# What is the preferred IUPAC name of o-toluidine?

Why does o-toluidine have 2-aminomethylbenzene as its IUPAC name?

Why isn't it named as a derivative of amine (like 2-methylaniline)? Aren't amines given a higher priority than alkanes in nomenclature?

See the Priority Chart given on Master Organic Chemistry for reference.

In principle, your rationale is correct.

Generally, the selection of a preferred parent structure is based on the seniority of classes, which gives priority first to characteristic groups expressed as suffixes. Since o-toluidine (note that this name is no longer recommended) contains only one characteristic group $(\ce{{}-NH2})$, it is named as amine.

According to Subsection P-62.2.1.1.1 of the current version of Nomenclature of Organic Chemistry – IUPAC Recommendations and Preferred Names 2013 (Blue Book), the retained name “aniline” is the preferred IUPAC name (PIN) for the functional parent compound. The systematic name “benzenamine” may be used in general nomenclature.

For the retained name “aniline”, substitution is permitted at any position. Substitution of such functional parent compounds is limited to substituent groups having a lower seniority than that denoted by the (explicitly expressed or implied) suffix of the functional parent compound. Anyway, in this case, this limitation is not relevant since o-toluidine contains only one characteristic group and carbon compounds (rings or chains) have a lower seniority than amines.

Therefore, the PIN for o-toluidine is “2-methylaniline”. The systematic name “2-methylbenzenamine” may be used in general nomenclature.

• But why is it "2-amino-1-methylbenzene" rather than "1-amino-2-methylbenzene"? – SendersReagent Mar 7 '16 at 20:21
• @DGS This appears to be a separate question. The name “2-amino-1-methylbenzene” is unambiguous and describes the correct structure; however, it is not in accordance with IUPAC recommendations for several reasons. Even if we ignore the higher seniority of amines and name the compound as “benzene”, the numbering would be “1-amino-2-methylbenzene” rather than “2-amino-1-methylbenzene” since both names correspond to the locant set “1,2” (all prefixes considered together in a series of increasing numerical order) and “amino” is cited first as a prefix in the name (see Subsection P-14.4, f and g). – Loong Mar 7 '16 at 20:40
• That's what I thought. I think it fit in with his question (yes, he specifically mentions aniline in one of his questions), but maybe it should get its own question. – SendersReagent Mar 7 '16 at 20:44
• I don't understand your answer,can you explain more? – Sana Apr 23 '18 at 4:48