Alcohols nomenclatural priority

In Section 8.1, we saw that a carbonyl group has a higher nomenclature priority than an alcohol or an amine group. However, all carbonyl compounds do not have the same priority. The nomenclature priorities of the various carbonyl groups are shown in Table 18.1. 

As with ketones, the carbonyl of an aldehyde has a higher priority than the C=C of an alkene, the C C of an alkyne, or the OH of an alcohol. Note that the aldehyde unit is drawn using a shorthand version, -CHO. When the aldehyde unit (-CHO) is attached to a ring, a major modification in the name is required. Aldehyde 71, for example, is not named cyclohexanal. This name makes no sense because an aldehyde must have the CHO carbon labeled as Cl. Instead, the term carboxal-dehyde is used and 71 is named cyclohexanecarboxaldehyde. This nomenclature rule should be used for all molecules where the CHO unit is attached to a ring. 

Where the molecule contains both a double bond and an OH group, the OH group takes precedence in numbering the chain (at the end of this section on nomenclature, there will be a complete list of priorities). So 4.18 is named as 2-propene-l-ol. Its common name is allyl alcohol. We have already met phenol, 4.19—when there is more than one OH group or where the OH group does not have the highest priority and is named as a substituent, we use the term hydroxy. So, 4.20 is 1,3-dihydroxybenzene. 

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