Hydroperoxides or peroxols are compounds of the form ROOH, where R stands for any group, typically Organic compound, which contain the hydroperoxy functional group (). Hydroperoxide also refers to the hydroperoxide anion () and its salts, and the neutral hydroperoxyl (•OOH) consist of an unbond hydroperoxy group. When R is organic, the compounds are called organic hydroperoxides. Such compounds are a subset of , which have the formula ROOR. Organic hydroperoxides can either intentionally or unintentionally initiate explosive polymerisation in materials with saturated bond.
Properties
The
bond length in peroxides is about 1.45 Å, and the angles (R =
Hydrogen,
Carbon) are about 110° (
water-like). Characteristically, the
are about 120°. The bond is relatively weak, with a bond dissociation energy of , less than half the strengths of , , and bonds.
Hydroperoxides are typically more volatile than the corresponding alcohols:
Miscellaneous reactions
Hydroperoxides are mildly
acidic. The range is indicated by 11.5 for to 13.1 for .
Hydroperoxides can be redox to alcohols with lithium aluminium hydride, as described in this idealized equation:
This reaction is the basis of methods for analysis of organic peroxides.
Another way to evaluate the content of peracids and peroxides is the volumetric titration with
such as
sodium ethoxide.
The
and tertiary phosphines also effect reduction:
Uses
Precursors to epoxides
"The single most important synthetic application of alkyl hydroperoxides is without doubt the metal-catalysed epoxidation of alkenes." In the
Halcon process tert-butyl hydroperoxide (TBHP) is employed for the production of
propylene oxide.
Of specialized interest, chiral are prepared using hydroperoxides as reagents in the Sharpless epoxidation.
Production of cyclohexanone and caprolactone
Hydroperoxides are intermediates in the production of many organic compounds in industry. For example, the cobalt catalyzed oxidation of cyclohexane to
cyclohexanone:
, as found in many paints and varnishes, function via the formation of hydroperoxides.
Hock processes
Compounds with
and
C−H bonds are especially susceptible to oxygenation.
[.] Such reactivity is exploited industrially on a large scale for the production of
phenol by the
Cumene process or Hock process for its
cumene and cumene hydroperoxide intermediates.
Such reactions rely on radical initiators that reacts with oxygen to form an intermediate that abstracts a hydrogen atom from a weak C-H bond. The resulting radical binds , to give hydroperoxyl (ROO•), which then continues the cycle of H-atom abstraction.
Formation
By autoxidation
The most important (in a commercial sense) peroxides are produced by
autoxidation, the direct reaction of with a hydrocarbon. Autoxidation is a radical reaction that begins with the abstraction of an H atom from a relatively weak
C-H bond. Important compounds made in this way include tert-butyl hydroperoxide, cumene hydroperoxide and ethylbenzene hydroperoxide:
Auto-oxidation reaction is also observed with common
, such as
diethyl ether, diisopropyl ether,
tetrahydrofuran, and 1,4-dioxane. An illustrative product is diethyl ether peroxide. Such compounds can result in a serious explosion when distilled.
[
To minimize this problem, commercial samples of THF are often inhibited with butylated hydroxytoluene (BHT). Distillation of THF to dryness is avoided because the explosive peroxides concentrate in the residue.
]
Although ether hydroperoxide often form adventitiously (i.e. autoxidation), they can be prepared in high yield by the acid-catalyzed addition of hydrogen peroxide to vinyl ethers:
From hydrogen peroxide
Many industrial peroxides are produced using hydrogen peroxide. Reactions with aldehydes and ketones yield a series of compounds depending on conditions. Specific reactions include addition of hydrogen peroxide across the C=O double bond:
In some cases, these hydroperoxides convert to give cyclic diperoxides:
Addition of this initial adduct to a second equivalent of the carbonyl:
Further replacement of alcohol groups:
Triphenylmethanol reacts with hydrogen peroxide gives the unusually stable hydroperoxide, .[Bryant E. Rossiter and Michael O. Frederick "Triphenylmethyl Hydroperoxide" E-EROS Encyclopedia of Reagents for Organic Synthesis, 2013. ]
Naturally occurring hydroperoxides
Many hydroperoxides are derived from fatty acids, steroids, and Terpene. The biosynthesis of these species is affected extensively by enzymes.
Inorganic hydroperoxides
Although hydroperoxide often refers to a class of organic compounds, many inorganic or metallo-organic compounds are hydroperoxides. One example involves sodium perborate, a commercially important bleaching agent with the formula . It acts by hydrolysis to give a boron-hydroperoxide:[Alexander McKillop and William R Sanderson (1995): "Sodium perborate and Sodium Percarbonate: Cheap, safe and versatile oxidising agents for organic synthesis". Tetrahedron, volume 51, issue 22, pages 6145-6166. ]
This hydrogen peroxide then releases hydrogen peroxide:
Several metal hydroperoxide complexes have been characterized by X-ray crystallography, for example: triphenylsilicon and triphenylgermanium hydroperoxides can be obtained by reaction of initial chlorides with excess of hydrogen peroxide in presence of base.
- ( refers to other ligands bound to the metal)
Some transition metal dioxygen complexes abstract H atoms (and sometimes protons) to give hydroperoxides:
