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

ほとんどの製品には不純物が含まれています。これらの製品を精製する1つの方法は、再結晶することです。再結晶は、不純な製品を飽和溶液を形成するのに十分な高温の溶媒に溶解することから始まります。そこでは、できるだけ多くの溶質が溶媒に溶解されます。追加の溶質は溶解しないように見えます。
溶融性は、通常、温度とともに溶解度が増加するため、高温溶媒が使用されます。温度が上昇すると、溶媒に溶解できる溶質の量が増加します。溶液が冷えると、生成物の溶解度が低下し、溶質分子が集まって核と呼ばれる小さな安定な結晶を形成します。
これが結晶化の最初のステップであり、核生成と呼ばれます。溶質分子は、新しい結晶を形成するよりも既存の溶質結晶を結合する親和性が高いため、原子核上で追加の結晶成長が起こります。溶液中に可溶性不純物が残ります。
結晶化は自発的に起こることもあれば、フラスコの内側を引っ掻いたり、溶液を攪拌したり、化合物の種結晶を加えたりすることで促進することもできます。これらはすべて、さらなる成長のための表面を提供します。
大きくて純粋な規則的な形状の結晶への再結晶は、適切な溶媒を使用した場合にのみ機能します。化合物は、室温では溶媒に不溶で、高温では可溶でなければなりません。理想的には、不純物は高温で溶媒に不溶であるか、室温で溶媒に可溶である必要があります。
不純物が高温の溶媒に不溶性の場合、結晶化する前にろ過されます。再結晶後、結晶をろ過し、冷たい溶媒で洗浄して表面から不純物を取り除きます。これで、結晶の純度を分析できます。
溶媒を選択するときは、高温と低温の溶解度の差が大きいほど、溶質が冷却されて結晶を形成するときに溶質が溶液から出てくる可能性が高くなることに注意してください。再結晶では、冷却速度も重要です。
急速冷却は、多くの核形成サイトの形成と小さな結晶の成長を促進しますが、冷却はゆっくりと少ない核形成サイトの形成を誘発し、より小さく、より純粋で少数の結晶の成長を誘導します。したがって、冷却が遅いことが望ましいです。
このラボでは、アセトアニリドとトランス桂皮酸という2つの不純な有機化合物を再結晶し、その融点範囲を文献の値と比較することにより、回収された化合物の純度を評価します。

再 結晶 化

再 結晶 化
多くの場合、化学反応の望ましい生成物は、溶媒、出発物質、および不純物で構成できる、より複雑な反応混合物の一部です。有機化合物を適切に精製する方法を学ぶことは、有機化学の貴重な技術です。再結晶は、目的の化合物の溶解度と溶媒中の不純物の違いを利用して、目的の製品を固体として精製します。精製には、蒸留、抽出、再結晶の3つの標準的な方法があります。
可溶性
物質の溶解度は、特定の温度で特定の溶媒の一定体積に溶解する最大量です。溶質が異なれば、溶解度も異なり、異なる溶媒に溶解します。溶質は、再結晶に利用されるのに適した明確な特性を持っている可能性があります。化合物は、溶媒中で以下のいずれかの挙動を示します。まず、化合物はすべての温度で溶媒に不溶性であるか、または非常に低い溶解度を有する可能性があります。第二に、化合物はより高い温度で溶媒に可溶性であり得る。第三に、化合物はすべての温度で溶媒に可溶であり得る。
溶質が溶媒に溶解して溶液を形成するかどうかを決定する主な要因は、溶質と溶媒の間の分子間力の強度と種類です。一般的な経験則では、「同類は同類に溶解する」、つまり、分子間力が類似したタイプを持つ物質は互いに溶解します。たとえば、食卓塩(NaCl)などの極性物質は、極性水によく溶けます。
溶解性を向上させるもう一つの重要な要素は温度です。多くの物質では、溶解度は高温で大幅に増加します。これは、高温で運動エネルギーが増加すると、分子を一緒に保持する溶質分子間力が破壊されるという事実によるものです。これは日常生活で見られます。たとえば、食卓塩(NaCl)は水によく溶けることがわかっています。ただし、低温よりも高温で多く溶解します。
定性的には、溶解した溶質の最大量にまだ達していない場合、溶液は不飽和と見なされます。可能な最大溶質が溶解すると、溶液は飽和します。過飽和溶液には、一般的な条件下で可能な最大量よりも多くの溶質が含まれています。
<h4>再 結晶 化 </h4>
再結晶は、所望の製品と高温での汚染物質との間の溶解度の違いを利用します。再結晶の最初のステップは、製品混合物を最小限の加熱溶媒に溶解することですが、それでも飽和溶液が得られますが、過飽和ではありません。次に、溶液を室温まで冷却し、目的の化合物と不純物の両方の溶解度を低下させます。
溶液が冷えると、純粋な成分の結晶化が始まりますが、まだ溶ける不純物は結晶化しません。これは、対象成分が不純物よりも大幅に高い濃度にある場合に発生します。まず、核形成段階では、溶媒が溶質分子のランダムな凝集を開始し、シードまたは核と呼ばれる最初の結晶を形成します。次に、粒子の成長または結晶化段階では、より多くの分子がシードに追加され、結晶が形成されます。結晶には純粋な化合物が含まれていますが、不純物は溶媒中に残ります。
核形成は、過飽和溶液中での粒子成長よりも速く進行します。種子が多いほど、各結晶は小さくなります。したがって、溶液が過飽和ではなく飽和の場合、形成される種子が少なくなり、結晶が大きくなります。溶液を高温に加熱してから室温に冷却すると、高濃度の溶質を溶解でき、過飽和状態が減少します。さらに、急速冷却により、迅速な核形成が起こり、多くの小さな結晶が形成され、結晶の内部に不純物が閉じ込められます。ゆっくりと冷却することは、より少ない、より大きな結晶を達成するために好まれます。
溶液が室温まで冷却され、結晶が形成されたら、溶液を真空ろ過を使用してろ過します。その後、結晶を乾燥させます。回収率は、回収された製品の質量を粗製品の質量で割ることによって計算されます。
<img src="/CDNSource/lm/labs/58/58_Concepts_1.jpg">
低温での化合物の溶解度が化合物の結晶化量を支配するため、回収率が100%になることはめったにありません。
<h4>溶媒の選択</h4>
結晶化を効果的に行うためには、最適な溶媒を使用する必要があります。目的の製品は、選択した溶媒への室温での溶解度は低いが、高温での溶媒への溶解度は高いものでなければなりません。理想的には、不純物はすべての温度で溶媒に可溶であるべきです。したがって、混合物を高温で溶媒に添加すると、目的の生成物と不純物が容易に溶解します。
溶液が冷却されると、目的の製品の溶解度が低下し、結晶化が起こり始め、精製された製品が形成されます。場合によっては、不純物が高温でも選択した溶媒に不溶性のままである場合があります。溶解した製品を含む溶液の高温重力ろ過により、固体不純物を除去できます。その後、サンプルを冷却することで生成物を再結晶させることができます。
理想的には、使用する溶媒は、目的の製品の融点をはるかに下回る温度で沸騰できる必要があります。また、溶媒は不活性で、目的の精製製品と反応してはなりません。
<h4>参照</h4>
<ol>
	<li style="font-size:17px">Kotz, J.C., Treichel Jr, P.M., Townsend, J.R. (2012).化学および化学反応性。カリフォルニア州ベルモント:ブルックス/コール、Cengage Learning。</li>
	<li style="font-size:17px">Silberberg, M.S. (2009) Chemistry: The Molecular Nature of Matter and Change.マサチューセッツ州ボストン:マグロウヒル。</li>
	<li style="font-size:17px">ハリス、ワシントンD.C.(2015年)。定量的化学分析。 ニューヨーク州ニューヨーク:WHフリーマンアンドカンパニー。</li>
</ol>

再 結晶 化
多くの場合、化学反応の所望の生成物は、溶媒で構成することができるより複雑な反応混合物の一部です...

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JoVE Lab Manual

Chemistry

化学

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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recrystallization

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...

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identifying alcohols

Alcohol Tests for Primary, Secondary, and Tertiary

Alcohol tests can help sort alcohols by structure and reactivity. Alcohols are organic molecules with a hydroxyl group attached to an alkyl or aryl group, written as ROH. If the carbon with the hydroxyl group has one R group, the alcohol is primary. If it has two R groups, it is secondary. If it has three R groups, it is tertiary. Alcohols can also be aromatic when they contain a benzene ring. Phenol is the simplest aromatic alcohol. To identify an alcohol, chemists can use its chemical...

chemistry

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Learn how recrystallization uses solubility, temperature, and slow cooling to purify organic solids and separate impurities from crystals.

Purifying Crystals by Solubility | JoVE K12

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Chemistry の 再 結晶 化 学生プロトコル - 54K+ 回の視聴 | JoVE Lab Manual

Purifying Crystals by Solubility

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