Synthesis of Azo Dyes

One of the first examples of micro reactors used for manufacturing was reported by Cellular Process Chemistry (CPC) [97] and involved a three-step process for the synthesis of a diazo pigment (a) diazotization, (b) coupling, and (c) pigmentation. 

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Uses. (9-Nitrochlorobenzene is used in the synthesis of azo dye intermediates such as o-chloroaniline (Fast YeUow G Base), i9-nitroani1ine (Fast Orange GR Base), o-anisidine (Fast Red BB Base), o-phenetidine, and (9-aminophenol (see Azo dyes). It also is used in corrosion inhibitors, pigments, and agriculture chemicals. -Nitrochlorobenzene is used principally in the production of intermediates for azo and sulfur dyes. Other uses include pharmaceuticals (qv), photochemicals, mbber chemicals (qv), and insecticides (see Insectcontroltechnology). Typical intermediates manufactured from the para isomer are -lutioaruline (Fast Red GC Base), anisidine, -aminophenol, -nitrophenol, -phenylenediamine, 2-chloro-/)-anisidine (Fast Red R Base), 2,4-dinitrochlorobenzene, and l,2-dichloro-4-nitrobenzene. 

C-coupling is of outstanding importance in the azo coupling reaction for the synthesis of azo dyes and pigments. An aromatic or heteroaromatic diazonium ion reacts with the so-called coupling component, which can be an aromatic primary, secondary, or tertiary amine, a phenol, an enol of an open-chain, aromatic, or heteroaromatic carbonyl compound, or an activated methylene compound. These reactions at an sp2-hybridized carbon atom will be discussed in Chapter 12. In the... 

It is worth noting, however, that up until fairly recently the reduction of aromatic nitrocompounds to the corresponding amines was commonly performed on an industrial scale, e.g. in the synthesis of azo dyes, with a mixture of iron and hydrochloric acid. This so-called Bechamp reduction has an E factor of ca. 15 compared to 1 for catalytic hydrogenation. 

The synthesis of carbonyl colorants uses a wide diversity of chemical methods, in which each individual product essentially has its own characteristic route. This is in complete contrast to the synthesis of azo dyes and pigments (Chapter 3) where a common reaction sequence is universally used. The subject is too vast to attempt to be comprehensive in a text of this type. The following section, therefore, presents an overview of some of the fundamental synthetic strategies which may be used to prepare some of the more important types of carbonyl colorants. 

Naphthylamine-5,7-disulphonic acid and 2-naphthylamine-l-sulphonic acid, which are intermediate products in Scheme 4.28, as well as 2-naphthylamine-l,5-disulphonic acid (obtained by careful low-temperature sulphonation of Tobias acid), are all used in the synthesis of azo dyes. 

Due to the great variability of different types of cationic azo dyes chemical constitution and synthesis are outlined together in this section. The general synthesis of azo dyes is described in Section 2.1. 

Azo dyes are by far the largest family of organic dyes. They play a prominent role in acid, direct, reactive, azoic, and disperse dye structures, as shown previously, and include structures that cover the full color spectrum. Generally, the synthesis of azo dyes involves a process known as diazo coupling. In this... 

The synthesis of azo dyes can be illustrated using the following letter designations ... 

A common use of diazonium salts is in the synthesis of azo dyes. In dyestuffs chemistry, the amine that is diazotized is referred to as the diazonium component and the compound it reacts with is known as the coupling component. The whole reaction is known as azo coupling. About 50yo of all commercial dyestuffs, of which two examples are shown in 19 and 20, contain the azo group. 

In the same year Caro [27] produced methylene blue, basing his process on the sulphuretted hydrogen reaction observed by Lauth in 1876 [28]. Almost simultaneously malachite green was discovered by E. and O. Fischer. [29] and by Doebner [30] and O. N. Witt and Roussin introduced the synthesis of azo-dyes on a manufacturing scale, which proved afterwards to be of such great importance. 

Lewis D M, ShenX M and Tapley KN, NIR FT-Raman spectroscopy for monitoring the synthesis of azo dyes , Mikrochimica Acta [Supplement , 1997,14, 751-754. 

In some methods the colour system is a product of a synthesis reaction. Examples of such methods are formation of an indophenol dye in a method for determination of ammonia, the synthesis of azo dyes in the determination of nitrite, the formation of Methylene Blue in the determination of sulphur, the pararosaniline method for determination of sulphite, and the benzidine-pyridine method for determination of cyanide. 

Scheme 6.3 Reaction scheme illustrating the protocol used for the continuous flow synthesis of azo dyes.

A one-pot method for the synthesis of azo dyes in high yields by using MWI for a very short time in the absence of solvent was developed. Thus, an equimolar mixture of the aniline derivatives 349, sodium nitrite, potassium hydrogen sulfate, and 8-hydroxyquinoline 347 (R = H) was irradiated for 30-60 s in an MW oven, followed by addition of a drop of water to give about 80-85% yields of the azo dyes 350 and 351 (Scheme 70) (00SC829). 

Recently, we reported the synthesis of azo dye-functionalized organoiron complexes from the reaction of the bimetallic organoiron complex 3 with azo dyes 4 (Scheme 7.2). These monomers displayed excellent solubility in polar organic solvents and were isolated as vibrantly colored powders ranging from orange to purple in good yields. 

Caldarelli, M. Baxendale, I. R. Ley, S. V. Clean and efficient synthesis of azo dyes using polymer-supported reagents. Green Chem. 2000, 2, 43 6. 

Schomburg, C. Woehrle, D. In situ synthesis of azo dyes in mesoporous Y zeolites. Zeolites 1996,17, 232-236. 

Propose a synthesis from benzene of 3-aminobenzenesulfonic acid [metanilic acid, used in the synthesis of azo dyes (Section 22-11), such as Metanil yellow, and certain sulfa drugs (Section 15-10)]. 

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