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: Six patients ( 3 . 6 % ) in the methadone group presented torsades de pointes .
### Output Text:
Six patients ( 3 . 6 % ) in the <span class="chemical">methadone</span> group presented torsades de pointes . 