Determine no. of Structural Isomers

 

 

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[Bad News] There is no mathematical formula available to calculate the number of structural isomers. You have to draw out all possible structures.

 

[Good News] There is a general approach for that.

 

APPROACH

  1. Draw the structure with the longest possible chain

  2. If a functional group is present, shift the position of the group to get all the different positional isomers

  3. After all positional isomers have been identified, draw the structure of the next longest chain (minus 1 C)

  4. Shift the position of the functional group and/ or substituent to get all the different isomers

  5. Repeat steps 3 & 4 until no more variations

 

Example : Determine the number of structural isomers for C4H10Cl.

 

 

There are 4 structural isomers for C4H10Cl.