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Pent-3-enal. In the structure of pent-3-enal, the third carbon atom has aldehyde group or double bond? Also is the priority same for all other functional groups?

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According to the current version of Nomenclature of Organic Chemistry – IUPAC Recommendations and Preferred Names 2013 (Blue Book), locants are placed immediately in front of the part of the name to which they refer.

P-14.3.2 Position of locants

Locants (numerals and/or letters) are placed immediately before that part of the name to which they relate, except in the case of the traditional contracted names when locants are placed at the front of their names.

Therefore, the locant ‘3’ in ‘pent-3-enal’ refers to the ending ‘ene’ (not to the suffix ‘al’).

The class name ‘aldehyde’ refers to compounds containing the group $\ce{{}-CH=O}$ attached to another carbon atom. In systematic substitutive nomenclature, the suffix ‘al’ is used for $\ce{{}-(C)HO}$ (see Subsection P-66.6.1). Thus, the aldehyde carbon atom is always a terminal carbon atom.

The characteristic group cited as suffix (here: ‘-al’) receives the lowest locant possible (here: ‘1’). However, terminal locants are omitted in preferred IUPAC names (PINs) for such aldehydes.

P-14.3.4.1 Terminal locants are not cited in names for mono- and dicarboxylic acids derived from acyclic hydrocarbons and their corresponding acyl halides, amides, hydrazides, nitriles, aldehydes, amidines, amidrazones, hydrazidines, and amidoximes, when unsubstituted or substituted on carbon atoms.

Therefore, the PIN for the example given in the question is ‘pent-3-enal’ (not ‘pent-3-en-1-al’).

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The third carbon has the double bond, not the aldehyde. Aldehydes exist on terminal carbons, so the aldehyde group is on the first carbon atom. The aldehyde takes priority over the alkene (double bond). See the attached structure.enter image description here

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The number 3 indicates the position of the double bond in this case and since there is no number before the -al suffix it means the aldehyde must be present at a terminal position i.e 1. Always when there is a presence of a functional group, it is given preference over other unsaturations and other substituents and lowest locant number is assigned to it.

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