CHAPTER 11: Unit 11. Physical properties: Alkenes and Alkynes

Learning Objective

1. Identify the physical properties of alkenes and describe trends in these properties.

The physical properties of alkenes are similar to those of the alkanes. Table below “Physical Properties of Some Selected Alkenes” shows that the boiling points of straight-chain alkenes increase with increasing molar mass, just as with alkanes. For molecules with the same number of carbon atoms and the same general shape, the boiling points usually differ only slightly, just as we would expect for substances whose molar mass differs by only 2 u (equivalent to two hydrogen atoms). Like other hydrocarbons, the alkenes are insoluble in water but soluble in organic solvents.

Alkenes and alkanes have similar properties. They are nonpolar, insoluble in water, less dense than water, and soluble in nonpolar solvents. The melting point of alkene is usually lower than that of alkane with the same number of carbon atoms.

Physical Properties of Some Selected Alkenes

IN CLASS PRACTICE PROBLEM

1. Briefly describe the physical properties of alkenes. How do these properties compare to those of the alkanes?
2. Without consulting tables, arrange the following alkenes in order of increasing boiling point: 1-butene, ethene, 1-hexene, and propene.Homework Exercise
   1. Without referring to a table or other reference, predict which member of each pair has the higher boiling point.a. 1-pentene or 1-buteneb. 3-heptene or 3-nonene
   2. Which is a good solvent for cyclohexene, pentane or water?
      Answer
   1. a. 1-penteneb. 3-nonene