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 use of thiopentone was significantly associated with an eight - fold - higher risk for delirium compared to propofol ( 57 . 1 % vs . 7 . 1 % , RR = 8 . 0 , X2 = 4 . 256 ; df = 1 ; 0 . 05 < p < 0 . 02 ) .
### Output Text:
The use of <span class="chemical">thiopentone</span> was significantly associated with an eight - fold - higher risk for delirium compared to <span class="chemical">propofol</span> ( 57 . 1 % vs . 7 . 1 % , RR = 8 . 0 , X2 = 4 . 256 ; df = 1 ; 0 . 05 < p < 0 . 02 ) . 