[Chemistry]What is Naphthol?-1
Updated: May 19, 2020
The naphthols are insoluble in water. They are phenols, soluble in alkaline solution and substantive to cotton, particularly in the presence of salt. In the presence of strong alkali they are converted to the corresponding naphtholate ions and are water soluble (Fig.1). The naphtholate ions are always coplanar and preferably have elongated molecular structures. These relatively small molecules are of low to moderate substantivity for cotton. The naphthols may be classified into four groups depending on their affinity for cotton (Shenai, 1993).
(a) Low substantivity naphthols: Naphthol AS, Naphthol ASD, Naphthol AS-OL, Naphthol AS-IRG, Naphthol AS-PH, etc. (b) Medium substantivity naphthols: Naphthol AS-BG, Naphthol AS-LT Naphthol AS-RL, etc. (c) High substantivity naphthols: Naphthol AS-BS, Naphthol AS-SW Naphthol AS-BI, Naphthol AS-BO, etc. (d) Very high substantivity naphthols: Naphthol AS-BT, Naphthol AS-S Naphthol AS-BT, Naphthol AS-BR, etc.
In general, the higher the substantivity the better the rubbing fastness since less azo pigment forms on the fibre surfaces. The substantivity increases with increase in the molecular size of the naphtholate ion, but the diffusion rate in the fibres and solubility in dilute aqueous alkali decrease. Addition of salt promotes better exhaustion of the bath, more being needed for naphthols of lower substantivity. For the dissolution of naphthols usually caustic soda solution of 72°TW, i.e. 38°Be, is used. The first ACC, which was employed in the commercial synthesis of azoic colour, was the 2-hydroxy-naphthalene, i.e. Beta-naphthol. The same was later replaced by more substantive arylamides of 2-hydroxy-3-naphthoic acid discovered in 1911 in Germany. This product is known today as Naphthol AS. With different substitutions on the basic structure, several other substantive naphthols were subsequently developed in the following years which were named as members of the Naphthol AS series (Table 19.1).
A correlation between the chemical structures of ACCs and their affinity for cotton has been attempted. The theoretically computed correlation equations of the practical affinity with the van der Waals surface, the molecular volume and the hydrophobicity are reported in the literature (Dă escu and Hă dă Ruga, 1999). Though reversing the position of amino groups in the conventional AS series of naphthols was found to enhance the brightness of the developed shade, significant advantages over conventional products were not achieved (Hall et al., 1980).