Question

unknown identification:
pure substance is clear, yellow liquid with pungent but sweet smell
b.p. 248-249 celsius at 718atm
insoluble in water, NaOH, HCl
soluble in sulfuric acid and compound turns into black foamy solid

Answer 1

b.p. 248-249 celsius at 718 mm (wrong unit given in the question) means actual bpt is 251 ^oC at 760 mm (or 1 atm pressure).

When an liquid organic compound is insoluble in water, NaOH, HCl but soluble in sulfuric acid and compound turns into black foamy solid it means that it must be an aldehyde or ketone (because it cannot be sugar as it is liquid).

Since it is a yelllow liquid compoun with a sweet smell having b pt  251 ^oC at atmospheric pressure , it must be cinnamaldehyde whose structure is as follows