Choosing principal chain in the nomenclature of organic compounds

  • #1
arham_jain_hsr
25
9
Homework Statement
Write the preferred IUPAC name for the following acyclic compound.
Relevant Equations
N/A
structure.png


In this question, I am not sure as to which of the following two chains should be considered as the principal chain.

Approach 1 (7 carbon chain, 2 substituents):
method 1.png

4-methyl-4-(1,1-dimethylethyl)heptane

Approach 2 (6 carbon chain, 4 substituents):
method 2.png

2,2,3-trimethyl-3-propylhexane

Clearly, the carbon chain is the longest in the first approach, but approach 2 has more substituents. In such scenarios, which of the two criteria takes precedence?
I found several conflicting responses on this subject, and would certainly appreciate confirmation. Thanks!
 
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  • #2
For alkanes it is always longest first.

It gets more complicated for other classes of compounds.
 
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  • #3
I too thought so. But, this is what I found in the IUPAC Blue Book:

"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)."

Further, it is stated in P-44.3,
"P-44.3 SENIORITY OF ACYCLIC CHAINS (THE PRINCIPAL CHAIN)
In an acyclic compound consisting of individual atoms, alike or different (an acyclic chain), the chain on which the nomenclature and numbering is based is called the ‘principal chain’. When there is a choice for the principal chain, the following criteria are applied, in the order listed, until a decision is reached.
...
P-44.3.2 The principal chain has the greater number of skeletal atoms [criterion (b) in P-44.3]"

From what I understand, the above excerpts suggest that the number of substituents holds precedence over length of the chain.
Am I missing something here?
 
  • #4
Sorry, I think I had misinterpreted this point. The longest carbon chain has precedence over a carbon chain with greater substituents. But, above both of those, a chain with more number of principal characteristic group as substituents is given priority.

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

Based on this, I can conclude that 4-methyl-4-(1,1-dimethylethyl)heptane is the preferred name.
 

FAQ: Choosing principal chain in the nomenclature of organic compounds

1. What is the principal chain in organic nomenclature?

The principal chain in organic nomenclature is the longest continuous chain of carbon atoms in a molecule. Identifying this chain is crucial because it serves as the base name for the compound and determines the numbering for substituents.

2. How do you choose the principal chain when there are multiple chains of the same length?

When there are multiple chains of the same length, the principal chain is chosen based on the following criteria in order: the chain with the greatest number of substituents, the chain with the lowest set of locants (numbers assigned to substituents), and the chain that results in the substituents being cited in alphabetical order.

3. What is the role of substituents in determining the principal chain?

Substituents play a significant role in determining the principal chain. If there are multiple chains of the same length, the chain with the most substituents is preferred. This ensures that the principal chain includes as many functional groups as possible, which can affect the compound's properties and reactivity.

4. How do you number the principal chain?

The principal chain is numbered in such a way that the substituents receive the lowest possible numbers. Numbering starts from the end of the chain nearest to a substituent. If there is a tie, the chain is numbered to give the lowest number at the first point of difference.

5. What happens if the principal chain includes a functional group with priority?

If the principal chain includes a functional group with priority, such as a carboxylic acid, aldehyde, or ketone, the chain must be numbered to give the functional group the lowest possible locant. This ensures that the functional group is given precedence in the naming process, reflecting its importance in the compound's structure and reactivity.

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