Alkenes are hydrocarbons that contain one or more carbon-carbon double bonds. Alkenes
are sometimes called olefins, particularly in older literature and in the chemical industry.
Ethylene is the simplest alkene.
Because compounds containing double or triple bonds have fewer hydrogens than the cor-
responding alkanes, they are classified as unsaturated hydrocarbons, in contrast to alka-
nes, which are classified as saturated hydrocarbons.
This chapter covers the structure, bonding, nomenclature, and physical properties of
alkenes. Then, using a few alkene reactions, some of the physical principles are discussed
that are important in understanding the reactivities of organic compounds in general.
4.1 STRUCTURE AND BONDING IN ALKENES
The double-bond geometry of ethylene is typical of that found in other alkenes. Ethylene
follows the rules for predicting molecular geometry (Sec. 1.3B), which require each carbon
of ethylene to have trigonal planar geometry; that is, all the atoms surrounding each carbon
lie in the same plane with bond angles approximating 120°. The experimentally determined
structure of ethylene agrees with these expectations and shows further that ethylene is a
planar molecule. For alkenes in general, the carbons of a double bond and the atoms at-
tached to them all lie in the same plane.
Models of ethylene are shown in Fig. 4.1, and a comparison of the geometries of ethyl-
ene and propene with those of ethane and propane is given in Fig. 4.2. Notice that the
(substitutive name: ethene)
Introduction to Alkenes:
Structure and Reactivity
CHAPTER 4 •
INTRODUCTION TO ALKENES: STRUCTURE AND REACTIVITY
carbon-carbon double bonds of ethylene and propene (1.330 Å and 1.336 Å, respectively)
are shorter than the carbon-carbon single bonds of ethane and propane (1.536 Å and
1.54 Å, respectively). This illustrates the relationship of bond length and bond order
(Sec. 1.3B): double