
In
chemistry
Chemistry is the scientific study of the properties and behavior of matter. It is a physical science within the natural sciences that studies the chemical elements that make up matter and chemical compound, compounds made of atoms, molecules a ...
, azoxy compounds are a group of
organic compound
Some chemical authorities define an organic compound as a chemical compound that contains a carbon–hydrogen or carbon–carbon bond; others consider an organic compound to be any chemical compound that contains carbon. For example, carbon-co ...
s sharing a common
functional group
In organic chemistry, a functional group is any substituent or moiety (chemistry), moiety in a molecule that causes the molecule's characteristic chemical reactions. The same functional group will undergo the same or similar chemical reactions r ...
with the general structure . They are considered
N-oxides of
azo compound
Azo compounds are organic compounds bearing the functional group diazenyl (, in which R and R′ can be either aryl or alkyl groups).
IUPAC defines azo compounds as: "Derivatives of diazene (diimide), , wherein both hydrogens are substituted ...
s. Azoxy compounds are
1,3-dipoles and
cycloadd to double bonds. Most azoxy-containing compounds have
aryl
In organic chemistry, an aryl is any functional group or substituent derived from an aromatic ring, usually an aromatic hydrocarbon, such as phenyl and naphthyl. "Aryl" is used for the sake of abbreviation or generalization, and "Ar" is used ...
substituents.
Preparation
Azoxybenzene and its derivatives are typically prepared by
reduction of
nitrocompounds, such as the reduction of
nitrobenzene
Nitrobenzene is an aromatic nitro compound and the simplest of the nitrobenzenes, with the chemical formula C6H5 NO2. It is a water-insoluble pale yellow oil with an almond-like odor. It freezes to give greenish-yellow crystals. It is produced ...
with arsenous oxide. Such reactions are proposed to proceed via the intermediacy of the
nitroso compounds and
hydroxylamines
Hydroxylamine (also known as hydroxyammonia) is an inorganic compound with the chemical formula . The compound exists as hygroscopic colorless crystals.Greenwood and Earnshaw. ''Chemistry of the Elements.'' 2nd Edition. Reed Educational and Prof ...
, e.g.
phenylhydroxylamine and
nitrosobenzene (Ph =
phenyl
In organic chemistry, the phenyl group, or phenyl ring, is a cyclic group of atoms with the formula , and is often represented by the symbol Ph (archaically φ) or Ø. The phenyl group is closely related to benzene and can be viewed as a benzene ...
, ):
PhNHOH + PhNO -> PhN(O)NPh + H2O
Nitrosocarbamate
ester
In chemistry, an ester is a compound derived from an acid (either organic or inorganic) in which the hydrogen atom (H) of at least one acidic hydroxyl group () of that acid is replaced by an organyl group (R). These compounds contain a distin ...
s decarboxylate in strong base to an azotate susceptible to strong alkylation agents:
:–N(H)CO
2R + 2
NO2 → –N(N=O)CO
2R + HNO
3
:–N(N=O)CO
2R + K
OR → –N=NO
−K
+ + CO
2 + R
2O
:–N=NO
−K
+ +
R3O+BF → –N(N=O)R + R
2O + KBF
4
An alternative route involves oxidation of
azobenzene
Azobenzene is a photoswitchable chemical compound composed of two phenyl rings linked by a azo compound, N=N double bond. It is the simplest example of an aryl azo compound. The term 'azobenzene' or simply 'azo' is often used to refer to a wide c ...
s with
peroxy acid
A peroxy acid (often spelled as one word, peroxyacid, and sometimes called peracid) is an acid which contains an acidic group. The two main classes are those derived from conventional mineral acids, especially sulfuric acid, and the peroxy deri ...
s.
Structure

Azoxybenzene compounds are more stable as their
trans isomer
Trans- is a Latin prefix meaning "across", "beyond", or "on the other side of".
Used alone, trans may refer to:
Sociology
* Trans, a sociological term which may refer to:
** Transgender, people who identify themselves with a gender that dif ...
isomer. In Ph
2N
2O, the N-N and N-O bond lengths are is 1.24 and 1.255 Å respectively, corresponding to some
double bonds character. The CNNC and CNNO are near 176°.
''trans''-
Azoxydibenzene's
resonance form with a negative
formal charge
In chemistry, a formal charge (F.C. or ), in the covalent view of chemical bonding, is the hypothetical charge assigned to an atom in a molecule, assuming that electrons in all chemical bonds are shared equally between atoms, regardless of rela ...
on oxygen (–N=N
+(O
−)–) corresponds to a theoretical 6
D dipole moment. However, the observed moment is only 4.7 D, suggesting a substantial resonance contribution in which the other nitrogen bears negative charge (–N
−–N
+(=O)–).
Reactions
Under
ultraviolet light
Ultraviolet radiation, also known as simply UV, is electromagnetic radiation of wavelengths of 10–400 nanometers, shorter than that of visible light, but longer than X-rays. UV radiation is present in sunlight and constitutes about 10% of th ...
transazobenzene compounds isomerize to their cis isomers, analogous to azobenzene. Similar reaction conditions can instead cause isomery to an ''ortho''-azophenol, or migration of the oxygen atom across the two nitrogens.
Unlike
azo compound
Azo compounds are organic compounds bearing the functional group diazenyl (, in which R and R′ can be either aryl or alkyl groups).
IUPAC defines azo compounds as: "Derivatives of diazene (diimide), , wherein both hydrogens are substituted ...
s, azoxy compounds do not fragment thermally to lose
nitrous oxide
Nitrous oxide (dinitrogen oxide or dinitrogen monoxide), commonly known as laughing gas, nitrous, or factitious air, among others, is a chemical compound, an Nitrogen oxide, oxide of nitrogen with the Chemical formula, formula . At room te ...
; the process is believed
forbidden by orbital symmetries. Correspondingly, the reaction is possible under UV light with wavelength approximately 220 nm.
The azoxy group is
electron-withdrawing, but in non-oxidizing media, aliphatic
α hydrogens must be situated between two azoxy groups to appreciably dissociate.
Alkyllithia replace the hydrogens, but with reduction:
:–N
+(O
−)=NC(H)< + 2LiR → –N=NC(R)< + Li
2O↓ + RH
Basic
oxidant
An oxidizing agent (also known as an oxidant, oxidizer, electron recipient, or electron acceptor) is a substance in a redox chemical reaction that gains or "Electron acceptor, accepts"/"receives" an electron from a (called the , , or ''electr ...
s abstract α hydrogens in a
free-radical process, leading to dimerization.
Azoxyarenes ''ortho'' to a benzylic carbon with good
leaving group
In organic chemistry, a leaving group typically means a Chemical species, molecular fragment that departs with an electron, electron pair during a reaction step with heterolysis (chemistry), heterolytic bond cleavage. In this usage, a ''leaving gr ...
eliminate the leaving group to give an indazolone (the
Davis-Beirut reaction).
Azoxy compounds are weak bases, and unstable to strong acids. Azoxyarenes undergo the
Wallach rearrangement to ''para''-azophenols; primary and secondary azoxyaliphates
tautomerize to a
hydrazamide.
The two aryl groups in azoxyarenes undergo
electrophilic aromatic substitution
Electrophilic aromatic substitution (SEAr) is an organic reaction in which an atom that is attached to an aromatic ring, aromatic system (usually hydrogen) is replaced by an electrophile. Some of the most important electrophilic aromatic substitut ...
differently. In the case of azoxybenzene, PhNN(O)– reacts at the ''meta'' position, while PhN(O)N– reacts at the ''ortho'' and ''para'' positions.
Electrochemical reduction converts azoxyarenes to
azo compounds
Azo compounds are organic compounds bearing the functional group diazenyl (, in which R and R′ can be either aryl or alkyl groups).
IUPAC defines azo compounds as: "Derivatives of diazene (diimide), , wherein both hydrogens are substituted ...
.
Single-electron reductants give a deep-blue radical anion, which dimerizes in
aqueous solution
An aqueous solution is a solution in which the solvent is water. It is mostly shown in chemical equations by appending (aq) to the relevant chemical formula. For example, a solution of table salt, also known as sodium chloride (NaCl), in water ...
to the corresponding azo compound and
hydrogen peroxide
Hydrogen peroxide is a chemical compound with the formula . In its pure form, it is a very pale blue liquid that is slightly more viscosity, viscous than Properties of water, water. It is used as an oxidizer, bleaching agent, and antiseptic, usua ...
. Strong reducing agents such as
lithium aluminum hydride also
hydrogenolyze electronegative ''ortho'' or ''para'' arene substituents, but
catalytic hydrogenation selects out the azoxy link. Conversely,
sodium borohydride
Sodium borohydride, also known as sodium tetrahydridoborate and sodium tetrahydroborate, is an inorganic compound with the formula (sometimes written as ). It is a white crystalline solid, usually encountered as an aqueous basic solution. Sodi ...
preserves the azoxy group even as it reduces arene substituents.
Safety
Alkyl azoxy compounds, e.g.
azoxymethane are suspected to be genotoxic.
[Guideline On The Limits Of Genotoxic Impurities ]
See also
*
azobenzene dioxide
References
{{Authority control
Functional groups
Azo compounds
Azoxy compounds