Naming and drawing alcohols and ethers
The -ol suffix, numbering chains that carry an OH group, diols and triols, the hydroxy prefix, and alkoxy names for ethers.
Alcohols contain an –OH group bonded to an sp3 carbon. Ethers contain an oxygen bonded to two carbons, C–O–C. They can have the same molecular formula: ethanol and methoxymethane are both C2H6O. So reading the name carefully decides which one you draw.
Alcohols: the -ol suffix
Replace the final -e of the alkane with -ol and put the locant of the OH carbon in front of the suffix: propan-1-ol, propan-2-ol. The parent chain must contain the carbon that carries the OH, and it is numbered to give that carbon the lowest possible locant, before any alkyl or halogen substituents are considered.
Worked example: 4-methylpentan-2-ol
Draw five carbons, put the OH on C2 and a methyl on C4. Numbering from the other end would put the OH on C4, so this direction wins even though it gives the methyl a higher number: the OH group decides the numbering first.
Several OH groups
With two or three OH groups the suffix becomes -diol or -triol, and the final "e" of the alkane is kept:ethane-1,2-diol (ethylene glycol) and propane-1,2,3-triol (glycerol). Each OH needs its own locant, and normally no carbon carries more than one OH.
When OH is a prefix: hydroxy
A molecule can have only one suffix. If it also contains a group that ranks higher, such as a carboxylic acid, aldehyde or ketone, the OH group is named as the prefix hydroxy-. For example, 4-hydroxybutan-2-one is a ketone (suffix -one) with an OH on C4.
Primary, secondary and tertiary alcohols
As for haloalkanes, the class depends on how many carbons are bonded to the carbon bearing the OH: one (primary), two (secondary) or three (tertiary). Propan-1-ol is primary, propan-2-ol secondary, and 2-methylpropan-2-ol tertiary. The class predicts how the alcohol behaves with oxidising agents.
Ethers: alkoxy substituents
In a systematic ether name the oxygen and the smaller group form an alkoxy substituent (methoxy –OCH3, ethoxy –OCH2CH3), attached to the larger alkane, which is the parent. Methoxyethane is CH3–O–CH2CH3; 2-methoxypropane has the oxygen on the middle carbon of propane.
Many ethers are better known by functional-class names that list both alkyl groups: diethyl ether is ethoxyethane, and methyl tert-butyl ether (MTBE) is 2-methoxy-2-methylpropane.
Common mistakes
- Drawing an ether for an alcohol, or the reverse. "-ol" means O–H; "alkoxy" means C–O–C. The checker reports which group you drew.
- Counting the oxygen as a chain atom. In 2-methoxypropane the parent is propane (three carbons); the oxygen belongs to the substituent.
- Putting two OH groups on one carbon for a 1,1-diol. Such compounds exist but are unstable; the locants in the names here always point to different carbons.
- Numbering for the alkyl group first. The OH gets the lowest locant before substituents are considered.