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: Despite popular use of piperacillin , the dire neurotoxicity associated with piperacillin still goes unrecognized , leading to a delay in appropriate management .
### Output Text:
Despite popular use of <span class="chemical">piperacillin</span> , the dire neurotoxicity associated with <span class="chemical">piperacillin</span> still goes unrecognized , leading to a delay in appropriate management . 