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 isomers of propranolol exhibited similar local anaesthetic potencies on an isolated frog nerve preparation at a level approximately three times that of procaine .
### Output Text:
The isomers of <span class="chemical">propranolol</span> exhibited similar local anaesthetic potencies on an isolated frog nerve preparation at a level approximately three times that of <span class="chemical">procaine</span> . 