Classification, nomenclature and structure
Alcohols carry -OH on an sp³ carbon, phenols on an aromatic ring carbon, and ethers have C-O-C. Name them as alkanols, phenols and alkoxyalkanes.
Classification
| Class | Feature | Example |
|---|---|---|
| Monohydric, dihydric, trihydric | One, two or three -OH groups | CH₃CH₂OH; HOCH₂CH₂OH; HOCH₂CH(OH)CH₂OH |
| 1°, 2° and 3° alcohols | -OH on an sp³ carbon joined to one, two or three other carbons | CH₃CH₂OH; (CH₃)₂CHOH; (CH₃)₃COH |
| Allylic alcohol | -OH on an sp³ carbon next to a C=C | CH₂=CH-CH₂OH |
| Benzylic alcohol | -OH on an sp³ carbon next to an aromatic ring | C₆H₅CH₂OH |
| Vinylic alcohol | -OH on an sp² carbon of a C=C | CH₂=CH-OH |
| Phenol | -OH directly on a benzene ring carbon (sp²) | C₆H₅OH |
| Simple (symmetrical) ether | The two groups on oxygen are the same | C₂H₅OC₂H₅ |
| Mixed (unsymmetrical) ether | The two groups on oxygen are different | CH₃OC₂H₅, C₆H₅OCH₃ |
Common and IUPAC names
| Formula | Common name | IUPAC name |
|---|---|---|
| CH₃OH | Methyl alcohol (wood spirit) | Methanol |
| CH₃CH₂CH₂OH | n-Propyl alcohol | Propan-1-ol |
| CH₃CH(OH)CH₃ | Isopropyl alcohol | Propan-2-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 |
| CH₃C₆H₄OH (OH at 1, CH₃ at 2) | o-Cresol | 2-Methylphenol |
| C₆H₄(OH)₂ (OH at 1,2 / 1,3 / 1,4) | Catechol / resorcinol / hydroquinone (quinol) | Benzene-1,2-diol / benzene-1,3-diol / benzene-1,4-diol |
| CH₃OCH₃ | Dimethyl ether | Methoxymethane |
| C₂H₅OC₂H₅ | Diethyl ether | Ethoxyethane |
| C₆H₅OCH₃ | Anisole | Methoxybenzene |
| C₆H₅OCH₂CH₃ | Phenetole | Ethoxybenzene |
Naming rules
- 1Alcohols: choose the longest chain that carries the -OH. Number it from the end nearer the -OH. Change the final -e of the alkane to -ol: CH₃CH(OH)CH₃ is propan-2-ol.
- 2Two or three -OH groups: keep the -e and add diol or triol: ethane-1,2-diol.
- 3★Example: (CH₃)₂CHCH₂CH(OH)CH₃. The longest chain has 5 carbons. Numbering from the -OH end puts OH on C-2 and CH₃ on C-4. Name: 4-methylpentan-2-ol.
- 4Phenols: the ring carbon with -OH is C-1, so a CH₃ on C-3 gives 3-methylphenol.
- 5Ethers: the smaller group becomes an alkoxy prefix on the larger alkane: CH₃OCH₂CH₃ is methoxyethane.
★ key step
Structures of the functional groups
- Alcohols: oxygen is joined to an sp³ carbon and has two lone pairs. The C-O-H angle (108.9° in methanol) is slightly less than tetrahedral (109°28′), because the lone pairs repel the bond pairs.
- Phenols: the -OH is on an sp² carbon of the ring. The C-O bond (136 pm) is shorter than in methanol (142 pm). Two reasons: (i) oxygen's lone pair is shared with the ring (conjugation), giving partial double bond character; (ii) the sp² carbon holds electrons more tightly.
- Ethers: the two bond pairs and two lone pairs on oxygen are arranged roughly tetrahedrally. But the C-O-C angle (111.7° in methoxymethane) is slightly more than tetrahedral, because the two bulky alkyl groups repel. The C-O bond (141 pm) is about the same as in alcohols.
How to draw the structures with bond angles
- Methanol: draw H₃C on the left, O in the middle with two lone pairs (two pairs of dots), and H on the right, bent at O. Mark the C-O-H angle 108.9° and the C-O bond 142 pm.
- Phenol: draw a benzene ring with O-H on one corner. Mark the C-O bond 136 pm and write 'partial double bond'.
- Methoxymethane: draw H₃C-O-CH₃ bent at O, with two lone pairs on O. Mark the C-O-C angle 111.7° and C-O 141 pm.
Glycerol (propane-1,2,3-triol), sold at chemists as glycerine for dry skin and cracked heels, keeps skin moist because its three -OH groups hold water. Ethane-1,2-diol is the antifreeze in vehicle radiators.
2 marks: “Write the IUPAC names of (i) (CH₃)₃COH and (ii) C₆H₅OC₂H₅.” (i) 2-Methylpropan-2-ol. (ii) Ethoxybenzene.
2 marks: “Why is the C-O-H angle in methanol slightly less than 109°28′, while the C-O-C angle in methoxymethane is slightly more?” In methanol, the two lone pairs on O repel the bond pairs and squeeze the angle to 108.9°. In the ether, the two bulky methyl groups repel each other and widen the angle to 111.7°.
2 marks: “The C-O bond in phenol is shorter than in methanol. Give two reasons.” Conjugation of oxygen's lone pair with the ring gives partial double bond character; the carbon is sp² hybridised, not sp³. So 136 pm against 142 pm.
Lone pairs squeeze (108.9°), bulky groups widen (111.7°), the ring shortens (136 pm).