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When should we apply least sum locant rule while nomenclature? Is it only valid for compounds containing 2 locant?

When should we apply least sum locant rule while nomenclature? Is it only valid for compounds containing 2 locant?

Grade:11

1 Answers

jyoti bhatia
202 Points
7 years ago
The numbers that define positions of principal functional group and substituents are called locants. Compounds are numbered from end of longest continuous chain.Locants are assigned such that principal functional group gets lowest possible locant.If this results in a ‘tie’ then first point of difference rule is applied so that the first time a difference in numbering occurs, then the method that gives lower number at this first difference is used.In the event that there is no first point of difference then alphabetization is used.Under Basic Principles, it is stated under instruction 6.1 that numbering is done according to which order (right to left or vice versa):‘Has lowest locant (or sum of locants) for suffix functional group. Locants are numbers on carbons to which the substituent is directly attached.’In the example of 18-bromo-12-butyl-11-chloro-4,8-diethyl-5-hydroxy-15-methoxytricos-6,13-dien-19-yne-3,9-dione, it is stated in example 2.2.that: ‘Numbering from left to right, ketone groups get numbered 3 and 9. When numbering from right to left, ketone groups get numbered 15 and 21. 3 is less than 15, thus, the numbering is done to right and ketones are numbered 3 and 9. The lesser number is always is always used not the sum of constituents numbers.So as to give lowest locants to the following structural features considered successively in the order listed until a decision is reached. It is not specific whether to give minimum available value or lowest sum. The lowest value is the correct one.The ‘Least Sum Rule’ is a special case. Least sum rule is valid only to chains with two substituents, a special case. However, use of Least Sum Rule is not advisable when there are more than two substituents since it may violate actual rule of first point of difference.

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