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: The children had a medical history significant for attention deficit disorder previously treated with methylphenidate without success .
### Output Text:
The children had a medical history significant for attention deficit disorder previously treated with <span class="chemical">methylphenidate</span> without success . 