Auxochrome
Encyclopedia
An auxochrome is a group of atoms attached to a chromophore
which modifies the ability of that chromophore to absorb light. Examples include the hydroxyl group (-OH), the amino group (-NH2), and an aldehyde
group (-CHO).
An auxochrome is a functional group
of atoms with nonbonded electrons which, when attached to a chromophore, alters both the wavelength
and intensity of absorption. If these groups are in direct conjugation with the pi-system of the chromophore, they may increase the wavelength at which the light is absorbed and as a result intensify the absorption. A feature of these auxochromes is the presence of at least one lone pair
of electrons which can be viewed as extending the conjugated system by resonance
.
It increases the color of any organic compound
. For example, benzene
does not display color as it does not have a chromophore; but nitrobenzene
is pale yellow color because of the presence of a nitro group (-NO2). Para-hydroxynitrobenzene exhibits a deep yellow color. Here an auxochrome (-OH) is conjugated with the chromophore -NO2. Similar behavior is seen in azobenzene
which has a red color, but para-hydroxy azobenzene is dark red in color.
There are mainly two types of auxochromes:
The presence of an auxochrome in the chromogen molecule is essential to make a dye
. However, if an auxochrome is present in the meta position to the chromophore, it does not affect the color.
An auxochrome is known as a compound that produces a red shift
because it increases the wavelength of absorption, therefore moving closer to infrared light. Woodward-Fieser rules estimate the shift in wavelength of maximum absorption for several auxochromes attached to a conjugated system
in an organic molecule.
An auxochrome helps a dye to bind to the object that is to be colored. Electrolytic dissociation of the auxochrome group helps in binding and it is due to this reason a basic substance takes an acidic dye.
The presence of certain groups which are not chromophores themselves, but deepen the colour of chromogens.such supporting groups are calld auxochromes.auxochromes may be acidic(phenolic)or basic.some important auxochromes are -OH,NH2,-NHR,NR2.
The presence of an auxochrome in the chromogen molecule is essential to make it a dye.however, if an auxochrome is present in the metaposition to the chromophore,it doesnot affect the colour.for example,in the compound p-Hydroxyazobenzene(a bright red eye).
Chromophore
A chromophore is the part of a molecule responsible for its color. The color arises when a molecule absorbs certain wavelengths of visible light and transmits or reflects others. The chromophore is a region in the molecule where the energy difference between two different molecular orbitals falls...
which modifies the ability of that chromophore to absorb light. Examples include the hydroxyl group (-OH), the amino group (-NH2), and an aldehyde
Aldehyde
An aldehyde is an organic compound containing a formyl group. This functional group, with the structure R-CHO, consists of a carbonyl center bonded to hydrogen and an R group....
group (-CHO).
An auxochrome is a functional group
Functional group
In organic chemistry, functional groups are specific groups of atoms within molecules that are responsible for the characteristic chemical reactions of those molecules. The same functional group will undergo the same or similar chemical reaction regardless of the size of the molecule it is a part of...
of atoms with nonbonded electrons which, when attached to a chromophore, alters both the wavelength
Wavelength
In physics, the wavelength of a sinusoidal wave is the spatial period of the wave—the distance over which the wave's shape repeats.It is usually determined by considering the distance between consecutive corresponding points of the same phase, such as crests, troughs, or zero crossings, and is a...
and intensity of absorption. If these groups are in direct conjugation with the pi-system of the chromophore, they may increase the wavelength at which the light is absorbed and as a result intensify the absorption. A feature of these auxochromes is the presence of at least one lone pair
Lone pair
In chemistry, a lone pair is a valence electron pair without bonding or sharing with other atoms. They are found in the outermost electron shell of an atom, so lone pairs are a subset of a molecule's valence electrons...
of electrons which can be viewed as extending the conjugated system by resonance
Resonance (chemistry)
In chemistry, resonance or mesomerism is a way of describing delocalized electrons within certain molecules or polyatomic ions where the bonding cannot be expressed by one single Lewis formula...
.
It increases the color of any organic compound
Organic compound
An organic compound is any member of a large class of gaseous, liquid, or solid chemical compounds whose molecules contain carbon. For historical reasons discussed below, a few types of carbon-containing compounds such as carbides, carbonates, simple oxides of carbon, and cyanides, as well as the...
. For example, benzene
Benzene
Benzene is an organic chemical compound. It is composed of 6 carbon atoms in a ring, with 1 hydrogen atom attached to each carbon atom, with the molecular formula C6H6....
does not display color as it does not have a chromophore; but nitrobenzene
Nitrobenzene
Nitrobenzene is an organic compound with the chemical formula C6H5NO2. It is a water-insoluble pale yellow oil with an almond-like odor. It freezes to give greenish-yellow crystals. It is produced on a large scale as a precursor to aniline. Although occasionally used as a flavoring or perfume...
is pale yellow color because of the presence of a nitro group (-NO2). Para-hydroxynitrobenzene exhibits a deep yellow color. Here an auxochrome (-OH) is conjugated with the chromophore -NO2. Similar behavior is seen in azobenzene
Azobenzene
Azobenzene is a chemical compound composed of two phenyl rings linked by a N=N double bond. It is the best known example of an azo compound. The term 'azobenzene' or simply 'azo' is often used to refer to a wide class of molecules that share the core azobenzene structure, with different chemical...
which has a red color, but para-hydroxy azobenzene is dark red in color.
There are mainly two types of auxochromes:
- Acidic -COOH, -OH, -SO3HSulfonateA sulfonate is a salt or ester of a sulfonic acid. It contains the functional group R-SO2O-.- Sulfonate salts:Anions with the general formula RSO2O− are called sulfonates. They are the conjugate bases of sulfonic acids with formula RSO2OH. As sulfonic acids tend to be strong acids, the...
- BasicBase (chemistry)For the term in genetics, see base A base in chemistry is a substance that can accept hydrogen ions or more generally, donate electron pairs. A soluble base is referred to as an alkali if it contains and releases hydroxide ions quantitatively...
-NHR, -NR2, -NH2
The presence of an auxochrome in the chromogen molecule is essential to make a dye
Dye
A dye is a colored substance that has an affinity to the substrate to which it is being applied. The dye is generally applied in an aqueous solution, and requires a mordant to improve the fastness of the dye on the fiber....
. However, if an auxochrome is present in the meta position to the chromophore, it does not affect the color.
An auxochrome is known as a compound that produces a red shift
Red shift
-Science:* Redshift, the increase of wavelength of detected electromagnetic radiation with respect to the original wavelength of the emission* Red shift, an informal term for a bathochromic shift...
because it increases the wavelength of absorption, therefore moving closer to infrared light. Woodward-Fieser rules estimate the shift in wavelength of maximum absorption for several auxochromes attached to a conjugated system
Conjugated system
In chemistry, a conjugated system is a system of connected p-orbitals with delocalized electrons in compounds with alternating single and multiple bonds, which in general may lower the overall energy of the molecule and increase stability. Lone pairs, radicals or carbenium ions may be part of the...
in an organic molecule.
An auxochrome helps a dye to bind to the object that is to be colored. Electrolytic dissociation of the auxochrome group helps in binding and it is due to this reason a basic substance takes an acidic dye.
The presence of certain groups which are not chromophores themselves, but deepen the colour of chromogens.such supporting groups are calld auxochromes.auxochromes may be acidic(phenolic)or basic.some important auxochromes are -OH,NH2,-NHR,NR2.
The presence of an auxochrome in the chromogen molecule is essential to make it a dye.however, if an auxochrome is present in the metaposition to the chromophore,it doesnot affect the colour.for example,in the compound p-Hydroxyazobenzene(a bright red eye).