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6-bromo-5-hydroxycyclohex-3-ene-1-carbonitrile

Going by the rules that were given to me by my professor:

  1. I first decide the seniormost group and its parent chain. In this case, cyclohexene carbonitrile.
  2. Then, I decide direction to traverse the ring to assign locants by the first point of difference. Accordingly, I move towards bromine.

The final name I obtain is thus "2-bromo 3-hydroxy cyclohex-4-enecarbonitrile". The answer given in my workbook however is "6-bromo 5-hydroxy cyclohex-3-enecarbonitrile". Apparently, they decided to traverse the ring in reverse direction.

Obviously, this must be some special case. Please explain the exact rule involved here. Thanks!

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  • 3
    $\begingroup$ Actually, the preferred name is 6-bromo-5-hydroxycyclohex-3-ene-1-carbonitrile. $\endgroup$ – Loong Nov 27 '17 at 18:32
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For the numbering:

P-14.4 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: [...]

(f) detachable alphabetized prefixes, all considered together in a series of increasing numerical order [...]

As correctly stated, carbonitrile therefore gets priority for the low locant, in this case '1'. The next group to be considered is the alkene. Since the locant '3' is lower than the locant '4', the numbering is therefore chosen as follows:

numbering system

Numbering aside, Loong's comment points out two other issues with your name.

  1. There are hyphens, not spaces, between substituents.

  2. The locant '1' in carbonitrile is also required, because of P-14.3.3 (see Loong's comment). There is no rule saying that it can be assumed to have the locant '1'.

Compare this with "butanoic acid", for example, where the locant '1' for the -oic acid suffix can be dropped. This is because

P-14.3.4.1 Terminal locants are not cited in names for mono- and dicarboxylic acids derived from acyclic hydrocarbons and their corresponding acyl halides, amides, hydrazides, nitriles, aldehydes, amidines, amidrazones, hydrazidines, and amidoximes, when unsubstituted or substituted on carbon atoms.

This rule is very prescriptive on when the locant '1' can be dropped. Since "butanoic acid" is a monocarboxylic acid derived from an acyclic hydrocarbon (butane), the locant '1' can be dropped, and the PIN is butanoic acid instead of butan-1-oic acid. My personal interpretation of this rule - or what I think is the rationale behind it - is that this is because "butan-2-oic acid" simply does not make chemical sense.

However, in this case, "6-bromo-5-hydroxycyclohex-3-ene-2-carbonitrile" is still a name which can generate a valid structure which is different from yours.

putative isomer

This numbering is obviously not in line with IUPAC rules, because it does not assign low locants to the nitrile, but that's beside the point.

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  • $\begingroup$ I think even for functional groups for nitrile,which doesn't that necessarily have to be terminal position needn't be specified in case of cyclic compounds as we can always start counting from it unlike in the case of a straight chain compound where we cant always start from functional group $\endgroup$ – ULTIMATEGAMER07 Nov 28 '17 at 14:26
  • $\begingroup$ No, that's not how it works. $\endgroup$ – orthocresol Nov 28 '17 at 15:55
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    $\begingroup$ The locant 1 in 6-bromo-5-hydroxycyclohex-3-ene-1-carbonitrile is a result of rule P-14.3.3 “In preferred IUPAC names, if any locants are essential for defining the structure of the parent structure or of a unit of structure as defined by its appropriate enclosing marks, then all locants must be cited for the parent structure or that structural unit. For example, the omission of the locant '1' in 2-chloroethanol, while permissible in general usage, is not allowed in preferred IUPAC names, thus the name 2-chloroethan-1-ol is the PIN. …” $\endgroup$ – Loong Nov 28 '17 at 18:49
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This is because alkene has higher preference than alkyl halides. So when counting clockwise position of ene is less than that when counting anticlockwise. Hence the answer is as stated.

Check out Rule 6.2 of basic principles on Wikipedia.

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