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: In Group 1 the rats were trained under the influence of pentobarbital to run to the same shelf as in the normal state .
### Output Text:
In Group 1 the rats were trained under the influence of <span class="chemical">pentobarbital</span> to run to the same shelf as in the normal state . 