Figure 4-46. Recombination of radicals leads to termination of the polymerization process.
Radical polymerization of ethylene furnishes so-called low-density polyethylene
, which is rather flexible and is used to manufacture such products as thin grocery bags, clear sandwich bags, and squeeze bottles. Much harder and stronger high-density polyethylene
for making plastic gallon milk jugs and thick glossy shopping bags (among many other things) cannot be produced by radical polymerization. High-density polyethylene is made by polymerizing ethylene in a non-radical catalytic process employing special Ziegler-Natta catalysts
. These catalysts are named after Karl Ziegler (Germany) and Giulio Natta (Italy) who won the 1963 Nobel Prize in Chemistry for their groundbreaking developments of new polymerization reactions and catalysts.4.3.8. Natural Occurrence, Production, and Applications of Alkenes.
Ethylene is biosynthesized by plants and, as mentioned above, is an important hormone that accelerates the ripening and regulates many other processes in plants. Alkenes are present in crude oil, albeit in vastly smaller quantities than alkanes.
Alkenes and especially ethylene are paramount for the production of various chemicals and materials. Ethylene is manufactured on a scale that is unrivaled in the chemical industry of organic compounds and polymers. The petrochemical process called cracking
is used to produce ethylene, propylene, and other light alkenes from longer chain alkanes and cycloalkanes distilled off crude oil.
Over one half of all ethylene produced worldwide is used to make polyethylene of various grades. Large quantities of ethylene are also consumed in the production of ethylene glycol (HOCH2
OH) and acetaldehyde (CH3
CHO). Ethylene glycol is one of the two key compounds needed to make polyester. Acetaldehyde serves as a precursor to many industrially important chemicals. Propylene, the second most industrially important alkene after ethylene, is used to make polypropylene and many chemicals that are vital to several industries. 4.3.9. Exercises.
1. Ethylene (a) is dangerous because it is toxic while being odorless; (b) is a nontoxic odorless low-boiling liquid; (c) has a strong pungent odor; (d) is a flammable gas that is nontoxic and has a faint odor; (e) is a solid polymer used to make plastic bags and milk bottles. Answer
2. Ethylene is planar due to sp2
hybridization of the carbon atoms. True or false? Answer
3. The high reactivity of alkenes originates from (a) the C-H bonds at an sp2
-hybridized C atom being highly polarizable and consequently reactive; (b) the σ-bond being polarizable and reactive; (c) the π-bond being polarizable and reactive. Answer
4. Using the hybridization concept, explain why ethylene is planar. [Answer: See 4.3.2]
5. Draw structural formulas for: 1-pentene; cis
-3-hexene; 2-methyl-1-hexene; 2,3-dimethyl-2-butene; cis
-3,6-diethyl-4-octene; cyclohexene; isobutene.
6. Name the following alkenes: