halogens

Examples of halogens in the following topics:

• Alkyl Halide Occurrence

  • Halogen containing organic compounds are relatively rare in terrestrial plants and animals.
  • However, the halogen rich environment of the ocean has produced many interesting natural products incorporating large amounts of halogen.
  • Many subsequent chemical and biological processes produce poly-halogenated methanes.
  • Synthetic organic halogen compounds are readily available by direct halogenation of hydrocarbons and by addition reactions to alkenes and alkynes.
  • Some halogen compounds, shown in the box. have been used as pesticides, but their persistence in the environment, once applied, has led to restrictions, including banning, of their use in developed countries.

• Halogen Compounds

  • The halogens form many compounds with metals.
  • The halogens react with each other to form interhalogen compounds.
  • Some properties, however, are found in neither parent halogen.
  • 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.
  • Polyhalogenated compounds are industrially created compounds substituted with multiple halogens.

• Reactions of Alkanes

  • Alkanes are generally unreactive, but can participate in oxidation, halogenation, and cracking reactions.
  • With the addition of a halogen gas and energy, alkanes can be halogenated with the reactivity of the halogens proceeding in the following order: Cl2>Br2>I2.
  • In this reaction, UV light or heat initiates a chain reaction, cleaving the covalent bond between the two atoms of a diatomic halogen.
  • The halogen radicals can then abstract protons from the alkanes, which can then combine or react to form more radicals.
  • Alkanes can be halogenated at a number of sites, and this reaction typically yields a mixture of halogenated products.

• Alkyl Halide Reactions

  • The functional group of alkyl halides is a carbon-halogen bond, the common halogens being fluorine, chlorine, bromine and iodine.
  • With the exception of iodine, these halogens have electronegativities significantly greater than carbon.
  • Consequently, this functional group is polarized so that the carbon is electrophilic and the halogen is nucleophilic, as shown in the drawing below.
  • The strongest of the carbon-halogen covalent bonds is that to fluorine.
  • The carbon-chlorine covalent bond is slightly weaker than a carbon-carbon bond, and the bonds to the other halogens are weaker still, the bond to iodine being about 33% weaker.

• The Halogens (Group 17)

  • The halogens are a series of highly reactive, nonmetal elements from Group 17 of the periodic table.
  • The halogens are nonmetallic elements in Group 17 of the periodic table.
  • Ununseptium, which is not a naturally occurring element, is also often considered a halogen.
  • The halogens, as a group, are extremely reactive.
  • The halogens can also react with each other to form interhalogen compounds.

• Properties of the Halogens

  • The halogens are a series of non-metal elements from group 17 of the periodic table (formerly VII).
  • The halogens include fluorine (F), chlorine (Cl), bromine (Br), iodine (I), and astatine (At).
  • The artificially created element 117 (ununseptium) may also be considered a halogen.
  • In hydrogen halides (HX, where X is the halogen), the H-X bond gets longer as the halogen atoms get larger.
  • Halogens can gain an electron by reacting with atoms of other elements.

• Halogenation

  • Halogenation is the replacement of one or more hydrogen atoms in an organic compound by a halogen (fluorine, chlorine, bromine or iodine).
  • Halogenation reactions may be conducted in either the gaseous or liquid phase.
  • When alkanes larger than ethane are halogenated, isomeric products are formed.
  • The halogenation of propane discloses an interesting feature of these reactions.
  • If all these hydrogen atoms were equally reactive, halogenation should give a 3:1 ratio of 1-halopropane to 2-halopropane mono-halogenated products, reflecting the primary/secondary numbers.

• Reactions at the α-Carbon

  • This difference may be used to facilitate the alpha-halogenation of carboxylic acids.
  • The final product is the alpha-halogenated acid, accompanied by a trace of the acyl halide.
  • This halogenation procedure is called the Hell-Volhardt-Zelinski reaction.
  • In a similar fashion, acetic anhydride serves as a halogenation catalyst for acetic acid (first equation below).
  • Acetic anhydride serving as a halogenation catalyst for acetic acid (i) and substitution with malonic acid compound (ii)

• Addition Reactions Initiated by Electrophilic Halogen

  • The halogens chlorine and bromine add rapidly to a wide variety of alkenes without inducing the kinds of structural rearrangements noted for strong acids (first example below).
  • We can account both for the high stereoselectivity and the lack of rearrangement in these reactions by proposing a stabilizing interaction between the developing carbocation center and the electron rich halogen atom on the adjacent carbon.
  • The stabilization provided by this halogen-carbocation bonding makes rearrangement unlikely, and in a few cases three-membered cyclic halonium cations have been isolated and identified as true intermediates.

• Halogen Uses

  • Although halogens and their compounds can be toxic, some are essential for the human body's functioning and are used in everyday products.
  • In drug discovery, the incorporation of halogen atoms into a lead drug candidate results in analogues that are usually more lipophilic and less water-soluble.
  • Therefore, halogen atoms are used to improve penetration through lipid membranes and tissues.
  • It follows that there is a tendency for some halogenated drugs to accumulate in adipose tissue.
  • Polyhalogenated compounds (PHCs) are of particular interest and importance because halogens are generally highly reactive and bioaccumulate in humans.