Name: purifying crystals by solubility
Uploaded: 2020-03-26T07:22:27.000Z
Duration: 3 min 12 s
Channel: JoVE

大多数产物含有杂质。提纯这些产物的方法之一是重结晶。重结晶首先将不纯的产物溶解在恰好足够的热溶剂中，形成饱和溶液，即溶剂中溶解了尽可能多的溶质。任何额外的溶质将不再明显溶解。
使用热溶剂是因为溶解度通常随温度升高而增加。随着温度升高，溶剂中可溶解的溶质量也随之增加。当溶液冷却时，产物的溶解度降低，溶质分子聚集形成称为晶核的小而稳定的晶体。
这是结晶的第一步，称为成核。溶质分子更倾向于加入已有的溶质晶体而非形成新的晶体，因此晶体的进一步生长发生在晶核上。可溶性杂质则留在溶液中。
结晶可以自发发生，也可以通过刮擦烧瓶内壁、搅拌溶液或加入该化合物的晶种来促进，这些方法均可提供晶体进一步生长的表面。
只有使用合适的溶剂，才能将化合物重结晶为大块、纯净且形状规则的晶体。该化合物在室温下应对溶剂不溶，而在高温下可溶。理想情况下，杂质应在高温下不溶于溶剂，或在室温下可溶于溶剂。
如果杂质在热溶剂中不溶解，则在结晶前将其过滤除去。重结晶后，将晶体过滤并用冷溶剂洗涤，以去除表面的杂质。此时，可对晶体的纯度进行分析。
选择溶剂时需注意，高温与低温下溶解度差异越大，溶质在冷却过程中析出形成晶体的可能性就越高。在重结晶过程中，冷却速率也十分重要。
快速冷却有利于形成大量成核位点并生长出较小的晶体，而缓慢冷却则促使形成较少的成核位点，从而生长出数量较少但更大且更纯净的晶体。因此，更倾向于采用缓慢冷却。
在本实验中，您将对两种不纯的有机化合物——乙酰苯胺和反式-肉桂酸进行重结晶，然后通过将回收产物的熔点范围与文献值进行比较，评估其纯度。

重结晶

重结晶
通常，化学反应的目标产物是复杂反应混合物的一部分，该混合物可能包含溶剂、起始原料和杂质。正确掌握有机化合物的纯化技术是有机化学中一项重要的实验技能。重结晶利用目标化合物与杂质在溶剂中溶解度的差异，将目标产物以固体形式纯化。常见的三种标准纯化方法为：蒸馏、萃取和重结晶。
溶解度
一种物质的溶解度是指在给定温度下，该物质在固定体积的特定溶剂中所能溶解的最大量。不同的溶质具有不同的溶解度，并且可在不同的溶剂中溶解。溶质可能具有某些特定性质，这些性质可用于实现重结晶。化合物在溶剂中表现出以下行为之一：第一，该化合物在所有温度下均不溶或在溶剂中溶解度极低；第二，该化合物在较高温度下可溶于溶剂；第三，该化合物在所有温度下均可溶于溶剂。
决定溶质是否能在溶剂中溶解并形成溶液的一个主要因素是溶质与溶剂之间分子间作用力的强度和类型。通常的经验法则是“相似相溶”，即具有相似类型分子间作用力的物质能够相互溶解。例如，像食盐（NaCl）这样的极性物质能很好地溶解在极性水分子中。
另一个能够提高溶解度的关键因素是温度。对于许多物质而言，温度升高时其溶解度显著增加。这是因为温度升高导致分子动能增加，从而破坏了溶质分子间维持其聚集状态的分子间作用力。这一现象在日常生活中随处可见。例如，我们知道食盐（NaCl）易溶于水；然而，在较高温度下溶解的量要多于在较低温度下的溶解量。
定性而言，如果溶液中溶解的溶质尚未达到最大量，则该溶液被视为未饱和。当溶质溶解量达到最大可能值时，溶液为饱和状态。过饱和溶液所含的溶解溶质超过通常条件下可能达到的最大量。
<h4>重结晶</h4>
重结晶利用了目标产物与杂质在高温下溶解度的差异。重结晶的第一步是将产物混合物溶于最小体积的加热溶剂中，形成饱和但不析出的溶液。随后，将溶液冷却至室温，从而降低目标化合物和杂质的溶解度。
随着溶液冷却，纯组分开始结晶，而仍可溶的杂质则不会析出。这种情况发生在目标组分的浓度显著高于杂质时。首先，在成核阶段，溶剂促使溶质分子随机聚集，形成最初的晶体，称为晶核或种子。接着，在晶体生长或结晶阶段，更多的分子不断添加到晶核上，逐渐形成晶体。该晶体包含纯化合物，而杂质则保留在溶剂中。
在过饱和溶液中，成核速度比颗粒生长速度更快。晶核越多，每个晶体的尺寸就越小。因此，如果溶液处于饱和状态而非过饱和状态，则形成的晶核较少，从而得到更大的晶体。在冷却至室温之前将溶液加热至更高温度，可使溶质溶解度升高，从而降低过饱和度。此外，快速冷却会导致迅速成核，形成大量细小晶体，并将杂质包裹在晶体内部。为了获得数量较少但尺寸较大的晶体，应采用缓慢冷却的方式。
待溶液冷却至室温且晶体析出后，采用减压过滤法对溶液进行过滤。随后，使晶体自然干燥。通过将回收产物的质量除以粗产物的质量，计算百分比回收率。
<img src="/CDNSource/lm/labs/58/58_Concepts_1.jpg" alt="百分比回收率公式；用于计算化学产率过程效率的方程式。" data-altupdatedat="1761830439000" />
回收率很少达到100%，因为化合物在低温下的溶解度决定了有多少化合物会结晶析出。
<h4>选择溶剂</h4>
要使结晶有效，必须使用最佳溶剂。目标产物在所选溶剂中于室温下应具有较低的溶解度，而在较高温度下应具有较高的溶解度。理想情况下，杂质在所有温度下均应溶于该溶剂。因此，当将混合物在高温下加入溶剂时，目标产物和杂质均可迅速溶解。
当溶液冷却时，目标产物的溶解度降低，开始发生结晶，从而形成纯化后的产品。有时，杂质可能在所选溶剂中即使在高温下也保持不溶解状态。通过热重力过滤含有已溶解产物的溶液，可去除这些固体杂质。随后通过冷却样品使产物重新结晶。
理想情况下，所用溶剂的沸点应远低于目标产物的熔点。该溶剂还应具有惰性，不与目标纯化产物发生反应。
<h4>参考文献</h4>
<ol>
	<li style="font-size:17px">Kotz, J.C., Treichel Jr, P.M., Townsend, J.R. (2012). 化学与化学反应性。加利福尼亚州贝尔蒙特：Brooks/Cole, Cengage Learning。</li>
	<li style="font-size:17px">Silberberg, M.S. (2009) 化学：物质的分子本质与变化。马萨诸塞州波士顿：McGraw Hill。</li>
	<li style="font-size:17px">Harris, D.C. (2015). 定量化学分析。纽约州纽约市：W.H. Freeman and Company。</li>
</ol>

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通常，化学反应的目标产物是复杂反应混合物的一部分，该混合物可能包含溶剂、……

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Recrystallization

purifying crystals by solubility

Purifying crystals by solubility is a key organic chemistry technique. Recrystallization uses differences in solubility to separate a desired solid product from starting materials, solvent, and impurities. It is one of the three standard purification methods, along with distillation and extraction.

Solubility is the maximum amount of a substance that can dissolve in a fixed volume of solvent at a given temperature. Different solutes dissolve in different solvents, and their behavior can vary widely. A compound may stay insoluble or only slightly soluble at all temperatures, it may dissolve better at high temperature, or it may remain soluble at all temperatures. The main reason is the strength and type of intermolecular forces between the solute and the solvent. The rule “like dissolves like” applies here, so substances with similar intermolecular forces tend to dissolve in each other. For example, polar table salt (NaCl) dissolves well in polar water. Temperature also matters. In many cases, solubility rises as temperature rises because added kinetic energy helps break the forces holding solute molecules together. A solution is unsaturated when more solute can still dissolve. It is saturated when it has reached the maximum amount. A supersaturated solution contains more dissolved solute than expected under normal conditions.

Recrystallization uses those solubility differences at high temperature and low temperature. The crude product is first dissolved in a minimal volume of heated solvent. The goal is a saturated solution, but not a supersaturated one. The solution is then cooled to room temperature. As it cools, the solubility of both the desired compound and the impurity drops. The pure compound begins to crystallize first, while impurities that are still soluble remain in the liquid. This works best when the compound of interest is present at a much higher concentration than the impurity.

Crystal formation happens in two stages. In nucleation, the solvent helps solute molecules gather into the first tiny crystal, called a seed or nucleus. In particle growth, more molecules attach to that seed and the crystal gets larger. The crystal contains the pure compound, while the impurity stays in the solvent. In a supersaturated solution, nucleation happens faster than particle growth, so more seeds form and the crystals stay small. If the solution is only saturated, fewer seeds form and the crystals become larger. Heating the solution more before cooling can reduce supersaturation by dissolving a higher concentration of solute. Slow cooling is also preferred because rapid cooling makes many small crystals and can trap impurity inside them.

After the crystals form, the solution is filtered by vacuum filtration and the crystals are allowed to dry. The percent recovery is found by dividing the mass of the recovered product by the mass of the crude product. Recovery is rarely 100% because the compound’s low-temperature solubility limits how much can crystallize. Choosing the right solvent is important for successful crystallization. The desired product should be poorly soluble at room temperature but highly soluble when hot. Ideally, impurities stay soluble at all temperatures, so they remain in solution as the product crystallizes. If some impurities do not dissolve even when hot, hot gravity filtration can remove them before the sample is cooled again. The solvent should also boil well below the melting point of the desired product and should not react with it.

Purifying Crystals by Solubility

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在这段 13 分钟的学生实验流程视频中，逐步学习 重结晶 的操作步骤。属于 JoVE Lab Manual 上的 Chemistry 系列。

Chemistry 中的 重结晶 学生实验流程 - 54K+ 观看次数 | JoVE Lab Manual

Purifying Crystals by Solubility

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