Hydroperoxide

File:FunktionelleGruppen Hydroperoxide.svg

The general structure of an organic hydroperoxide with the blue marked functional group, where R stands for any group, typically organic

Hydroperoxides or peroxols are compounds of the form ROOH, where R stands for any group, typically organic, which contain the hydroperoxy functional group (Template:Chem2). Hydroperoxide also refers to the hydroperoxide anion (Template:Chem2) and its salts, and the neutral hydroperoxyl radical (•OOH) consist of an unbond hydroperoxy group. When R is organic, the compounds are called organic hydroperoxides. Such compounds are a subset of organic peroxides, which have the formula ROOR. Organic hydroperoxides can either intentionally or unintentionally initiate explosive polymerisation in materials with saturated chemical bonds.

Properties

The Template:Chem2 bond length in peroxides is about 1.45 Å, and the Template:Chem2 angles (R = H, C) are about 110° (water-like). Characteristically, the Template:Chem2 dihedral angles are about 120°. The Template:Chem2 bond is relatively weak, with a bond dissociation energy of , less than half the strengths of Template:Chem2, Template:Chem2, and Template:Chem2 bonds.

Hydroperoxides are typically more volatile than the corresponding alcohols:

• tert-BuOOH (b.p. 36°C) vs tert-BuOH (b.p. 82-83°C)
• [[Methyl hydroperoxide|Template:Chem2]] (b.p. 46°C) vs [[Methanol|Template:Chem2]] (b.p. 65°C)
• cumene hydroperoxide (b.p. 153°C) vs cumyl alcohol (b.p. 202°C)

Miscellaneous reactions

Hydroperoxides are mildly acidic. The range is indicated by 11.5 for [[Methyl hydroperoxide|Template:Chem2]] to 13.1 for Template:Chem2.

Hydroperoxides can be reduced to alcohols with lithium aluminium hydride, as described in this idealized equation:

Template:Chem2

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 alkoxides such as sodium ethoxide. The phosphite esters and tertiary phosphines also effect reduction:

Template:Chem2

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 epoxides are prepared using hydroperoxides as reagents in the Sharpless epoxidation.

The Sharpless epoxidation

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:

Template:Chem2

Drying oils, as found in many paints and varnishes, function via the formation of hydroperoxides.

Hock processes

File:Hock-Phenol.png

Synthesis of cumene hydroperoxide

Compounds with allylic and benzylic 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 Template:Chem2, to give hydroperoxyl (ROO•), which then continues the cycle of H-atom abstraction.

File:Schenk-En-Reaktion.png

Synthesis of hydroperoxides of alkene and singlet oxygen in an Schenck ene reaction

Formation

By autoxidation

The most important (in a commercial sense) peroxides are produced by autoxidation, the direct reaction of Template:Chem2 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:

Template:Chem2

File:Tetrahydrofuran peroxide formation.svg

Auto-oxidation reaction is also observed with common ethers, 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:

Template:Chem2

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:

Template:Chem2

In some cases, these hydroperoxides convert to give cyclic diperoxides:

Template:Chem2

Addition of this initial adduct to a second equivalent of the carbonyl:

Template:Chem2

Further replacement of alcohol groups:

Template:Chem2

Triphenylmethanol reacts with hydrogen peroxide gives the unusually stable hydroperoxide, Template:Chem2.

Naturally occurring hydroperoxides

Many hydroperoxides are derived from fatty acids, steroids, and terpenes. The biosynthesis of these species is affected extensively by enzymes.

File:LyaseNonenalHemiAc.png

Illustrative biosynthetic transformation involving a hydroperoxide. Here cis-3-hexenal is generated by conversion of linolenic acid to the hydroperoxide by the action of a lipoxygenase followed by the lyase-induced formation of the hemiacetal.

Inorganic hydroperoxides

File:Karen'sOOH.svg

Structure of a square planar palladium hydroperoxide complex

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 Template:Chem2. It acts by hydrolysis to give a boron-hydroperoxide:

Template:Chem2

This hydrogen peroxide then releases hydrogen peroxide:

Template:Chem2

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. Some form by the reaction of metal hydrides with oxygen gas:

Template:Chem2 (Template:Chem2 refers to other ligands bound to the metal)

Some transition metal dioxygen complexes abstract H atoms (and sometimes protons) to give hydroperoxides:

Template:Chem2