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Structural Isomers

Structural isomers are different ways of arranging the same atoms. The more carbons, the more possible ways there will be to arrange the atoms.

Remember that Lewis structures show only the way atoms are connected to each other, not how the atoms are arranged in space. It is possible for the Lewis structures to look differently but still represent the same molecule rather than one of its isomers.

>> Example 1

Which of the following pairs are structural isomers?

1. and CH₃CH₂CHCH₂
   
   
   
   
2. and CH₃CH₂CH₂CH₂CH₃
   
   
   
   
3. CH₃OCH₂CH₃ and CH₃CH₂CHH₂OH and (CH₃)₂COH
   
   
   
   
4. CH₂=CHCH₂CH₃ and CH₃CH₂CH=CH₂
   
   
   
   

Solution:

1. These are the same molecule, one written with line structures and one not. There must be a double bond between the last two carbons to explain the lack of hydrogens.
2. These are structural isomers. The first one branches at the next to last carbon, where it is bonded to three carbons and one hydrogen. In the other structure, there is no carbon bonded to three other carbons. However, both have the same number of carbons and hydrogens, so they are structural isomers.
3. All three are structural isomers, with the general formula C₃H₇O. However, the oxygen in the first structure is bonded to two carbons, whereas the oxygen in the other two structures is bonded to one hydrogen and one carbon. The one at the end has a carbon bonded to three other carbons, whereas the carbons of the middle one are bonded to no more than two other carbons.
4. These are the same molecule. Both have the same number of carbons with a double bond between the first and second carbons. The only difference is that one is flipped over.

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