I would like to know whether the blue or purple way of numbering is preferable? Following the two possible ways, I get the following names:

  • 4-(1,1-dichloro ethyl)-5-(1-chloro-2-bromo ethyl)-octane
  • 4-(1-chloro-2-bromo ethyl)-5-(1,1-dichloro ethyl)-octane

skeletal structure

In each case, the locant set is (4, 5), so from which side should I start numbering from?

  • 1
    $\begingroup$ Maybe this may help, but I can only guess. I took the liberty reworking your question, please check if I have introduced any errors.// I think that first you would need to order the substituents alphabetical, then apply the lowest set of locants, hence 4-(2-bromo-1-chloroethyl)-5-(1,1-dichloroethyl)octane, but I could be completely wrong. It should be noted, that all names are unambiguous, so while they are not the preferred IUPAC name (PIN) they are not false with respect to general nomenclature. $\endgroup$ Jun 14, 2017 at 6:36
  • $\begingroup$ Thanks for the link again Martin. There in the link the substituents were single atoms like Br, Cl, I. But here the substituents are chain themselves, so how do i arrange them alphabetically? Also the answer you mentioned is correct as that's what my book says. Also is it okay to name the other way? Thanks for editing my question and making it better. I appreciate your help $\endgroup$ Jun 14, 2017 at 7:18
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    $\begingroup$ @Martin-マーチン The rules of Section P-14, which are relevant in this case, are general rules. They shall be followed not only for preferred IUPAC names but also for other names used in general nomenclature. $\endgroup$
    – user7951
    Jun 14, 2017 at 16:46

1 Answer 1


If numbering from each end of the parent chain gives the same set of locants, the next criterion stipulates that lowest locants are assigned to the substituent that is cited first as a prefix in the name.

On this matter, the current version of Nomenclature of Organic Chemistry – IUPAC Recommendations and Preferred Names 2013 (Blue Book) reads as follows:


When several structural features appear in cyclic and acyclic 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. If no decision can be attained by application of P-14.4(f), the lowest set of locants is selected according to P-14.4(g) in their order of their citation in the name.

This means in effect that alphanumerical order is used to establish the numbering when a choice is possible since alphanumerical order is used to establish the order of citation of substituent prefixes in the name.


Alphanumerical order has been commonly called ‘alphabetical order’. As these ordering principles do involve ordering both letters and numbers, in a strict sense, it is best called ‘alphanumerical order’ in order to convey the message that both letters and numbers are involved

Alphanumerical order is used to establish the order of citation of detachable substituent prefixes (not the detachable saturation prefixes, hydro and dehydro), and the numbering of a chain, ring, or ring system when a choice is possible.


P-14.5.1 Simple prefixes (i.e., those describing atoms and unsubstituted substituents) are arranged alphabetically; multiplicative prefixes, if necessary, are then inserted and do not alter the alphabetical order already established.

For example ‘dibromo’ comes before ‘chloro’ since ‘bromo’ begins with ‘b’, ‘chloro’ begins with ‘c’, and the multiplicative prefix ‘di’ does not change the alphabetical order.

However, the example given in the question does not include simple prefixes but the compound substituents ‘1,1-dichloroethyl’ and ‘2-bromo-1-chloroethyl’. Such compound names are considered to begin with the first letter of their complete name.

P-14.5.2 The name of a prefix for a substituent is considered to begin with the first letter of its complete name.

Therefore, the correct name for the compound given in the question is 4-(2-bromo-1-chloroethyl)-5-(1,1-dichloroethyl)octane since ‘2-bromo-1-chloroethyl’ starts with ‘b’ and ‘1,1-dichloroethyl’ starts with ‘d’.



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