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Which name is the Preferred IUPAC Name of the above molecule? 3-methyl-4-oxobutanenitrile or 3-formylbutanenitrile?

I feel the second one is the PIN, since it's shorter than the first one. But, I haven't been able to navigate the IUPAC Blue Book, regarding such cases.

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The most important simplified criteria for the choice of a principal chain are:

  1. greater number of substituents corresponding to the suffix (principal characteristic group)
  2. longest chain
  3. greater number of multiple bonds
  4. lower locants for suffixes
  5. lower locants for multiple bonds
  6. greater number of prefixes
  7. lower locants for prefixes
  8. lower locants for substituents cited first as a prefix in the name

The corresponding wording of the rules taken from Nomenclature of Organic Chemistry – IUPAC Recommendations and Preferred Names 2013 (Blue Book) is as follows.

P-44.1 SENIORITY ORDER FOR PARENT STRUCTURES

When there is a choice, the senior parent structure is chosen by applying the following criteria, in order, until a decision is reached. These criteria must always be applied before those applicable to rings and ring systems (see P-44.2) and to chains (see P-44.3). Then criteria applicable to both chains and rings or ring systems given in P-44.4 are considered.

P-44.1.1 The senior parent structure has the maximum number of substituents corresponding to the principal characteristic group (suffix) or senior parent hydride in accord with the seniority of classes (P-41) and the seniority of suffixes (P-43).

(…)

P-44.3.2 The principal chain has the greater number of skeletal atoms [criterion (b) in P-44.3].

(…)

P-44.4.1 If the criteria of P-44.1 through P-44.3, where applicable, do not effect a choice of a senior parent structure, the following criteria are applied successively until there are no alternatives remaining. These criteria are illustrated in P-44.4.1.1 through P-44.4.1.12.

The senior ring, ring system, or principal chain:

(a) has the greater number of multiple bonds (P-44.4.1.1);

(b) has the greater number of double bonds (P-44.4.1.2);

(…)

(h) has the lower locant for an attached group expressed as a suffix (P-44.4.1.8);

(…)

(j) has the lower locant(s) for endings or prefixes that express changes in the level of hydrogenation, i.e., for ‘ene’ and ‘yne’ endings and ‘hydro/dehydro’ prefixes (P-44.4.1.10);

(…)

P-45.2.1 The preferred IUPAC name is based on the senior parent structure that has the maximum number of substituents cited as prefixes (other than ‘hydro/dehydro’) to the parent structure.

P-45.2.2 The preferred IUPAC name is based on the senior parent structure that has the lower locant or set of locants for substituents cited as prefixes (other than ‘hydro/dehydro’) to the parent structure.

P-45.2.3 The preferred IUPAC name is based on the senior parent structure that has the lower locant or set of locants for substituents cited as prefixes to the parent structure (other than ‘hydro/dehydro’ prefixes) in their order of citation in the name.

(…)

According to Rule P-44.1.1, the priority list of substituent groups has to be considered first in order to decide the parent chain for this compound. You have already done that in both of your suggested possibilities: The principal characteristic group that is expressed as a suffix is the nitrile group ($\ce{-(C)N}$).

The next rule (P-44.3.2) is looking at the longest chain. Therefore, the parent structure is a butanenitrile.

The following rules (P-44.4.1.1 and P-44.4.1.2) would be looking at multiple bonds, which is not relevant in this case.

Next, Rule P-44.4.1.8 is looking for a lower locant for the attached group that is expressed as a suffix (here: nitrile); however, the nitrile group has the lowest locant ‘1’ in both suggested cases; therefore, this rule doesn't help to make a choice.

The next rule (P-44.4.1.10) again is not relevant in this case.

Finally, Rule P-45.2.1 is looking for the structure that has the maximum number of substituents cited as prefixes to the parent structure. Therefore, the preferred IUPAC name is 3-methyl-4-oxobutanenitrile (two substituents) rather than ‘3-formylbutanenitrile’ (only one substituent).

3-methyl-4-oxobutanenitrile

By way of comparison, if you add more substituents to the methyl group, the principal chain can switch, for example 4,4-dichloro-3-formylbutanenitrile (three substituents) rather than ‘3-(dichloromethyl)-4-oxobutanenitrile’ (two substituents).

4,4-dichloro-3-formylbutanenitrile

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The Preferred IUPAC name is 3-methyl-4-oxobutanenitrile. Reference: Pubchem

Explanation:

This compound has two functional groups. The seniority table says that the Nitrile group has a higher precedence over the Aldehyde/Formyl group. While picking the longest chain, we have the following set of rules (From Wikipedia):

  1. It should have the maximum length.
  2. It should have the maximum number of substituents of the suffix functional group. By suffix, it is meant that the parent functional group should have a suffix, unlike halogen substituents. If more than one functional group is present, the one with highest precedence should be used.

  3. ...

Here, the longest chain in both cases has 4 carbon atoms. The chain should also contain the maximum number of substituents of the suffix functional group. We then choose the chain containing the oxygen atom attached to it, thereby obtaining methyl as a substituent on C3, giving us the name.

Note that oxo can be used even for an aldehyde (From Wikipedia)

If the compound is a natural product or a carboxylic acid, the prefix oxo- may be used to indicate which carbon atom is part of the aldehyde group; for example, $\ce{CHOCH2COOH}$ is named 3-oxopropanoic acid.

This logic is extendable to include any functional group that is above the aldehyde group in the seniority table.

Answering the doubt you posed regarding 3-formylpentanenitrile in the comments, here, the pentane chain has the most number of carbon atoms and also contains the suffix functional group attached to it, i.e. formyl, which is what makes it the PIN.

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  • $\begingroup$ "The chain should also contain the maximum number of substituents of the suffix functional group". Isn't the suffix functional group nitrile? If yes, then both the longest chains have maximum number of nitriles, i.e. 1. So, how can we decide which longest chain to choose? $\endgroup$ – Shashank Dec 30 '19 at 14:30
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    $\begingroup$ By suffix, it is meant that the parent functional group should have a suffix, unlike halogen substituents. Here, the possibilities are that it has nitrile + formyl or nitrile + oxo groups, in which case we consider the oxo group one. $\endgroup$ – Aniruddha Deb Dec 30 '19 at 15:59
  • $\begingroup$ The preferred IUPAC name 3-methyl-4-oxobutanenitrile is correct; however, the explanations are wrong. Oxo and formyl are not suffixes and have nothing to do with the principal characteristic group that is expressed as a suffix. $\endgroup$ – Loong Jan 7 at 10:03

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