以甲基為第三取代基的胺基苯酚類化合物，使用直接重氮化法形成重氮鹽時，如果其上的羥基取代基位在胺基的鄰位或對位，則會因羥基的作用，而甚難在一般的條件下與G酸（2-Naphthol-6，8-disulfonic acid）進行偶合反應。 以苯酚類化合物、對-乙醯胺基苯胺（p-Aminoacetanilide，PAA）及G酸製備雙偶氮化合物時，如先製備對-乙醯胺基苯胺和苯酚化合物的單偶氮化合物，再將其乙醯胺基水解成胺基，最後再於鹽酸水溶液中，以亞硝酸鈉將其重氮化成重氮鹽和G酸反應，則會因重氮鹽本身長共軛共振現象，以及G酸本身的反應性等因素，導致反應因難。必須尋求另一反應途徑，先以對-乙醯胺基苯胺和G酸形成單偶氮化合物，再將乙醯胺基水解成胺基，再重氮化後與苯酚類化合物偶成形成第二個偶氮基。 The direct method is commonly used to diazotize primary amines to corresponding diazonium salts. For the amines which have hydroxy group at the ortho or para position to the amino group. After diazotization, the diazonium salts usually convert into diazo-oxides in alkaline media. And because the stability and slow coupling of the diazo-oxides, reactions between them and G acid are hrad to occur under common conditions. In preparing disazo compounds with phenol derivatives、p-aminoacetanilide(PAA) and G acid. If we make monoazo compounds with phenol derivatives and PAA first, then hydrolyzing their acetamido groups to amino groups. After diazotization, the long chain diazo-oxides form, and still hard to react with G acid under common conditions. But if we take another reaction path, make the monoazo compound with PAA and G acid first. After hydrolysis and diazotization, The diazonium salt can react with G acid easily and rapidly
