Synthetic Biologics Inc., Rockville-based biotechnology company, recently sold 14.1 million units — each consisting of a share of common stock and a warrant to purchase a half-share — to select institutional investors for approximately $20.7 million. The units sold at $1.47 each. The warrant’s exercise price is $1.75 per share. Synthetic Biologics’ closing share price on the day of the sale, Oct. 9, was $1.42. Synthetic Biologics said it plans to use the proceeds primarily for clinical development of its C. difficile preventative and constipation-predominant irritable bowel syndrome therapeutic products.