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: Chloroacetaldehyde as a sulfhydryl reagent : the role of critical thiol groups in ifosfamide nephropathy .
### Output Text:
<span class="chemical">Chloroacetaldehyde</span> as a <span class="chemical">sulfhydryl</span> reagent : the role of critical <span class="chemical">thiol</span> groups in <span class="chemical">ifosfamide</span> nephropathy . 