In the sentence extracted from biomedical literature, identify all the chemical entities. The required answer format is the same sentence with HTML <span> tags to mark up specific entities. 
### Entity Markup Guides:
Use <span class="chemical"> to denote a chemical.

### Input Text: Both drugs impaired immediate free recall but the decrease was greater for diazepam than propranolol .
### Output Text:
Both drugs impaired immediate free recall but the decrease was greater for <span class="chemical">diazepam</span> than <span class="chemical">propranolol</span> . 