Hydroperoxides or peroxols are compounds containing the hydroperoxide functional group (ROOH). If the 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 unsaturated chemical bonds.
The O−O bond length in peroxides is about 1.45 Å, and the R−O−O angles (R = H, C) are about 110° (water-like). Characteristically, the C−O−O−H dihedral angles are about 120°. The O−O bond is relatively weak, with a bond dissociation energy of 45–50 kcal/mol (190–210 kJ/mol), less than half the strengths of C−C, C−H, and C−O bonds.
Use and reactions
- 4 ROOH + LiAlH4 → LiAlO2 + 2 H2O + 4 ROH
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:
- ROOH + PR3 → OPR3 + ROH
- C6H12 + O2 → (CH2)5CO + H2O
Many epoxides are prepared using hydroperoxides as reagents, such as the Halcon process for the production of propylene oxide. The Sharpless epoxidation is a related reaction conducted on laboratory scale. tert-Butyl hydroperoxide (TBHP) is an organic-soluble oxidant employed in these operations.
Drying oils, as found in many paints and varnishes, function via the formation of hydroperoxides.
Auto-oxidation reaction is 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:
- C2H5OCH=CH2 + H2O2 → C2H5OCH(OOH)CH3
Naturally occurring hydroperoxides
Many hydroperoxides are derived from fatty acids, steroids, and terpenes. The biosynthesis of these species is effected extensively by enzymes.
Compounds with allylic and benzylic C−H bonds are especially susceptible to oxygenation. Such reactivity is exploited industrially on a very 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 O2, to give hydroperoxyl (ROO.), which then continues the cycle of H-atom abstraction.
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