Common names are seldom used except for three simple alkenes: ethylene propylene, and isobutylene. The various alkenes of a given carbon number are,
however, sometimes referred to collectively as the pentylenes (amylenes), hexylenes, heptylenes, and so on. (One sometimes encounters the naming of alkenes as derivatives of ethylene: as, for example, tetramethylethylene for (CH3) 2C C(CH3)2 .) Most alkenes are named by the IUPAC system.
The rules of the IUPAC system are: 1 . Select as the parent structure the longest continuous chain that contains the carbon-carbon double bond; then consider the compound to have been derived from this structure by replacement of hydrogen by various alkyl groups. The parent structure is known ethene<propene,butene,pentene, and so on, depending
upon the number of carbon atoms; each name is derived by changing the endi
-ane of the corresponding alkane name to -ene: 2. Indicate by a number the position of the double bond in the parent chain. Although the double bond involves two carbon atoms, designate its position by the number of the first doubly-bonded carbon encountered when numbering from the end of the chain nearest the double bond; thus J-butene and 2-butene. 3. Indicate by numbers the positions of the alkyl groups attached to the parent chain.
Problem 5.2 Give the structural formula of: fa) 2,3-dimethyl-2-butene; (b) 3-bromo2-methylpropene; (c) m-2-methyl-3-heptene: (d) (>2-chloro-2-butene. Problem 5.3 Referring to your answer to Problem 5.1 (p. 151), give IUPAC
names for: (a) the isomeric pentylenes; (b) the isomerk chloropropenes.
5.9 Physical properties As a class, the alkenes possess physical properties that are essentially the same as those of the alkanes. They are insoluble in water, but quite soluble in nonpolar solvents like benzene, ether, chloroform, or ligroin. They are less dense than water. As we can see from Table 5.2, the boiling point rises with increasing carbon number; as with the alkanes, the boiling point rise is 20 30" for each added carbon, except for the very small homologs. As before, branching lowers the boiling point. A comparison of Table 5.2 with Table 3.3 (p. 86) shows that the boiling point of an alkene is very nearly the same as that of the alkane with the corresponding carbon skeleton.
Like alkanes, alkenes are at most only weakly polar. Since the loosely held rr electrons of the double bond are easily pulled or pushed, dipole moments than for alkanes. They are still small, however: compare the dipole moments shown for propylene and 1-butene, for example, with the moment of 1.83 r> for
methyl chloride. The bond joining the alkyl group to the doubly-bonded carbon has a small polarity, which is believed to be in the direction shown, that is, with the alkyl group releasing electrons to the doubly-bonded carbon. Since this polarity is not canceled by a corresponding polarity in the opposite direction, it gives a net dipole moment to the molecule. cw-2-Butene, with two methyl groups on one side of the molecule and two hydrogens on the other, should have a small dipole moment. In mws-2-butene, on the other hand, with one methyl and one hydrogen on each side of the molecule, the bond moments should cancel out. Although the dipole moments have not been measured directly, a small difference in polarity is reflected in the higher boiling point of the c/j-isomer.
This same relationship exists for many pairs of geometric isomers. Because
of its higher polarity the cfr-isomer is generally the higher boiling of a pair;
because of its lower symmetry it fits into a crystalline lattice more poorly, and thus generally has the lower melting point.
The differences in polarity, and hence the differences in melting point and
boiling point, are greater for alkenes that contain elements whos electronegativiti
differ widely from that of carbon. For example: The relationship between configuration and boiling point or melting point is only a rule of thumb, to which there are many exceptions (for example, the boiling points of the diiodoethenes). Measurement of dipole moment, on the other hand, frequently enables us positively to designate a particular isomer as c/$ or trans.