Functional class names of alcohols are derived by naming the alkyl group that bears the hydroxyl substituent (–OH) and then adding alcohol as a separate word. The chain is always numbered beginning at the carbon to which the hydroxyl group is attached. Substitutive names of alcohols are developed by identifying the longest continuous chain that bears the hydroxyl group and replacing the -e ending of the corresponding alkane by the suffix -ol. The position of the hydroxyl group is indicated by number, choosing the sequence that assigns the lower locant to the carbon that bears the hydroxyl group.
Hydroxyl groups take precedence over (“outrank”) alkyl groups and halogen substituents in determining the direction in which a carbon chain is numbered. The OH group is assumed to be attached to C-1 of a cyclic alcohol, and is not numbered.
Alcohols occur with primary, secondary and tertiary structures. Whether an alcohol is primary, secondary or tertiary depends on the number of hydrogens attached to the carbon atom bearing the –OH group, as shown in the below figure. These differences in structure affect the way the alcohols react.
- Primary alcohols have two hydrogens on the OH–bearing carbon – this carbon is bonded to one other carbon.
- Secondary alcohols have two carbons and only one hydrogen substituted onto the OH–bearing carbon.
- Tertiary alcohols have three carbons and no hydrogens on the OH–bearing carbon.
(a) Some primary alchohols
(b) Some secondary alchohols
(c) Some tertiary alchohols
As well as having varying numbers of alkyl or aryl groups within the molecules, alcohols may have varying numbers of –OH groups. Alcohols with more than one –OH group are known as polyhydric alcohols or polyols and they are of particular importance in biological systems. Molecules with two –OH groups are known as diols and those with three –OH groups as triols.
Vicinal diols are diols that have their hydroxyl groups on adjacent carbons. Two commonly encountered vicinal diols are 1,2-ethanediol and 1,2-propanediol.
NOMENCLATURE OF FENOLS
An old name for benzene was phene, and its hydroxyl derivative came to be called phenol (the systematic name for phenol is benzenol). This, like many other entrenched common names, is an acceptable IUPAC name. Likewise, o-, m-, and p-cresol are acceptable names for the various ring-substituted hydroxyl derivatives of toluene. More highly substituted compounds are named as deriv- atives of phenol. Numbering of the ring begins at the hydroxyl-substituted carbon and proceeds in the direction that gives the lower number to the next substituted carbon. Substituents are cited in alphabetical order.
Pyrocatechol is often called catechol. The graphic that opened this chapter is a molecular model of phenol that shows planar structure and electro-I static potential. The three dihydroxy derivatives of benzene may be named as 1,2-, 1,3-, and 1,4- benzenediol, respectively, but each is more familiarly known by the common name indicated in parentheses below the structures shown here. These common names are per- missible IUPAC names.
Carboxyl and acyl groups take precedence over the phenolic hydroxyl in deter- mining the base name. The hydroxyl is treated as a substituent in these cases.
NOMENCLATURE OF ETHERS
Ethers are named, in substitutive IUPAC nomenclature, as alkoxy derivatives of alkanes. Functional class IUPAC names of ethers are derived by listing the two alkyl groups in the general structure ROR’ in alphabetical order as separate words, and then adding the word ether at the end. When both alkyl groups are the same, the prefix di- precedes the name of the alkyl group.
Ethers are described as symmetrical or unsymmetrical depending on whether the two groups bonded to oxygen are the same or different. Unsymmetrical ethers are also called mixed ethers. Diethyl ether is a symmetrical ether; ethyl methyl ether is an unsymmetrical ether. Cyclic ethers have their oxygen as part of a ring–they are heterocyclic compounds. Several have specific IUPAC
In each case the ring is numbered starting at the oxygen. The IUPAC rules also permit oxirane (without substituents) to be called ethylene oxide. Tetrahydrofuran and tetrahy- dropyran are acceptable synonyms for oxolane and oxane, respectively.