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: Hepatonecrosis and cholangitis related to long - term phenobarbital therapy : an autopsy report of two patients .
### Output Text:
Hepatonecrosis and cholangitis related to long - term <span class="chemical">phenobarbital</span> therapy : an autopsy report of two patients . 