1. INTRODUCTION

2. OBSERVATIONS  
2.1. Mid-IR Classification of Sample

3. RESULTS  
3.1. Dust Temperatures and $H_{2}$ Column Densities  
3.2. Determination of Molecular Abundances

4. DISCUSSION  
4.1. Dust Temperatures and $H_{2}$ Column Densities  
4.3. Comparison to Predictions of Chemical Evolution  
4.4. Can Molecular Properties be used as Chemical Clocks?

5. CONCLUSIONS