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: Cocaine generated more abnormal behaviors in the brainstem perturbation group , especially the electrically perturbated subjects .
### Output Text:
<span class="chemical">Cocaine</span> generated more abnormal behaviors in the brainstem perturbation group , especially the electrically perturbated subjects . 