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: He had been taking disulfiram for alcohol dependence for the preceding 3 years .
### Output Text:
He had been taking <span class="chemical">disulfiram</span> for alcohol dependence for the preceding 3 years . 