# Omission of locants for functional groups in IUPAC names

I'm having a bit of conflict while naming molecules as per IUPAC the nomenclature rules. If we take the following compound for example,

IUPAC would say that that correct name is 2-phenylacetaldehyde. But then for this molecule,

IUPAC designate it the name is chloroacetaldehyde.

Why does the aldehyde with a benzene ring as a functional group receive a '2' while the aldehyde with a halide as a functional group does not? I figured the molecules mentioned above wouldn't require a number to designate their functional groups given they're both substituents to an ethane parent chain.

Also, would it be acceptable to drop 'aldehyde' and replace it with 'ethanal'. I've always been taught to add '-al' with a '-CHO' functional group?

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There is only one possible phenylacetaldehyde, so its IUPAC name is in fact phenylacetaldehyde. 2-Phenylacetaldehyde is its systematic IUPAC name, but many chemicals' official and systematic IUPAC names differ, like propan-2-one for example. The reasoning for this is often historical and to reflect what is most commonly used in the scientific community.

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Chloroacetaldehyde is quite obviously an aldehyde, but switching the position of the hydrogen and the chlorine as you suggested results in a completely different class of molecule known as an acyl chloride. There is therefore only one possible chloroacetaldehyde and no need to specify the chlorine's location, though its systematic IUPAC name is indeed 2-chloroethanal.

• The answer is correct; however, I should like to make it clear that the preferred IUPAC names are indeed “phenylacetaldehyde”, “chloroacetaldehyde”, and “acetyl chloride”, respectively. – Loong Sep 3 '16 at 15:35