If the position of the double bond is uncertain, so that choice cannot be made between the last two structures, then the name is: O-Ethyl-N-phenylisourea.
Examples to Rule C-972.1
972.2 - The radical NH=C(OH)-NH- is named "1-isoureido-", and NH2-C(OH)=N- is named "3-isoureido-". "Isoureido-", without a numerical prefix, denotes a radical in which the position of the double bond is not proved. These names are used as a basis for designation of derivatives in which another group is to be cited as principal group.
See Recommendations'93 Table 31(a)
Example to Rule C-972.2
Published with permission of the IUPAC by Advanced Chemistry Development, Inc., www.acdlabs.com, +1(416)368-3435 tel, +1(416)368-5596 fax. For comments or suggestions please contact firstname.lastname@example.org