Wize University Organic Chemistry Textbook > Stereochemistry & Isomers
Constitutional Isomers
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Constitutional Isomers
Constitutional or Structural Isomers
Isomers are two structures with the same molecular formula but difference in bond connectivity or three-dimensional (3D) space occupation. Constitutional (or structural) isomers differ in their bond connectivity.
Two examples are shown below.
Index of Hydrogen Deficiency (IHD)
The index of hydrogen deficiency (IHD, sometimes called double bond equivalents or DBE) is a calculation to determine how many degrees of unsaturation are in a molecule. A degree of unsaturation is a double bond (1), triple bond (2), or ring (1).
Wize Concept
A saturated alkane will have the general formula CnH2n+2
To calculate the index of hydrogen deficiency, use the formula below.
C = carbons; N = nitrogen; H = hydrogen; X = halogen
When drawing multiple isomers of carbon chain compounds:
- Assess the IHD (determine if you need a ring or multiple bond)
- Draw all the possible chain lengths and then look at branching sites
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Example: Constitutional Isomers
Which of the following pairs are constitutional isomers?
Recall, constitutional isomers are structural isomers. This means they have different bond connectivity. Both of these sets of molecules are constitutional isomers.
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Example: Constitutional Isomers
Which of the following pairs are constitutional isomers?
Recall, constitutional isomers are structural isomers. This means they have different bond connectivity. Structures found in example 3 are not constitutional isomers because they have different molecular formulae. Structures found in example 4 are not constitutional isomers because they are identical.