Halogen Compounds

 

Learning Objective

  • Discuss¬†halogen compounds and their properties.

Key Points

    • Hydrogen halides are binary compounds of halogens with hydrogen. They are strong hydrohalic acids when dissolved in water, with the exception of HF. All of these acids are dangerous; some are widely used in chemical manufacturing plants.
    • Metal halides are compounds of halogens and metals. They include highly ionic compounds, monomeric covalent compounds, and polymeric covalent compounds. Metal halides can be obtained through direct combination or through neutralization of a basic metal salt with a hydrohalic acid.
    • Interhalogen compounds are formed when halogens react with each other. Some resemble the pure halogens in some respects, but mostly their properties and behaviors are intermediates of those of the two parent halogens. Some properties, however, are found in neither parent halogen.
    • Halogenated compounds, or organic halides, are organic compounds that contain halogen atoms. In the human body, some halogens perform multiple regulatory functions, while others are not essential. Organohalogens are synthesized through the nucleophilic abstraction reaction.
    • Compounds substituted with multiple halogens are known as polyhalogenated compounds. Many of them are very toxic and bioaccumulate in humans, but they have many potential applications.

Terms

  • polyhalogenated compoundA compound with multiple atoms of halogens.
  • interhalogenA binary compound of two different halogens.
  • halideA compound of a halogen and one or more elements.

Hydrogen Halides

The halogens all form binary compounds with hydrogen, and these compounds are known as the hydrogen halides: hydrogen fluoride (HF), hydrogen chloride (HCl), hydrogen bromide (HBr), hydrogen iodide (HI), and hydrogen astatide (HAt). All of these except HF are strong chemical acids when dissolved in water. However, hydrofluoric acid does have quite destructive properties towards animal tissue, including that of humans.

When in aqueous solution, the hydrogen halides are known as hydrohalic acids. The names of these acids are as follows:

  • hydrofluoric acid
  • hydrochloric acid
  • hydrobromic acid
  • hydroiodic acid

All of these acids are dangerous and must be handled with great care. Some of these acids are also widely used in chemical manufacturing plants. Hydrogen astatide should also be a strong acid (hydroastatic acid), but it is seldom included in presentations about hydrohalic acids because of the extreme radioactivity of astatine (via alpha decay) and the fact that it readily decomposes into its constituent elements (hydrogen and astatine).

Metal Halides

The halogens form many compounds with metals. These include highly ionic compounds such as sodium chloride, monomeric covalent compounds such as uranium hexafluoride, and polymeric covalent compounds such as palladium chloride. Metal halides are generally obtained through direct combination or, more commonly, through neutralization of a basic metal salt with a hydrohalic acid.

Silver ChlorideSilver chloride is the precipitate formed when silver nitrate solution is added to chloride solution.

Interhalogen Compounds

The halogens react with each other to form interhalogen compounds. Diatomic interhalogen compounds such as BrF, ICl, and ClF bear resemblance to the pure halogens in some respects. The properties and behavior of a diatomic interhalogen compound tend to be intermediates of those of its parent halogens. Some properties, however, are found in neither parent halogen. For example, Cl2 and I2 are soluble in CCl4, but ICl is not since it is a polar molecule (due to the electronegativity difference between I and Cl).

Organic Halides

Many synthetic organic compounds, such as plastic polymers, as well as a few natural organic compounds, contain halogen atoms; these are known as halogenated compounds, or organic halides. Chlorine is by far the most abundant of the halogens and is the only one needed (as chloride ions) in relatively large amounts by humans. For example, chloride ions play a key role in brain function by mediating the action of the inhibitory transmitter GABA. They are also used by the body to produce stomach acid. Iodine is needed in trace amounts for the production of thyroid hormones, such as thyroxine. On the other hand, neither fluorine nor bromine is believed to be essential for humans. Organohalogens are also synthesized through the nucleophilic abstraction reaction.

Polyhalogenated Compounds

Polyhalogenated compounds are industrially created compounds substituted with multiple halogens. Many of them are very toxic and bioaccumulate in humans, but they have many possible applications. Polyhalogenated compounds include the much publicized PCBs, PBDEs, and PFCs, as well as numerous other compounds.