Onium compound

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In chemistry, an onium ion, is a cation formally obtained by the protonation of mononuclear parent hydride of a pnictogen (group 15 of the periodic table), chalcogen (group 16), or halogen (group 17). The oldest-known onium ion, and the namesake for the class, is ammonium, NH+
4
, the protonated derivative of ammonia, NH
3
.[1][2]

The name onium is also used for cations that would result from the substitution of hydrogen atoms in those ions by other groups, such as organic radicals, or halogens; such as tetraphenylphosphonium, (C
6
H
5
)
4
P+
. The substituent groups may be divalent or trivalent, yielding ions such as iminium and nitrilium.[1][2]

A simple onium ion has a charge of +1. A larger ion that has two onium ion subgroups is called a double onium ion, and has a charge of +2. A triple onium ion has a charge of +3, and so on.

Compounds of an onium cation and some other negative ion are known as onium compounds or onium salts.

Onium ions and onium compounds are inversely analogous to -ate ions and ate complexes:

  • Lewis bases form onium ions when the central atom gains one more bond and becomes a positive cation.
  • Lewis acids form -ate ions when the central atom gains one more bond and becomes a negative anion.[3]

Simple onium cations (hydrides with no substitutions)[edit]

Group 15 (pnictogen) onium cations[edit]

Group 16 (chalcogen) onium cations[edit]

Group 17 (halogen) onium cations, halonium ions, H2X+ (protonated hydrogen halides)[edit]

Pseudohalogen onium cations[edit]

Group 14 (carbon group) onium cations[edit]

All are carbonium ions.

Group 13 (boron group) onium cations[edit]

Group 18 (noble gas) onium cations[edit]

Hydrogen onium cation[edit]

  • hydrogenonium, better known as trihydrogen cation, H+
    3
    (protonated [molecular] or [diatomic] hydrogen), found in ionized hydrogen and interstellar space

Onium cations with monovalent substitutions[edit]

Onium cations with polyvalent substitutions[edit]

  • secondary ammonium cations having one double-bonded substitution, R=NH2+
  • quaternary ammonium cations having one double-bonded substitution and two single-bonded substitutions, R=NR2+
  • quaternary ammonium cations having two double-bonded substitutions, R=N=R+
  • tertiary ammonium cations having one triple-bonded substitution, R≡NH+
  • tertiary ammonium cations having two partially double-bonded substitutions, RNH+R
  • quaternary ammonium cations having one triple-bonded substitution and one single-bonded substitution, R≡NR+
  • tertiary oxonium cations having one triple-bonded substitution, R≡O+
  • tertiary oxonium cations having two partially double-bonded substitutions, RO+R
  • tertiary sulfonium cations having one triple-bonded substitution, R≡S+

Double onium dications[edit]

Enium cations[edit]

The extra bond is added to a less-common parent hydride, a carbene analog, typically named -ene or -ylene, which is neutral with 2 fewer bonds than the more-common hydride, typically named -ane or -ine.

External links[edit]

References[edit]

  1. ^ a b IUPAC definition
  2. ^ a b George A. Olah (1998), Onium Ions. John Wiley & Sons, 509 pages. ISBN 9780471148777
  3. ^ Advanced Organic Chemistry: Reactions and mechanisms, Maya Shankar Singh, 2007, Dorling Kindersley, ISBN 978-81-317-1107-1