It definitely looks like sulfuric acid is the most reasonable choice for Venusian microbes in the area where the phosphine was detected (50km-60km altitude, 50C-60C temperature, ~1atm, see
>>12123275). Based on freezing and boiling points, there are several other chemicals that could be contenders (formamide, hydrazine, water), but they are only found in tiny amounts whereas sulfuric acid is abundant. [1]
"In a biochemistry that used sulfuric acid as a solvent, the alkene group (C=C), with two carbon atoms joined by a double bond, could function analogously to the carbonyl group (C=O) in water-based biochemistry." [2] See attached pic. Precedence already exists here on Earth. Acidic C=C reactions is used some plants to create fragrances. [1]
>[1] https://www.nap.edu/read/11919/chapter/8#73>[2] https://en.wikipedia.org/wiki/Hypothetical_types_of_biochemistry#Other_solvents_or_cosolvents 
