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Đáp án:

21. $C_6H_5CH_3+Br_2\xrightarrow{{Fe, t^o}} HBr+\left[ \begin{array}{l} o-Br-C_6H_4CH_3\\ p-Br-C_6H_4CH_3\end{array} \right.$ 

22. $C_6H_5CH_3+HONO_{2_đ}\xrightarrow{{H_2SO_4 đ, t^o}} H_2O+\left[ \begin{array}{l} o-O_2N-C_6H_4CH_3\\ p-O_2N-C_6H_4CH_3\end{array} \right.$

23. $C_6H_5CH_3+2KMnO_4\xrightarrow{{t^o}} C_6H_5COOK+2MnO_2+KOH+H_2O$ 

24. $C_6H_5CH=CH_2+Br_2\to C_6H_5CHBr-CH_2Br$ 

25. $3C_6H_5CH=CH_2+2KMnO_4+4H_2O\to 3C_6H_5CH(OH)-CH_2(OH)+2MnO_2+2KOH$  

26. $nC_6H_5CH=CH_2\xrightarrow{{t^o, p, xt}} (\kern-6pt-CH_2-CH(C_6H_5)-\kern-6pt)_n$