二氧化錳在有機化學(xué)中十分有用，被用于氧化物的二氧化錳的形態(tài)不一，因為二氧化錳有多個結(jié)晶形態(tài)，化學(xué)式方面可以寫成MnO2-x(H2O)n，其中x介于0至0.5之間，而n可以大于0。二氧化錳可在不同pH下的高錳酸鉀（KMnO4）和硫酸錳（MnSO4）的反應(yīng)之中產(chǎn)生。啡色的二氧化錳沉淀物很活潑。最有效的有機溶劑包括芳香性物質(zhì)、氯化碳、醚、四氫呋喃和酯類等。
活性MnO2廣泛用于氧化α,β-不飽和基團(tuán)（三鍵，雙鍵、芳香環(huán)）的醇，可選擇性氧化烯丙式醇，條件溫和，不會引起雙鍵的異構(gòu)化MnO2的活性及溶劑的選擇對反應(yīng)至關(guān)重要，常用的溶劑有二氯甲烷、乙醚、石油醚、己烷、丙酮等。
極性溶劑中反應(yīng)較慢，一般用二氯甲烷和過量的二氧化錳。由于是非均相反應(yīng)，反應(yīng)的后處理只需要簡單地過濾。中性條件的緣故，不發(fā)生副反應(yīng)。
反應(yīng)實例
A 500-mL, round-bottomed flask, equipped with a Teflon-coated magnetic stirring bar, is charged with the 11.2 g of allylic alcohol and 300 mL of dichloromethane, and the resulting vigorously stirred solution is treated with 33 g of active manganese dioxide (380 mmol). Additional 2–5 g lots of the oxidant are added every 2–3 hr until the reaction is complete. The reaction mixture is vacuum-filtered through a pad of diatomaceous earth, and the pad is washed with 200 mL of dichloromethane. The resulting clear filtrate is concentrated carefully using a rotary evaporator, and the residual oil is purified by bulb-to-bulb distillation at 0.3 mm (pot temperature 100 °C) affording 8.43–8.71 g (87–90%) of enone as a pale yellow oil that solidifies when cooled below 15°C. Crystallization of the crude product from pentane at ?70 °C gives (4S)-(?)-tert-butyldimethylsiloxy-2-cyclopenten-1-one as colorless needles having mp 32–33 °C, [α]D23 ?65.1° (CH3OH, c 0.94).
Reference: Organic Syntheses, Coll. Vol. 9, p.136; Vol. 73, p.44
A 100-mL, one-necked, round-bottomed flask is fitted with an efficient reflux condenser and arranged for magnetic stirring and heating. The flask is charged with 50 mL of pentane and 2.0 g (13 mmol) of alcohol. To the rapidly stirred solution is added 16 g (180 mmol) of activated manganese (IV) oxide in small portions over 5 min. The solution is heated at reflux for 18 hr and then an additional 8 g (90 mmol) of activated manganese (IV) oxide is added in portions. After being heated at reflux for 24 hr, the reaction mixture is filtered through a 2-cm Celite filter pad. The filtered manganese oxides are thoroughly washed with about 200–300 mL of dichloromethane. Evaporation of solvent from the combined filtrates leaves 1.4–1.6 g of a light yellow oil. Bulb-to-bulb distillation at 100°C/0.1 mm gives 1.27–1.40 g (8.4–9.3 mmol, 64–71% yield) of an oil 3-Butyroyl-1-methylpyrrole.
Reference: Organic Syntheses, Coll. Vol. 7, p.102; Vol. 62, p.111
活性二氧化錳的制備方法【J. Chem. Soc. 1952, 1094】
高錳酸鉀(960g)的水溶液(6000mL)加熱攪拌、在一小時中同時慢慢加入硫酸錳(II)四水和物(1110g)的水溶液(1500mL)以及氫氧化鈉水溶液(40%, 1170mL）。能看到二氧化錳以茶色沉淀析出。繼續(xù)攪拌一小時，離心機分離，收集固體，水洗至洗液到無色為止。得到的固體在空氣中(100-120℃)干燥，碾成粉末狀備用(920g)。
實驗技巧
※ 注意不同的公司販賣的二氧化錳活性純度不同。
※失活的主要原因是二氧化錳能結(jié)合一分子的結(jié)晶水，處理方法是碾碎后兩百度以上高溫長時間加熱，能恢復(fù)活性。
參考文獻(xiàn)
一、化學(xué)空間：http://www.chem-station.com/cn/reactions/2014/05/活性二氧化錳-activated-manganese-dioxide-mno2.html
二、藥明標(biāo)準(zhǔn)操作
