CH₃CHCHCH₃ A colorless, highly flammable gas, used to make butadiene and in the synthesis of four- and five-carbon organic molecules; the cis form, boiling point 3.7_C, is insoluble in water, soluble in organic solvents, and is also known as high-boiling butene-2; the trans form, boiling point 0.88_C, is insoluble in water, soluble in most organic solvents, and is also known as low-boiling butene- 2.