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: Groups 1 and 3 remained untreated while groups 2 and 4 received enalapril .
### Output Text:
Groups 1 and 3 remained untreated while groups 2 and 4 received <span class="chemical">enalapril</span> . 