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.