# Nearest branch numbering

If the longest carbon chain is symmetric numbering-wise (meaning the numbering schemes are identical both ways) and has all alkyl substituents, should one prioritize the largest alkyl group?

• I am not sure what is your concern here, but if you are asking about naming a compound like $\ce{CH3CH2CH(CH3)(CH2)_4CH(C2H5)CH2CH3}$, I believe the priority is alphabetical (i.e ethyl>methyl), and hence this compound will be 3-ethyl 8-methyl decane. – stochastic13 Feb 4 '14 at 11:13
• Thanks - that's what I what I was looking for. I was confused about if one should take into consideration the size of the substituent. – Luke Chang Feb 5 '14 at 18:28

Priority is given to the substituent with the largest overall summed atomic weight. Compare successive "left-handed" amino acids with their CIP notation, then cysteine (heavy sulfur).

• This is not the priority the question was asking for as per OP’s comments. (I don’t know why it was accepted.) – Jan Dec 27 '16 at 21:22

If numbering from each end of the parent chain gives the same locants, the next criterion stipulates that lowest locants are assigned to the substituent that is cited first as a prefix in the name. This means in effect that alphanumerical order is used to establish the numbering when a choice is possible since alphanumerical order is used to establish the order of citation of substituent prefixes in the name.

On this matter, 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:

(…)

(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;

(…)

Note that Rule f takes precedence over Rule g. If no decision can be attained by application of P-14.4(f), the lowest set of locants is selected according to P-14.4(g) in their order of their citation in the name.

For example, a correct systematic name is 4-ethyl-5-methyloctane (not 5-ethyl-4-methyloctane) since ethyl is cited first as a prefix in the name.

Another correct systematic name is 4-ethyl-5-propyloctane (not 5-ethyl-4-propyloctane). Again, the reason is that ethyl is cited first as a prefix in the name. The size of the substituent is not relevant in this step.