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: Hallucinations occurred in 4 patients , and an unpleasant sensation ( " empty head " ) was also reported by 2 patients .
### Output Text:
Hallucinations occurred in 4 patients , and an unpleasant sensation ( " empty head " ) was also reported by 2 patients . 