Unsaturation Number (textbook error)

Chapter 4 introduces so-called “road map” problems. Problem 4.51 & 4.52 are typical examples. A “road map” problem gives you some information about the composition of a molecule and its chemical behavior (what you can turn it into, how you can make it from other substances). Your job is to figure out the molecule’s structure.

A key step in the solution of a “road map” problem is to calculate the molecule’s unsaturation number (also called “degree of unsaturation” and “number of units of unsaturation”). Loudon explains how to perform this calculation in section 4.3 and I won’t cover this material in lecture. I do want to call your attention, however, to an error in the book. The term “2C” should appear only once in equation 4.7 (compare equations 4.5, 4.6, and 4.7).

FYI – I learned a slightly different (but mathematically equivalent) equation as a student: U = #C + 1 – #H/2 + #N/2.

Whichever equation you learn, notice that:

  • the number of oxygens does not affect the unsaturation number
  • #H represents the number of hydrogens plus the number of halogens.
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