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一般的有机化学教科书中,在描述芳香族物质的取代作用时,指出:“在苯的取代作用中,第二个取代基进入对第一个取代基的三个可能的相对位置时,其难易程度是不一样的。在一些情况下,第一个取代基好像是把第二个取代基主要地导入邻位和对位,而在另一些情况下则主要是导入间位。”简而言之,也就是说已经存在苯核上的取代基的性质决定后一个取代基进入苯核的难易和将要占据的相对位置(指主要产物而言)。为了区别两类不同的取代基通常有以下数种经验性的规律:
In the general textbook of organic chemistry, describing the substitution effect of aromatic substances, it is stated that “in the substitution of benzene, when the second substituent enters the three possible relative positions for the first substituent, it is difficult In some cases, the first substituent seems to introduce predominantly the second substituent into the ortho and para positions, while in other cases it is primarily the meta position. ” In other words, the nature of the substituents already present on the benzene nucleus determines the relative ease with which the next substituent will enter the benzene nucleus (referring to the primary product). In order to distinguish two different types of substituents there are usually the following empirical rules: