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Are both 2-ethyltoluene and 1-ethyl-2-methylbenzene correct? If yes, do they belong to the same compound or belong to different compounds?

I'm confused when I have to put organic nomenclature for a compound if there is a methyl group. When should I use toluene and when should I use benzene?

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The names 2-ethyltoluene and 1-ethyl-2-methylbenzene are unambiguous and describe the same compound.

1-Ethyl-2-methylbenzene is a correct systematic name according to current IUPAC recommendations.

Various traditional names are retained for use in IUPAC nomenclature, though the number of retained names has been reduced with each succeeding edition of the IUPAC recommendations. In the current version of Nomenclature of Organic Chemistry – IUPAC Recommendations and Preferred Names 2013 (Blue Book), the name toluene is still retained as the preferred IUPAC name, but no substitution is allowed.

P-22.1.3 Parent hydrocarbons having retained names

Toluene, xylene, and mesitylene are specific parent hydrides that are composed of two components, one cyclic and the other acyclic and saturated. These names are retained due to a long and well established tradition. Toluene and xylene are preferred IUPAC names, but are not freely substitutable; toluene is substitutable under certain conditions, but only for general nomenclature (see P-15.1.8 for a general substitution rules for retained names).
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For the compound that is given in the question, the systematic name 1-ethyl-2-methylbenzene is the preferred IUPAC name.

1-ethyl-2-methylbenzene

In general nomenclature, toluene is substitutable; however, substitution is permitted only by groups listed in P-15.1.8.2.2.

P-15.1.8.2.2 Substitution rules for Type 2b retained names

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The following characteristic groups cited can be used to substitute parent structures of Type 2b (ring and the side chain if required): halides $\ce{-Br}$, $\ce{-Cl}$, $\ce{-F}$, $\ce{-I}$, pseudohalides $\ce{-N3}$, $\ce{-NCO}$ (and chalcogen analogues), $\ce{-NC}$, substituent groups derived from the halogen oxo acids $\ce{-ClO}$, $\ce{-ClO2}$, $\ce{-ClO3}$ (similarly for groups in which $\ce{Cl}$ is replaced by $\ce{Br}$ or $\ce{I}$), $\ce{-NO2}$ and $\ce{-NO}$, and $\ce{-OR}$ ($\ce{R}$ =alkyl groups), and chalcogen analogues, and-$\ce{SO-R}$ and $\ce{-SO2-R}$, and $\ce{Se}$ and $\ce{Te}$ analogues.

Therefore, the name 2-ethyltoluene is not in accordance with current IUPAC recommendations, neither for preferred IUPAC names nor for general nomenclature.

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I like to use ChemDraw program to help me to confirm in converting IUPAC name to molecular structure and vice versa. With your proposed two names, based on CD program, they are the same molecule.

enter image description here

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    $\begingroup$ While this provides a good answer to the first part of the question, you have completely ignored the second part: "I'm confused when I have to put organic nomenclature for a compound if there is a methyl group. When should I use toluene and when should I use benzene?" $\endgroup$ – airhuff Jul 30 '17 at 19:26
  • $\begingroup$ @airhuff My answer is clearly itself. They are the same molecule. Whatever name what you call this structure, as long as those name are right and can be refer to the same molecular structure, it's ok. On the other hand, if I provide the 30 students just its 2D structure of this chemical on their final exam and ask them to provide the name of this structure, would their answer be the same ? $\endgroup$ – rangsiman Jul 30 '17 at 19:34
  • $\begingroup$ All I'm saying is that your answer would be more complete if it explicitly and directly addressed the second part of the question. I feel like I can't give it a UV without that and I suspect others will think the same. Just my advice, take it or leave it. $\endgroup$ – airhuff Jul 30 '17 at 19:47
  • $\begingroup$ @airhuff Thank you for your advice. I got your point. $\endgroup$ – rangsiman Jul 30 '17 at 19:56
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The two compounds are the same. The difference in name comes from what you are taking as the base name. In the case of 2-ethyltoluene, you are using as a reference toluene (benzene with a $\ce{-CH3}$ group attached to it which is assumed then to be at position 1). In the second case, you are using benzene as the reference point and naming the substituents attached to it. While toluene is a common name it has been accepted by IUPAC as correct in systematic nomenclature. Thus, you can use either one.

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    $\begingroup$ Not really 2-ethyltoluene isn't correct according to IUPAC; toluene has only limited use as a basis for creating names. $\endgroup$ – Mithoron Mar 9 '17 at 16:17
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The latest IUPAC convention has given permission to use some 'commonly' used common names. Toluene is very common in its usage and also is permitted to use according to IUPAC. So both are correct. Answer according to options given in the question.

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