Numbering in IUPAC name of a trisubstituted cyclohexane

Question

This is the organic compound. CHO - aldehyde group. Br - Bromine

How do name this organic compound (IUPAC Standard)? Specifically, how do you number the carbon atoms? I thought the answer was 1-Bromo-3-Methyl-cyclohexane-2-carbaldehyde.
But that is not the right answer. I think I might have gone wrong in numbering the carbon atoms. Could someone explain why and what is the correct answer?

Answer 1

You have already correctly identified the principal characteristic group of the compound as carbaldehyde. You have also identified the senior parent structure as cyclohexane, which is correct since the compound has only one possible parent structure. The remaining substituents are correctly expressed as prefixes. The last problem is the correct numbering of locants. The most important simplified criteria for the numbering in such cases are:

1. lower locants for the principal characteristic group that is expressed as suffix
2. lower locants for multiple bonds
3. lower locants for prefixes
4. lower locants for substituents cited first as a prefix in the name

The corresponding actual wording in the current version of Nomenclature of Organic Chemistry – IUPAC Recommendations and Preferred Names 2013 (Blue Book) reads as follows:

P-14.4 NUMBERING

When several structural features appear in cyclic and acyclic compounds, low locants are assigned to them in the following decreasing order of seniority:

(…)

(c) principal characteristic groups and free valences (suffixes);

(…)

(e) saturation/unsaturation:

  (i) low locants are given to hydro/dehydro prefixes (…) and ‘ene’ and ‘yne’ endings;

  (ii) low locants are given first to multiple bonds as a set and then to double bonds (…);

(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;

(…)

The lowest locant is assigned first to the principal characteristic group according to rule (c), i.e. 1-carbaldehyde.

Next, low locants are assigned to the other substituents as a set accoding to Rule (f). However, this could be ‘2-bromo-6-methylcyclohexane-1-carbaldehyde’ as well as ‘6-bromo-2-methylcyclohexane-1-carbaldehyde’ since both names correspond to the locant set ‘2,6’.

According to Rule (g), the lower locant is assigned to the substituent that is cited first as a prefix in the name, i.e. ‘bromo’. Therefore, the correct name is 2-bromo-6-methylcyclohexane-1-carbaldehyde.