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 abnormal rhythm disappeared with the withdrawal of propranolol and when the drug was restarted a 2 / 1 S - A block was seen .
### Output Text:
The abnormal rhythm disappeared with the withdrawal of <span class="chemical">propranolol</span> and when the drug was restarted a 2 / 1 S - A block was seen . 