAC name of C₆H₅OH: NCERT calls it both its common name and “an accepted IUPAC name” (page 196). Beyond the NCERT text, IUPAC’s 2013 recommendations keep phenol as both the preferred IUPAC name and the name for general use, so benzenol, which the -ol rule would build, and hydroxybenzene are not the recommended names (Wikipedia, “Phenol”; PubChem gives phenol too). Its old common name is carbolic acid (NCERT page 201), and it is never benzyl alcohol, which is C₆H₅CH₂OH, phenylmethanol, an alcohol with its –OH on the CH₂ carbon next to the ring (reasoned).

What kind of isomers are catechol, resorcinol and quinol?

Catechol, resorcinol and quinol (hydroquinone) are position isomers: all three are C₆H₄(OH)₂, molecular formula C₆H₆O₂, and they differ only in where the second –OH sits on the ring, 1,2 (ortho), 1,3 (meta) or 1,4 (para), which is why their IUPAC names are benzene-1,2-, 1,3- and 1,4-diol (NCERT page 197). NCERT Class 11 calls compounds position isomers when they differ only in the position of a substituent or functional group on the same carbon skeleton (NCERT Class 11 Chemistry, Unit 8, page 270; structural isomerism).

Are cresol, anisole and benzyl alcohol isomers of one another?

The cresols, anisole and benzyl alcohol are all isomers: each has the molecular formula C₇H₈O, a benzene ring with one more carbon and one oxygen (reasoned). They are functional group isomers, because the oxygen is in a different functional group in each: a cresol is a phenol, with –OH and –CH₃ on the ring, anisole, C₆H₅OCH₃ (methoxybenzene), is an ether, and benzyl alcohol, C₆H₅CH₂OH (phenylmethanol), is an alcohol (NCERT pages 195 and 197; NCERT Class 11 Chemistry, Unit 8, page 270). The o-, m- and p-cresols, in turn, are position isomers of one another.

What is the IUPAC name of phenolphthalein?

The IUPAC name of phenolphthalein, C₂₀H₁₄O₄, is 3,3-bis(4-hydroxyphenyl)-2-benzofuran-1-one, a name beyond the NCERT text that PubChem gives: two 4-hydroxyphenyl groups, from two phenol molecules, on carbon 3 of a ring ester (a lactone). Its –OH groups take the prefix hydroxy-, not the suffix -ol, because NCERT’s order of priority puts esters and >C=O above –OH, as in NCERT’s hydroxyalkanones (NCERT Class 11 Chemistry, Unit 8, page 266). Phenol forms this dye in the phthalein dye test.

Practice questions

Try each question first, then open the answer to check your working.

1. Q1. Give the IUPAC name of CH₃CH(OH)CH(CH₃)CH₂CH₃.

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   A five-carbon chain numbered from the end nearer –OH: –OH on C2 and methyl on C3: 3-methylpentan-2-ol (NCERT page 196; reasoned).

2. Q2. Give the common and IUPAC names of C₆H₅OCH₂CH₂CH₃.

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   Common name: phenyl n-propyl ether (ph comes before pr in the alphabet). IUPAC name: phenyl, with six carbons, is larger than propyl, so benzene is the parent: propoxybenzene (reasoned from NCERT pages 197–198).

3. Q3. Write the structure of 3-methylbutan-1-ol and give its common name.

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   (CH₃)₂CHCH₂CH₂OH: a four-carbon chain with –OH on C1 and a methyl on C3. Its alkyl group is the isopentyl group of NCERT Table 7.2, so its common name is isopentyl alcohol (reasoned from NCERT pages 195 and 197).