Crystallization | 结晶操作

1. 原理

在特定温度下，将一种固体化合物完全溶解于一种（或多种）溶剂中，随后改变溶液的条件，使其对欲结晶的化合物达到过饱和状态，进而使该化合物从溶液中结晶析出。

利用溶剂对被提纯物质及杂质的溶解度不同，可以使被提纯物质从过饱和溶液中析出，而让杂质全部或大部分仍留在溶液中；若杂质在溶剂中的溶解度极小，则配成饱和溶液后被过滤除去，从而达到提纯目的。

• 优点：操作简单，产品纯度高。
• 缺点：产物损失较大。

注意：重结晶适宜于杂质含量在 5% 以下的固体有机混合物的提纯。

许多化合物可以以多种不同的多晶型形式结晶，所得到的多晶型取决于结晶过程所处的条件。多晶型具有不同的熔点和稳定性，因此如果在某一结晶体系中难以获得优质的晶体，可以尝试其他体系。

2. 产生过饱和溶液的方法

• 冷却结晶：将目标固体溶解在最少量的单一溶剂中，制成热饱和溶液，然后通过冷却诱导结晶。
  • 化合物在热溶剂中的溶解度通常远高于冷溶剂。粗略而言，温度每升高 20 °C，化合物的溶解度大约会翻倍。因此，对于高熔点化合物，通常更容易找到合适的单一结晶溶剂（使用沸点较高的溶剂）。
• 蒸发结晶：通过部分蒸发溶剂来诱导过饱和。
  • 可采用自然蒸发直至结晶出现，或蒸馏除去一部分溶剂后再通过冷却诱导结晶。
  • 若化合物在低沸点溶剂中溶解度较大，可使用混合溶剂进行蒸发结晶。
• 反溶剂结晶：如果单一溶剂的结晶方法不成功，通常可以加入反溶剂（化合物在其中溶解度较低的溶剂），可同时配合冷却，也可不冷却。

3. 简单重结晶步骤

3.1 选择合适的溶剂或混合溶剂

• 通过小规模试验寻找合适的溶剂。
• 最好选用沸点在 60 °C 以上的溶剂，但溶剂的沸点应至少比待结晶化合物的熔点低 10 °C，以防止溶质从溶液中"油析出"。

3.2 将化合物溶解在最少量的热溶剂中

• 将粗产物和几粒沸石放入装有回流冷凝管的锥形瓶中。
• 加入少量溶剂，加热混合物直至观察到温和回流。
• 分次加入更多热溶剂，直到所有粗产物完全溶解。
• 若使用反溶剂辅助结晶：分次加入热反溶剂，直至目标化合物刚开始沉淀（观察到浑浊），此时加入少量良溶剂溶解最初生成的沉淀，然后将热溶液静置冷却。

安全提示：大多数有机溶剂极易燃烧，且许多会产生剧毒蒸气！

3.3 热过滤除去不溶性杂质

• 仅在悬浮有大量不溶物时进行。
• 额外加入约 5% 过量溶剂。
• 将过滤装置预热至略低于溶剂沸点的温度。
• 若溶液颜色很深或含有少量焦油状杂质：稍冷却片刻，加入约 2% 重量的脱色活性炭，加热回流几分钟，然后过滤除去活性炭。

3.4 冷却溶液，使晶体析出

• 若出现油状物：最好重新加热溶液，再缓慢冷却。
• 尝试用玻璃棒摩擦瓶壁，或加入少量晶种以诱导结晶。
• 若仍无效：再加入适量溶剂，使结晶在更低的温度下开始析出。
• 若上述方法均不奏效：可将瓶口塞住，放入冰箱冷冻过夜——不要害怕尝试其他溶剂体系。

3.5 过滤并干燥晶体

• 结晶完成后，使用适当孔径的烧结玻璃漏斗过滤收集晶体。
• 一旦母液全部抽干，先解除真空，然后在晶体上倒入少量冷溶剂，必要时搅拌晶体，重复洗涤一至两次。
• 经过仔细洗涤后，让晶体在空气中短暂干燥，然后使用真空烘箱、干燥枪或真空线去除最后残留的溶剂。

4. 小规模重结晶步骤

4.1 热过滤

• 将样品放入球形瓶中，用几滴溶剂润洗，加热球形瓶使溶剂回流至砂板。
• 分多次少量加入更多溶剂，直至化合物完全溶解。
• 将装置从浴中取出，对装置加压，将热溶液快速过滤到一个洁净的接收器中。
• 当规模为 100 mg 或更少时，使用 Craig tube。

4.2 结晶与离心

• 将热溶液过滤到合适尺寸的 Craig tube 中，用铝箔盖住管口结晶。
• 结晶完成后，将配套的玻璃棒插入 Craig tube，用橡皮筋牢牢固定。
• 将组装好的装置倒置放入离心管中，离心几分钟。

注意：记得使用配平管！

• 离心使母液进入离心管中。
• 将 Craig tube 从离心管中取出，拔掉玻璃棒，轻敲管壁使晶体掉落至管底。
• 用铝箔盖住管口，在真空下干燥晶体。

5. 低温重结晶步骤

适用于低熔点固体或热不稳定固体的纯化

5.1 溶解与装置准备

• 在室温下将化合物溶解于最少量的溶剂中，然后过滤到两颈或三颈梨形瓶中。
• 在梨形瓶上安装惰性气体入口以及一个连接有起泡器的温度计转接头，该转接头内装有过滤棒。

5.2 惰性气体吹扫与冷却

• 先将过滤棒置于液面之上，用惰性气体吹扫梨形瓶。
• 将梨形瓶放入冷却浴中，通过向冷却浴的溶剂中逐渐加入冷却剂，使体系缓慢冷却。

5.3 过滤与洗涤

• 结晶完成后，将过滤棒与适当尺寸的接收瓶连接。
• 将过滤棒下降至溶液中，利用惰性气体施加的压力将母液压入接收瓶。
• 释放压力，通过三通阀用套管加入少量预冷的溶剂洗涤晶体（洗涤液可按同样方式移除）。
• 移去冷却浴，按常规方式分离并干燥晶体，或在同一烧瓶中重复低温结晶操作。

6. 低温小规模重结晶步骤

• 将待纯化物质在球形瓶中用适当溶剂溶解。
• 用惰性气体吹扫装置，并用一个小隔膜密封出口。
• 将烧瓶浸入冷却浴中。
• 结晶完成后，取下隔膜，在惰性气体压力下快速过滤悬浮液。
• 晶体可用预冷的溶剂洗涤。

1. Principles

A solid compound is fully dissolved in one or more solvents at a given temperature, after which the conditions of the solution are altered to render it supersaturated with respect to the desired compound, causing it to crystallize out of solution.

Differences in solubility between the target substance and impurities are exploited: the desired compound precipitates from the supersaturated solution while impurities remain largely dissolved; if impurities have very low solubility, they are removed by filtration of the saturated solution before crystallization.

• Advantages: Simple operation; high product purity.
• Disadvantages: Significant product loss.

Note: Recrystallization is suitable for purifying solid organic mixtures with impurity levels below 5%.

Many compounds can crystallize in multiple polymorphic forms; the polymorph obtained depends on crystallization conditions. Polymorphs differ in melting point and stability. If good crystals are difficult to obtain in one solvent system, try another.

2. Methods for Generating Supersaturated Solutions

• Cooling crystallization: Dissolve the target solid in the minimum amount of a single hot solvent, then induce crystallization by cooling.
  • Solubility in hot solvents is typically much higher than in cold solvents. As a rough guide, solubility approximately doubles for every 20 °C increase. High-melting compounds are therefore often easier to crystallize from a single solvent (using a higher-boiling solvent).
• Evaporative crystallization: Induce supersaturation by partially evaporating the solvent.
  • Either allow natural evaporation until crystallization occurs, or distill off a portion of solvent followed by cooling.
  • If the compound is highly soluble in a low-boiling solvent, mixed-solvent evaporative crystallization can be used.
• Anti-solvent crystallization: If single-solvent crystallization fails, add an anti-solvent (in which the compound is poorly soluble), with or without accompanying cooling.

3. Simple Recrystallization Procedure

3.1 Selecting a Suitable Solvent or Mixed Solvent

• Identify a suitable solvent through small-scale trials.
• Preferably choose a solvent with a boiling point above 60 °C, but at least 10 °C below the melting point of the compound to prevent “oiling out.”

3.2 Dissolving the Compound in a Minimum of Hot Solvent

• Place the crude product and a few boiling chips in a conical flask fitted with a reflux condenser.
• Add a small portion of solvent and heat the mixture to gentle reflux.
• Add additional portions of hot solvent until all crude product has dissolved.
• For anti-solvent-assisted crystallization: Add portions of hot anti-solvent until the target compound just begins to precipitate (turbidity observed); add a small amount of good solvent to redissolve the initial precipitate, then set the hot solution aside to cool.

Safety: Most organic solvents are highly flammable and many produce highly toxic vapors!

3.3 Hot Filtration to Remove Insoluble Impurities

• Only perform when substantial insoluble matter is present.
• Add approximately 5% excess solvent.
• Preheat the filtration apparatus to a temperature just below the solvent boiling point.
• If the solution is deeply colored or contains small amounts of tarry impurities: allow to cool briefly, add ~2 wt% decolorizing charcoal, heat at reflux for several minutes, then filter off the charcoal.

3.4 Cooling the Solution to Induce Crystallization

• If an oil forms: It is best to reheat the solution and cool more slowly.
• Try scratching the flask wall with a glass rod, or add a few seed crystals to induce crystallization.
• If still unsuccessful: Add more solvent so that crystallization begins at a lower temperature.
• If none of the above works: Stopper the flask and place in a freezer overnight — do not hesitate to try alternative solvent systems.

3.5 Filtering and Drying the Crystals

• Once crystallization is complete, collect the crystals by filtration through a sintered-glass funnel of appropriate porosity.
• After the mother liquor has been completely drawn off, release the vacuum, pour a small amount of cold solvent over the crystals, stir if necessary, and repeat the wash once or twice.
• After thorough washing, allow the crystals to air-dry briefly, then remove residual solvent using a vacuum oven, drying pistol, or vacuum line.

4. Small-Scale Recrystallization Procedure

4.1 Hot Filtration

• Place the sample in a pear-shaped flask, moisten with a few drops of solvent, and heat so that the solvent refluxes onto the sinter.
• Add further small portions of solvent until the compound has fully dissolved.
• Remove the apparatus from the bath, apply pressure, and rapidly filter the hot solution into a clean receiver.
• For scales of 100 mg or less, use a Craig tube.

4.2 Crystallization and Centrifugation

• Filter the hot solution into a Craig tube of appropriate size and cover the mouth with aluminum foil for crystallization.
• Once crystallization is complete, insert the mating glass rod into the Craig tube and secure firmly with a rubber band.
• Invert the assembled apparatus into a centrifuge tube and centrifuge for several minutes.

Note: Remember to use a balance tube!

• Centrifugation drives the mother liquor into the centrifuge tube.
• Remove the Craig tube from the centrifuge tube, withdraw the glass rod, and tap the tube wall to dislodge crystals to the bottom.
• Cover the mouth with aluminum foil and dry the crystals under vacuum.

5. Low-Temperature Recrystallization Procedure

For purification of low-melting solids or thermally labile solids

5.1 Dissolution and Apparatus Setup

• Dissolve the compound in the minimum amount of solvent at room temperature, then filter into a two- or three-necked pear-shaped flask.
• Fit the flask with an inert gas inlet and a thermometer adapter connected to a bubbler; the adapter houses a filter stick.

5.2 Inert Gas Purge and Cooling

• With the filter stick positioned above the liquid surface, purge the flask with inert gas.
• Place the flask in a cooling bath and cool the system slowly by gradually adding coolant to the bath solvent.

5.3 Filtration and Washing

• Once crystallization is complete, connect the filter stick to a receiver of appropriate size.
• Lower the filter stick into the solution and apply inert gas pressure to force the mother liquor into the receiver.
• Release the pressure, and through a three-way stopcock use a cannula to add a small amount of pre-cooled solvent to wash the crystals (the wash can be removed in the same manner).
• Remove the cooling bath, isolate and dry the crystals conventionally, or repeat the low-temperature crystallization in the same flask.

6. Low-Temperature Small-Scale Recrystallization Procedure

• Dissolve the substance to be purified in a pear-shaped flask with a suitable solvent.
• Purge the apparatus with inert gas and seal the outlet with a small septum.
• Immerse the flask in a cooling bath.
• Once crystallization is complete, remove the septum and rapidly filter the suspension under inert gas pressure.
• Wash the crystals with pre-cooled solvent.

Ref:

• 有机化学实验（学校资源） – Recrystallization video tutorial (Bilibili)
• Leonard, J., Lygo, B., & Procter, G. (2013). Advanced practical organic chemistry. CRC press.