By Ron B. Davis Jr., Georgetown University
In their pure forms, all four of the common halogens form diatomic molecules with a single bond. In the diatomic molecules, two atoms of a halogen come together and share a valence electron, fooling each atom into thinking that it has a full octet. Besides forming ionic bonds to many metals or strong acids in combination with hydrogen, halogens can also combine via covalent bonds with nonmetals.
Carbon: A Tricky Nonmetal
Carbon has been perhaps the trickiest of the nonmetals we combine with halogens.
Carbon’s electronegativity is 2.5, equal to iodine, and only somewhat lower than fluorine, chlorine, or bromine. So, carbon can form a covalent bond with halogens in which it partially shares electrons. This is sometimes called a ‘polar covalent bond’. Covalent bonds of carbon with halogens have offered inventors a variety of tempting properties—yet often with unwanted side effects when this bond breaks down under more extreme conditions.
In the popular anti-stick coating Teflon consisting of chains of carbon atoms, each carbon in the chain has two covalently bonded fluorine atoms. At normal cooking or baking temperatures, the tightly held electron clouds of the fluorine atoms interact very weakly with other materials, providing a non-stick surface that remains solid, since it is a large polymer molecule.
However, if scratched or heated above 600° F, Teflon may begin to break down.
Chlorine and Carbon’s Covalent Bond
Chlorine bonds covalently to carbon as well. Chloroform, for example, is a small molecule with 3 chlorines and 1 carbon that has many industrial uses as a solvent. But, it also has many harmful side effects, if workers are not sufficiently protected.
Polyvinyl chloride, known as PVC, is a plastic polymer widely used in piping. Each ‘vinyl’ in the polymer is just two carbons attached to a chloride ion. And there is also the C-PVC, which adds extra chlorine to keep the product unreactive all the way up to 200° Fahrenheit.
But higher temperatures are the Achilles heel, with PVC posing a known risk to firefighters when the chlorine is released inside a burning house.
The Curious Case of Chlorofluorocarbons
The now-infamous chlorofluorocarbons, or CFCs, are compounds that bring carbon, fluorine, and chlorine together to form small molecules formerly used in refrigeration systems.
Compounds like this were also formerly used in aerosol propellants because they can be compressed into a liquid under modest pressure but vaporize aggressively when released, for example through a spray nozzle.
Near the surface of the Earth, they are remarkably stable—almost like Teflon—and this is why they were largely regarded as harmless to release into the environment for decades.
CFCs Banned
Unfortunately, in the 1970s a few new facts about these compounds came to light. First, they do not always stay near Earth’s surface. Instead, they can slowly be carried into the upper atmosphere of the Earth by air currents.
Second, in the upper atmosphere, carbon-chlorine bonds can be broken by high-energy UV light from our sun. This releases a free chlorine atom with just seven valence electrons, launching fierce assaults to steal electrons from ozone molecules, breaking ozone down into ordinary oxygen. The protective ozone layer in our upper atmosphere can be significantly damaged in this way.
It is because of these harmful effects high in the atmosphere that CFCs were banned in the US in 1978, subject to a worldwide reduction treaty in 1989, and completely phased out by 2010.
So, instead of CFCs, refrigerants now often use HFCs. The chlorine is gone, while the tightly held fluorine is still there, in what is now ‘Hydrofluorocarbons’. The difference is that hydrogen substitutes for chlorine.
This article comes directly from content in the video series Understanding the Periodic Table. Watch it now, on Wondrium.
Bromine Compound
Similarly, bromine is used in a BFC compound called halon. This is a bromine-carbon-fluorine compound used to extinguish electrical fires in laboratories and aircraft, where the bromine atom gives halon a high density that keeps the extinguishing media near the base of a fire.
But halon’s special advantage comes after the fire is extinguished—because it is a gas, any unreacted halon is carried away slowly by the air. This leaves sensitive electronics intact without damaging residues that other extinguishing media might leave behind.
Unfortunately, this BFC gas with bromine has ozone-depleting properties similar to the CFCs.
The US banned the new production of the most widely used halon while allowing unused halon gases from older systems to be collected and reused in new systems. Recapture keeps halon out of the atmosphere.
Covalently Bonded Chlorine
Covalently bonded chlorine is also found in the sugar substitute sucralose, which is just ordinary sucrose made sweeter by the substitution of several chlorine atoms for other groups of atoms on the molecule.
Unfortunately, sucralose may not remain stable in high-temperature baking, prompting warnings of caution when using it as a sugar replacement in such a project.
Bromine
Bromine reacted with vegetable oil produces a compound that helps prevent the oil-based flavorings in soft drinks from separating out from the water-based drink over time.
However, Europe and Japan ban this additive, and US companies have gradually removed it over concerns that the bromine may disrupt important biological reactions involving its similarly sized halogen counterpart iodine.
Iodine-Carbon Bond
The food we eat contains iodine largely as ionically bonded iodide, but this is the raw material used by the thyroid to produce molecules that contain covalently bonded iodine. Iodine bonds to the carbon of amino acid, tyrosine, to make a powerful hormone called thyroxine.
Such iodine compounds play a crucial role in regulating human metabolism. Too many or too few of these compounds in our bodies can result in medical conditions that undermine healthy weight and energy levels.
Hence, covalent carbon bonds with halogens bring many unique properties to the fore.
Common Questions about Covalent Bonds of Carbon with Halogens
Carbon’s electronegativity is 2.5, equal to iodine, and only somewhat lower than fbluorine, chlorine, or Bromine. So, carbon can form a covalent bond with halogens in which it partially shares electrons.
The iodine-carbon covalent bond is vitally important in our biology. Iodine binds to the carbon of amino acid, tyrosine, to make a powerful hormone called thyroxine. Such iodine compounds play a crucial role in regulating human metabolism. Too many or too few of these compounds in our bodies can result in medical conditions that undermine healthy weight and energy levels.
The element chlorine bonds covalently to carbon as well. Chloroform, for example, is a small molecule with 3 chlorines and 1 carbon that has many industrial uses as a solvent.
