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: A 2 - year - old child with known neurologic impairment developed a dyskinesia soon after starting phenobarbital therapy for seizures .
### Output Text:
A 2 - year - old child with known neurologic impairment developed a dyskinesia soon after starting <span class="chemical">phenobarbital</span> therapy for seizures . 