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The IUPAC name for Acetophenone is : 1-Phenylethanone. An other name is : Methyl phenyl ketone. My question here why there's a space in "Methyl phenyl ketone" although there's no space in "1-Phenylethanone"?

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The two mentioned systematic names correspond to different types of nomenclature. The name “1-phenylethan-1-one” is formed according to substitutive nomenclature, and the name “methyl phenyl ketone” is formed according to functional class nomeclature. Both systematic names are in accordance with current IUPAC recommendations. However, the preferred IUPAC name (PIN) for such acyclic ketones is generated using substitutive nomenclature. Note that the omission of the locant “1” for the suffix “one” in “1-phenylethanone”, while permissible in general usage, is not allowed in PINs, thus the name “1-phenylethan-1-one” is the PIN. Furthermore, the name “acetophenone” is retained, but only for general nomenclature.

1-phenylethan-1-one

The corresponding subsection in the current version of Nomenclature of Organic Chemistry – IUPAC Recommendations and Preferred Names 2013 (Blue Book) reads as follows:

P-64.2.2.1 Acyclic ketones

Unsubstituted acyclic ketones are systematically named in two ways:

(1) substitutively, using the suffix ‘one’ and the prefix ‘oxo’; the presence of several ‘one’ characteristic groups is denoted by the numerical multiplying prefixes ‘di’, ‘tri’, etc.; the final letter ‘a’ of a numerical multiplying prefix is elided before the suffix ‘-one’, for example, ‘tetrone’;

(2) by functional class nomenclature using the class names ‘ketone’, ‘diketone’ etc.; substituent groups are placed, as separate words, in alphanumerical order before the class name.

Method (1) generates preferred IUPAC names.

Note that the rule for functional class nomeclature (method 2) stipulates that the groups shall be written as separate words.

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