Disproportionaatio

Disproportionaatio on kemiallisen hapetus-pelkistysreaktion erikoislaji, jossa saman alkuaineen ennestään samalla hapetusasteella olleista atomeista osa hapettuu, osa pelkistyy.

Esimerkkejä

Disproportionaatio esiintyy muun muassa joidenkin siirtymämetallien ioneilla, kun saman alkuaine voi esiintyä yhdisteissään eri hapetus­asteilla. Esimerkiksi kupari voi yhdisteissään esiintyä joko Cu+ tai Cu2+ -ioneina, mutta näistä vain Cu2+ -ionit ovat pysyviä vesi­liuoksissa. Jos jotakin Cu+-ioneja sisältävää suolaa liuotetaan veteen, osa ioneista hapettuu Cu2+ -ioneiksi ja osa pelkistyy metalliseksi kupariksi:

  • 2 Cu+ Cu2+ + Cu[1]

Tämä reaktio tapahtuu, koska reaktion Cu+ + e- Cu(s) elektrodipotentiaali on +0,52 V mutta reaktion Cu2 + e- Cu+ vain +0,159 V, joten Cu+ on voimakkaampi hapetin kuin Cu2+.[1]

Tällainen reaktio tapahtuu muun muassa, kun kupari(I)oksidia (CuO) lisätään rikkihappoliuokseen. Kokonais­reaktio on tällöin

  • Cu2O Cu(s) + CuO

tai ionireaktioksi kirjoitettuna

  • Cu2O (S) + 2H+ Cu(s) + Cu2+ + H2O.

Disproportionaatio voi esiintyä myös ko­va­lentti­silla yhdisteillä. Esimerkiksi dityppitetraoksidissa (N2O4) typpi on hapetus­asteella +IV, mutta tämän yhdisteen liuetessa veteen syntyy sekä typpihappoa, jossa typen hapetu­saste on +V, että typpihapoketta, jossa sen hapetu­saste on +III:

  • N2O4 + 2 H2O HNO2 + HNO3[2]

Vetyperoksidi (H2O2) on epävakaa yhdiste, joka hajoaa herkästi dis­pro­portinoi­tu­malla vedeksi ja vapaaksi hapeksi:

2 H2O2 2 H2O + O2.[3]

Myös orgaanisessa kemiassa esiintyy dis­pro­portio­naatio­reaktioita. Esimerkiksi Cannizzaro-reaktiossa väkevä kaliumhydroksidi voi saada bentsaldehydin dis­pro­portinoi­tu­maan niin, että osa siitä hapettuu bentsoe­hapoksi, osa pelkistyy bentsyylialkoholiksi:

  • 2 C6H5COH C6H5COOH + C6H5CH2OH[4]

Lähteet

  1. ↑ a b Antti Kivinen, Osmo Mäkitie: Kemia, s. 261. Otava, 1988. ISBN 951-1-10136-6
  2. ↑ Kivinen, Mäkitie, s. 351
  3. ↑ Kivinen, Mäkitie, s. 366
  4. ↑ Pentti Mälkönen: Orgaaninen kemia, s. 110. Otava, 1979. ISBN 951-1-05378-7

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