甲（乙）基在甲（乙）苯亲电取代反应中的定位作用和定位能力讨论

在有机化学的亲电取代反应定位法则教学中，甲基代表烷基，作为芳烃亲电取代反应定位效应的典型例子，但甲基不同于乙基等烷基，具有不同的定位能力，烷基是如何影响芳环的结构从而影响取代基定位，教材和教学中描述较简单，不便于学生领会和比较。该文就此问题进行探讨，结果表明，烷基苯RAr（R—Me、Et）的R在空间的位置影响烷基苯的能量和原子电荷密度，从而影响烷苯的亲电取代反应。（1）烷苯的R—Ar键旋转形成不同构象，其中的最大与最小体系能量值之差△E，△E（Me）〈△E（Et），{△E（Me）=0．0000084a．u，△E（Et）=0．0068029a．u}，甲苯比乙苯的R—Ar单键更容易自由旋转。在EtAr，C8-C7-C1平面与苯环垂直时，体系能量[E（90）=-308．661166a．u．]最低，为较稳定的优势构象。（2）甲基和乙基均向苯环供电子，虽甲（乙）苯环的碳原子所带电荷之和都不及苯，但甲苯比乙苯的环碳上电子密度大，∑Q（Me，30）=-1．02794〈∑Q（Et，90）=-1．010487。（3）甲（乙）苯化合物中，均为邻、对位碳原子电子密度大于苯环碳{甲基：邻位qc2（30）=qc6（30）=-0．212118，对位-0．20724～-0．20728；乙基：邻位qc2（90）=qc6（90）=-0．209509，对位q（90）（-0．20695）}，间位碳原子的电子密度小于苯环碳[甲基间位-0．19153～-0．19228；乙基间位q（90）=-0．192171]，甲、乙苯的邻、对位有利于亲电取代反应，且甲基大于乙基的定位能力。（4）亲电试剂Me^＋与苯环反应形成邻、对位的碳正离子中间体比闯位碳正离子中间体稳定，有利于生成邻、对位取代产物，因此，甲、乙基均为邻、对位定位基。

Orientation Effect of the Me and Et Group on the Reaction of Electrophilic Aromatic Substitution

In teaching orientation law of organic chemistry,Methyl is a typical example of alkyls on the reaction of electrophilic aromatic substitution. However, alkyls, such as methyl and ethyl, have different orientation abilities; and how alkyl affects the structure of aromatic ring which influences orientation of substituents; and these aspects are simply described in teaching and text books, so it is difficult for students to understand and master. This paper discussed related issues, and the results showed that the R special position of the alkyl-benzene RAr （R = Me, Et） influences its energy and atomic charging density, thus affecting electrophilic substitution reaction of alkyl-benzene. （1） the rotation of alkyl benzene R-Ar bond forms different eonfor mations, and the energy difference between the largest and the smallest system is △E, △E（Me） 〈 △E（Et）, {△E（Me）= 0. 000 0084a. u. , △E（Et）=0. 006 802 9a. u }. The R-Ar single bond of methybbenzene rotates more easily than that of ethyl benzene. When C8-C7-C1 plane is perpendicular to the benzene ring in EtAr, the system energy [E （90） =-308. 661 166au] is the lowest and the conformation is more stable. （2） Both methyl and ethyl supply electrons for benzene. The electric charges of benzene ring carbon atoms in methyl-benzene and ethyl-benzene are less than that of benzene. But electron density of methyl- benzene is more than that of ethyl benzene, ∑Q（Me,30） = =-1. 027 94 〈∑Q（Et,90） = -1.010 487. （3） Electron density of ortho-position and para-position carbon atoms in methyl and ethyl benzene compounds is more than that of benzene ring carbon atoms. { methyl： ortho-position Cqc2（30） = qc6（30） = -0.212 118, para-position -0. 207 24 --0.207 28; ethyl： or tho-position qc2 （90） = qc6 （90） = -0. 209 509, para-position q（90）（-0. 206 95） }. The electron density of meta-position ear bon atoms is less than that of benzene ring carbon atoms. [methyl meta-position： -0. 191 53 --0. 192 28; ethyl meta-position： q（90） = - 0. 192 171]. Ortbo-position and para-position of methyl and ethyl benzene are in favor of electrophilic substitution reactions, and the orientation ability of methyl is greater than that of ethyl . （4） Carbonium ions of ortho-position and para-position formed during the reaction between electropbilie reagents （Me^＋ ） and the benzene rings are more stable than carbonium ions of meta-position, which benefits to the formation of substitution products of ortho-position and para-position. Therefore, methyl and ethyl are orientation groups of ortho-position and para-position.