Organic Chemistry – Some Basic Principles and Techniques · Lesson 5 of 15
Naming Compounds with Functional Groups and Benzene Rings
“Use functional-group priority and ring substitution to move confidently between names and structures.”
• Recognise the principal functional group and select its suffix. • Name compounds with several groups or multiple bonds. • Derive structures from systematic names. • Name substituted benzene rings and distinguish phenyl from a benzene parent.
A functional group changes the naming priorities
A functional group often identifies the chemically characteristic part of a molecule. Its name may appear as a suffix when it is the principal group, or as a prefix when another group has higher priority. Begin by finding all functional groups, rather than numbering the chain from the nearest branch. Alcohols such as methanol, ethanol, and propan-2-ol illustrate a family whose OH group also gives similar reactions with sodium.
Choose a suitable parent containing the principal functional group and the relevant multiple bonds. Number it to give the principal group its required low locant, then consider unsaturation and substituents. Replace the appropriate ending of the parent name with the class suffix. For a simple alcohol, pentane becomes pentan-2-ol; the final e is omitted before the vowel-starting suffix ol.
| Family | Characteristic group | Prefix when subordinate | Characteristic suffix or naming form |
|---|---|---|---|
| Alkane | Single carbon framework | — | -ane |
| Alkene | — | -ene | |
| Alkyne | — | -yne | |
| Arene | Aromatic ring | Phenyl as a substituent | Benzene or retained parent |
| Halide | Fluoro-, chloro-, bromo-, iodo- | Prefix | |
| Alcohol | Hydroxy- | -ol | |
| Aldehyde | Formyl- or oxo-, as appropriate | -al | |
| Ketone | Oxo- | -one | |
| Nitrile | Cyano- | -nitrile | |
| Ether | Alkoxy- | Alkoxyalkane form | |
| Carboxylic acid | Carboxy- | -oic acid | |
| Carboxylate ion | — | -oate | |
| Ester | Alkoxycarbonyl- | Alkyl alkanoate | |
| Acyl halide | Halocarbonyl- | -oyl halide | |
| Amine | Amino- | -amine | |
| Amide | Carbamoyl- | -amide | |
| Nitro compound | Nitro- | Prefix | |
| Sulphonic acid | Sulpho- | -sulphonic acid |
This table is a recognition guide, not a reason to apply every possible naming rule at once. Ethers are commonly named as alkoxyalkanes, such as ethoxyethane. An ester name identifies the group on oxygen followed by the acid-derived part: methyl propanoate is an example. Halogens, nitro, alkoxy, alkyl, and phenyl groups are treated as prefixes in the examples here.
Handle several groups without losing the parent
For the functional classes in this chapter, decreasing naming priority runs from carboxylic acid through sulphonic acid, ester, acyl halide, amide, nitrile, aldehyde, ketone, alcohol, and amine, with multiple bonds considered after these characteristic groups. Priority is a naming convention; it does not claim that a higher-priority group is always more reactive. Groups not selected as the principal one are expressed by prefixes.
Problem
Name and .
- 1.For the alcohol, the longest chain containing OH has eight carbons. Number from the end giving OH locant 3 rather than 6; the methyl branch is then at 6. The name is 6-methyloctan-3-ol.
- 2.For the diketone, the six-carbon parent contains two ketone groups. Numbering gives 2 and 4 rather than 3 and 5. Use the multiplying suffix dione and retain the e of hexane: hexane-2,4-dione.
Problem
Name and .
- 1.In the first compound, carboxylic acid has priority over ketone. Include the carboxyl carbon as carbon 1 in a six-carbon chain. The ketone becomes oxo at carbon 5: 5-oxohexanoic acid.
- 2.In the second compound, ketone has priority over alcohol. Number the seven-carbon chain from the methyl end so the carbonyl is at 2. The OH is at 7: 7-hydroxyheptan-2-one.
- 3.The suffix follows the principal group even when another group would receive a smaller number under a different numbering direction.
Repeated identical groups use di or tri before the suffix, as in ethane-1,2-diol. Multiple double bonds similarly give diene or triene; the parent ending is adjusted to give buta-1,3-diene. When both double and triple bonds occur, both are indicated. In the source example, the selected direction gives hexa-1,3-dien-5-yne, with en expressed before yn in the name.
Problem
Draw pent-4-en-2-ol, cyclohex-2-en-1-ol, and 6-hydroxyheptanal.
- 1.Pent specifies five carbons. Place OH on carbon 2 and the double bond between 4 and 5. One condensed formula is .
- 2.For cyclohex-2-en-1-ol, draw a six-carbon ring, put OH at carbon 1, and place the double bond between carbons 2 and 3. OH controls the starting point.
- 3.Heptanal contains seven parent carbons including the aldehyde carbon. Count from CHO and put OH at 6, giving . Each name specifies both connectivity and position.
Numbering a substituted benzene ring
For a simple benzene derivative, write the substituent before benzene: methylbenzene, nitrobenzene, or bromobenzene. Some retained common names are widely used: toluene for methylbenzene, anisole for methoxybenzene, and aniline for aminobenzene. With two substituents, the relative positions may be 1,2; 1,3; or 1,4, corresponding to ortho, meta, and para.
With three or more substituents, use numbers rather than trying to extend ortho, meta, and para. Select the lowest appropriate locant sequence and list prefix substituents alphabetically. If a retained parent such as phenol, anisole, or aniline is used, its defining substituent fixes carbon 1. Thus 3,4-dimethylphenol starts its numbering at the carbon carrying OH.
Problem
Interpret o-ethylanisole, p-nitroaniline, and 2,3-dibromo-1-phenylpentane.
- 1.Anisole fixes the methoxy-bearing ring carbon as 1. Ortho places ethyl on the adjacent ring carbon: o-ethylanisole is 2-ethylanisole.
- 2.Aniline fixes the amino-bearing carbon as 1. Para places nitro at carbon 4, on the opposite side of the ring.
- 3.In 2,3-dibromo-1-phenylpentane, pentane is the named parent. A benzene ring missing one hydrogen is the phenyl group, attached at carbon 1; bromines occupy carbons 2 and 3. Phenyl is abbreviated Ph.
Do not give an alkyl branch a low locant at the expense of the principal functional group. The carbon of an aldehyde, carboxylic acid, or nitrile may belong to the parent chain and must be counted correctly. A prefix such as phenyl describes an attached group, not an extra parent chain.
Quiz
Which group has higher naming priority in a molecule containing both COOH and OH?
What relationship does meta describe on disubstituted benzene?
What is the correct name when a seven-carbon chain has ketone at 2 and OH at 7?
Which name identifies benzene acting as an attached substituent?
In pent-4-en-2-ol, between which carbons is the double bond?
Practice Problems
- Draw 2-chlorohexane, pent-4-en-2-ol, 3-nitrocyclohexene, cyclohex-2-en-1-ol, and 6-hydroxyheptanal.
- Name and explain why the alkene is numbered before the bromo substituent.
- Draw hexa-1,3-dien-5-yne and verify the valency of every carbon.
- Draw 2-chloro-4-methylanisole, 4-ethyl-2-methylaniline, and 1-chloro-2,4-dinitrobenzene.
- Write the structural formulas for o-ethylanisole, p-nitroaniline, and 4-ethyl-1-fluoro-2-nitrobenzene.
- Identify the principal and subordinate functional groups in 5-oxohexanoic acid and 7-hydroxyheptan-2-one.
Key Takeaways
• The principal functional group determines the characteristic suffix and numbering priority. • Count parent carbons carefully, including functional-group carbons when required. • Ortho, meta, and para correspond to 1,2; 1,3; and 1,4 positions. • Names and structures should be checked in both directions.