铁粉还原硝基制备胺是什么机理？

硝基的还原是一种常用的合成伯胺的方法特别是芳香伯胺，一般而言，最干净和简便的还原方法就是通过Pd/C或Raney Ni加氢，但是当分子内存在对加氢敏感的官能团，如：卤素（Cl, Br and I; F 对加氢不敏感），双键，三键，氰基，羰基时，我们不得不采用其他方法， 最为经典的要数铁粉的还原。铁粉还原一般在盐类电解质(氯化铵)或烯酸条件下进行，此条件可以将芳香硝基，脂肪硝基，亚硝基和羟胺等基团还原为氨基。铁粉还原芳香硝基化合物时，芳基上有吸电子基团时，有利于硝基获得电子，反应容易进行。

常用的还原条件是：一、铁粉/HCl，THF体系或乙醇体系；二、铁粉/AcOH；另外常用的条件还有，铁粉（5eq），氯化铵（10eq），在甲醇和水混合溶液（4：1）中回流，但此条件很容易形成偶氮化合物，致使反应变黑，氮气保护可以减少偶氮化合物的生成，上述两种酸性条件很少生成偶氮类副产物。此类反应的后处理，一般情况下都是趁热过滤，防止冷却后，产品吸附到剩余的铁粉和氧化铁上。

反应机理

铁粉还原为单电子转移机理，反应中需要转移六个电子，理论上还原硝基至少需要3倍当量铁粉，实际反应操作中一般加入4倍当量以上铁粉。

反应实例

To the starting material (3.1 g) was added acetic acid (24.4 mL) and then, reduced iron powder (9.15 g), and the mixture was stirred for 16 hours at room temperature. The mixture was filtered with Celite, and washed with ethyl acetate. The solvent was removed under reduced pressure, and water was added to the obtained residue, and the mixture was extracted with ethyl acetate. The organic layer was washed with saturated brine, and dried over magnesium sulfate. The solvent was removed under reduced pressure, and the obtained residue was purified by silica gel column chromatography, to give 1.4 g of the desired product.

To 80 percent ethanol (5.0 mL), the starting material (300 mg, 1.76 mmol) and concentrated hydrochloric acid (0.45 ml, 5.28 mmol) were added to stir for 10 minutes at 20°C. Then, iron powder (500 mg, 8.95 mmol) was added to the mixture to further stirring for 1 hour at 20°C. Insoluble matters were filtered off and the filtrate was concentrated, adding 0.5N NaOH (10 mL) to the residue, filtering the solution to further remove insoluble matters, extracting the filtrate with chloroform, drying the extract over anhydrous sodium sulfate, concentrating the extract, and distilling the resulting crude product to afford 224 mg of the desired compound (91% yield).

另外由于铁粉反应后氧化物的颜色较深，不好纯化，有时会用锌粉代替铁粉进行反应，条件类似，锌粉的另外一个优点是利用磁力搅拌器就可以进行较大量的反应。