Alkene Types and Structures
Alkenes are not saturated, which means they contain at least one carbon-to-carbon double bound. This double bond displaces two hydrogen atoms, which means that alkenes do not have the maximum number of hydrogen atoms per carbon and are hence termed unsaturated.
Like alkanes, alkenes derive their “alk” prefix from the fact that they contain alkyl groups. Their “-ene” ending lets you know that there is at least one double bond in these molecules.
Alkenes are also called olefins. Like alkanes, this can be confusing because there is also an olefin material. As in the case of paraffin wax and alkanes, olefin is just a type of fiber that is made from alkenes. Olefin is often made from polypropylene or polyethylene and is used in everything from rope to car interiors. It is useful because it is strong without being heavy, resistant to sunlight, and relatively easy to produce.
The double bound structure of alkenes changes their chemistry in comparison to alkanes. First, alkenes are more acidic than alkanes. They are also more reactive than alkanes and easily undergo polymerization reactions, which makes them highly valuable in industrial applications.
Ethene (C2H4) aka Ethylene
Ethene is the smallest alkene and is a colorless gas at room temperature. Unlike its cousin ethane, ethane does have an odor that is often described as “sweet and musky.” It is sometimes called ethylene, which is technically a misnomer, but is part of common parlance and so remains.
Ethene is a very important industrial chemical. Its primary use is in the production of polyethylene plastic, the mostly widely-used plastic in the world. Polyethylene is used to make everything from shopping bags to packing peanuts, lubricants, and even some detergents. More than half of all ethane is used to make polyethylene.
Oxidation of ethane produces various components including ethylene glycol, the primary ingredient in antifreeze. Halogenation of ethene produces a range of molecules such as ethyl chloride and ethylene dibromide. These molecules are found in produces like polyvinyl chloride, another widely used plastic. Finally, alkylation of ethane (the addition of an alkyl group), is the first step in the production of styrene, which is used to make the plastic polystyrene for insulation and packaging. Ethene, without modification, is also a plant hormone that produces a number of different effects including ripening of fruit, pollination, leaf abscission (falling off), seed germination, and more.
Polyethylene (units simply repeat)
Polyethylene is produced in a process that requires highly pure ethene and a metal catalyst. Most often a metal chloride is used such as titanium chloride. When polyethylene is made using a catalyst, high density polyethylene is produced. This molecule is 3 times stronger than its low density counterpart and is used in plastic milk bottles.
Polyethylene is also made through process is known as addition polymerization. When addition polymerization is performed at 500 degrees Celsius under high pressure, low density polyethylene is produced. This produces a transparent polyethylene that is flexible and finds use in plastic bags and wraps.
Propene (C3H6) aka Propylene
Propene is a colorless gas with a weak, but unpleasant smell that is described as somewhat sweet. It is a byproduct of oil refining, though it can be produced in laboratory settings as well.
Propene serves as the raw product for a number of industrial chemicals. Most notably, it can be polymerized to form polypropylene, which accounts for two-thirds of all propene used. Polypropylene is the primary component in a number of textiles including ropes, carpet, and thermal underwear. It is used in plastic containers, automotive parts, and constitutes a €50 billion industry. Propene is also used to make acetone and isopropyl alcohol (rubbing alcohol).
The production of polypropylene occurs in much the same way as polyethylene above.
Butene (C4H8) aka Butylene
Butene is a colorless gas with four structural isomers (different forms), all of which have somewhat sweet odors. Its major use is in the production of polymers for synthetic rubber tires. Its isobutylene isomer is used in the production of methyl tert-butyl ether (MBTE), which is added to gasoline to boost octane.
Alkenes above butane are generally broken down into their smaller constitutes for use in the production of polymers like polyethylene and polypropylene. The exception to this is heptene, which is used as an additive in some lubricants. Alkenes become liquid when they contain 5 or more carbon atoms and solid when they have 15 or more carbon atoms.
Polystyrene is a modification of polyethylene. It contains a benzene ring and its structure is shown below.
Polyvinyl Chloride (PVC)<
PVC is a modification of polyethylene. It is similar in structure to polystyrene, with benzene groups being replaced with chloride ions. PVC is used in pipes (roughly half of all PVC is used for pipes), as insulation in electric cables, in vinyl siding, windows, and in clothing and furniture (to create a faux leather).
PVC is made by first halogenating ethylene to produce the following vinyl chloride molecule.