^ abNomenclature of Organic Chemistry : IUPAC Recommendations and Preferred Names 2013 (Blue Book). Cambridge: The Royal Society of Chemistry. 2014. p. 375. doi:10.1039/9781849733069-FP001. ISBN978-0-85404-182-4. The name allene, for CH2=C=CH2, is retained for general nomenclature only; substitution is allowed, but not by alkyl or any other group that extends the carbon chain, nor characteristic groups expressed by suffixes. The systematic name, propadiene, is the preferred IUPAC name.