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I am trying to understand how the locants work for the following molecule (locants added by me).

benzimidazole

I added the locants following IUPAC Blue Book rules from 2013, P-14.4. Basically, first numbering heteroatoms in the ring, assigning the highest locant to the one with an indicated hydrogen, and then number clockwise around the ring (red locants).

I assumed the same would go for the second ring, so the name would be:

$1^1H,2^1H-1^2,2^7-\text{bibenzoimidazole}$

However, Chemdraw says the name is:

$1H,3'H-2,4'-\text{bibenzo[d]imidazole}$

I get the differences between the prime and composite locants (P-14.3.1), however it shows a numbering more like:

enter image description here

Is Chemdraw wrong or am I missing something here? If Chemdraw is right then I'd assume that for some reason the ring assembly has preference for a lower locant, but I can't find the rule that states this in the Blue Book.

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Your understanding of the numbering of the individual cyclic systems is correct. If we consider the following arbitrarily simplified separate compounds, the numbering is in accordance with your expectations; low locants are given first to the indicated hydrogen and then to the substituent according to the usual rule P-14.4.

2-bromo-1H-benzimidazole7-bromo-1H-benzimidazole

In ring assemblies of two identical cyclic systems that are joined with a single bond, however, low locants are given first to the junction according to Rule P-28.2.1.

P-28.2.1 Ring assemblies with a single bond junction

(…)

Each cyclic system is numbered in the traditional way, one with unprimed locants, the other with primed locants. Lowest possible locants must be used to denote the positions of attachment. These locants must be cited in preferred IUPAC names; they can be omitted in general nomenclature when no ambiguity would result.
(…)

After that, indicated hydrogen is named and numbered according to P-28.2.3.

Therefore, the systematic name is 1​H,3'H-2,4'-bibenzimidazole and not 1​H,1'H-2,7'-bibenzimidazole since the locants 2,4' for the junction are lower than 2,7'.

1H,3'H-2,4'-bibenzimidazole

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