2-Chloro-3,3-dimethylbutane or 3-Chloro-2,2-dimethylbutane?
According to these sources, both are correct:
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1 Answer
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The current IUPAC recommendations (2013) read as follows:
Numbering
When several structural features appear in cyclic and acylic compounds, low locants are assigned to them in the following decreasing order of seniority.
(…)
(f) detachable alphabetized prefixes, all considered together in a series of increasing numerical order;
(g) lowest locants for the substituent cited first as a prefix in the name;
(…)
Note that Rule f takes precedence over Rule g.
Furthermore,
The lowest set of locants is defined as the set that, when compared term by term with other locant sets, each cited in order of increasing value, has the lowest term at the first point of difference (…)
Therefore, the example is named as 3-chloro-2,2-dimethylbutane rather than 2-chloro-3,3-dimethylbutane since the locant set ‘2,2,3’ is lower than ‘2,3,3’.
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2$\begingroup$ but the iupac naming rules also say that if there are 2 substituent groups at the same position in a compound alphabetical priority comes into play!!! $\endgroup$– geek101Mar 25, 2015 at 15:29
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$\begingroup$ @user166748 Alphabetical order is used to establish the order of citation of detachable prefixes (that is why ‘chloro’ is cited before ‘methyl’). For numbering, however, all prefixes are considered together (see item f above). Only when there is a choice between equivalent numberings, the lowest locant is assigned to the substituent cited first as a prefix in the name. $\endgroup$– user7951Mar 25, 2015 at 15:49
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$\begingroup$ yaa so that means both the names are correct one by locant rule and one by alphabetical priority? sorry for the punctuations $\endgroup$– geek101Mar 25, 2015 at 16:49
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5$\begingroup$ No. First, the alphabetical order is used to establish the order of citation of the prefixes; thus, the name is x-chloro-y,y-dimethylbutane (not x,x-dimethyl-y-chloro-butane). Then, low locants are assigned using Rule f considering all prefixes together; thus, the numbers are 2,2,3 (not 2,3,3). Therefore, the name is 3-chloro-2,2-dimethylbutane and we do not need any further rules. – For example, 2-chloro-3-methylbutane would be a different case, which would also require Rule g (and thus implicitly the alphabetical order of prefixes) in order to assign the numbers. $\endgroup$– user7951Mar 25, 2015 at 17:31
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3$\begingroup$ @Loong You should add this last comment into the front of your answer. Reading simple english is much easier than the IUPAC rule book. Thanks! $\endgroup$ Feb 2, 2018 at 13:31