Protonolysis is the cleavage of a
chemical bond
A chemical bond is the association of atoms or ions to form molecules, crystals, and other structures. The bond may result from the electrostatic force between oppositely charged ions as in ionic bonds or through the sharing of electrons a ...
by acids. Many examples are found in
organometallic chemistry
Organometallic chemistry is the study of organometallic compounds, chemical compounds containing at least one chemical bond between a carbon atom of an organic molecule and a metal, including alkali, alkaline earth, and transition metals, and so ...
since the reaction requires polar M
δ+-R
δ- bonds, where δ+ and δ- signify partial positive and negative charges associated with the bonding atoms. When compounds containing these bonds are treated with acid (HX), these bonds cleave:
:M-R + HX → M-X + H-R
Hydrolysis (X
− = OH
−) is a special case of protonolysis. Compounds susceptible to hydrolysis often undergo protonolysis.
Hydrides
The
borohydride anion is susceptible to reaction with even weak acids, resulting protonolysis of one or more B-H bonds. Protonolysis of
sodium borohydride with
acetic acid
Acetic acid , systematically named ethanoic acid , is an acidic, colourless liquid and organic compound with the chemical formula (also written as , , or ). Vinegar is at least 4% acetic acid by volume, making acetic acid the main compone ...
gives
triacetoxyborohydride:
:NaBH
4 + 3 HO
2CCH
3 → NaBH(O
2CCH
3)
3 + 3 H
2
Related reactions occur for hydrides of other electropositive elements, e.g.
lithium aluminium hydride.
Alkyls
The alkyl derivatives of many metals undergo protonolysis. For the alkyls of very electropositive metals (zinc, magnesium, and lithium), water is sufficiently acidic, in which case the reaction is called hydrolysis. Protonolysis with
mineral acids is sometimes used to remove organic ligands from a metal center.
Nitrides, phosphides, silicides and related species
Inorganic materials with highly charged anions are often susceptible to protonolysis. Derivatives of nitride (N
3−), phosphides (P
3−), and silicides (Si
4−) hydrolyze to give
ammonia
Ammonia is an inorganic chemical compound of nitrogen and hydrogen with the chemical formula, formula . A Binary compounds of hydrogen, stable binary hydride and the simplest pnictogen hydride, ammonia is a colourless gas with a distinctive pu ...
,
phosphine
Phosphine (IUPAC name: phosphane) is a colorless, flammable, highly toxic compound with the chemical formula , classed as a pnictogen hydride. Pure phosphine is odorless, but technical grade samples have a highly unpleasant odor like rotting ...
, and
silane. Analogous reactions occur with molecular compounds with M-NR
2, M-PR
2, and M-SiR
3 bonds.
[Greenwood, N. N.; & Earnshaw, A. (1997). Chemistry of the Elements (2nd Edn.), Oxford:Butterworth-Heinemann. .]
References
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Chemical reactions