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For to apply first point of difference rule how long should the carbon chain be?

For to apply first point of difference rule how long should the carbon chain be? 

Grade:10

1 Answers

jyoti bhatia
202 Points
5 years ago

First point of difference rule

This is probably the most difficult of the rules associated with nomenclature to master.

The factors that influence the numbering according to the first point of difference rule are:
  • the principal functional group is given the lowest possible locant
  • substituents are then assigned locants based on this numbering scheme in such a way as to give the lowest locant number at the first time there is a difference
  • in the event that there is no first point of difference in the locants, substituents are numbered  based on alphabetisation giving the first substituent the lowest possible locant number.

  • In order to work with the first point of difference rule, try the approach shown below based on the comparision of 1,1,2 vs 1,2,2:

  • for each possible numbering scheme, list out the locants in numerical order
  • compare the "lists" to locate the first point of difference, i.e. the first time you encounter a low number
  • at the first point of difference, select the lower locant choice.
  • 2-methylpentane
    not
    4-methylpentane.
     
    Here the methyl group is given the lowest number by numbering from the right (2- rather than 4-).

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