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: There are no previous reports of clonidine - associated cardiac arrest in a child undergoing induction of anesthesia .
### Output Text:
There are no previous reports of <span class="chemical">clonidine</span> - associated cardiac arrest in a child undergoing induction of anesthesia . 