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2-Chloro-3,3-dimethylbutane or 3-Chloro-2,2-dimethylbutane?

According to these sources, both are correct:

  1. NIST/TRC Web Thermo Tables (WTT)
  2. ChemSpider

nomenclature discrepancy

Melanie Shebel
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geek101
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2 Answers2

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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’.

3-chloro-2,2-dimethylbutane

Community
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    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!!! – geek101 Mar 25 '15 at 15:29
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    @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. –  Mar 25 '15 at 15:49
  • yaa so that means both the names are correct one by locant rule and one by alphabetical priority? sorry for the punctuations – geek101 Mar 25 '15 at 16:49
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    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. –  Mar 25 '15 at 17:31
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    @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! – Gaurang Tandon Feb 02 '18 at 13:31
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What I understand in simple terms is:

See that longest chain is butane.

Now we can number the carbon atoms of main chain either:

(A) 1, 2, 3, 4: In this case, the numbering of substituents is 2, 3, 3.

(B) 4, 3, 2, 1: In this case, numbering on substituents is 2, 2, 3.

Now, we apply "First point of difference rule". Using this rule, we see that the first 2's are same, but in the second number, there is a difference. So we select that numbering which has the lower number.

So, we follow numbering (B).

But to write the name, we write the chloro part first and dimethyl later because alphabetically c of chloro comes before m of methyl.

So the correct name is:

3-Chloro-2,2-dimethylbutane

NOTE: The first point of difference rule is used before the alphabetical rule. Like if there was symmetry, say the molecule was 2-Chloro-3-bromobutane, then its correct name would be 2-Bromo-3-chlorobutane.

Naman Goyal
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