C₅H₁₀ Either of two colorless, flammable, volatile liquid isomers; soluble in alcohol, insoluble in water: 3- methyl-1-butene boils at 20°C, is used as a chemical intermediate and in the manufacture of high-octane fuel, and is also known as isopropylethylene; 3- methyl-2-butene boils at 38°C, is used as an anesthetic and high-octane fuel and as a chemical intermediate, and is also known as trimethylethylene.