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

대부분의 제품에는 불순물이 포함되어 있습니다. 이러한 제품을 정제하는 한 가지 방법은 재결정화입니다. 재결정화는 불순한 생성물을 포화 용액을 형성하기에 충분한 뜨거운 용매에 용해시키는 것으로 시작되며, 여기서 가능한 한 많은 용질이 용매에 용해됩니다. 추가적인 용질은 용해되지 않는 것처럼 보입니다.
용해도는 일반적으로 온도에 따라 증가하기 때문에 고온 용매가 사용됩니다. 온도가 상승함에 따라 용매에 용해될 수 있는 용질의 양이 증가합니다. 용액이 냉각됨에 따라 생성물의 용해도가 감소하고 용질 분자가 함께 모여 핵이라고 하는 작고 안정적인 결정을 형성합니다.
이것은 핵형성이라고 하는 결정화의 첫 번째 단계입니다. 핵에서 추가적인 결정 성장이 일어나는데, 이는 용질 분자가 새로운 결정을 형성하는 것보다 기존 용질 결정을 결합하는 데 더 큰 친화력을 갖기 때문입니다. 용해성 불순물은 용액에 남아 있습니다.
결정화는 자발적으로 발생하거나 플라스크 내부를 긁거나, 용액을 교반하거나, 화합물의 종자 결정을 추가하여 촉진할 수 있으며, 이 모든 것은 추가 성장을 위한 표면을 제공합니다.
크고 순수하며 규칙적인 모양의 결정으로의 재결정화는 적절한 용매가 사용될 때만 작동합니다. 화합물은 실온에서 용매에 불용성이어야 하며 고온에서 용해되어야 합니다. 이상적으로, 불순물은 고온에서 용매에 불용성이거나 실온에서 용매에 용해되어야 합니다.
불순물이 뜨거운 용매에 불용성인 경우 결정화 전에 여과됩니다. 재결정화 후, 결정은 여과되고 차가운 용매로 세척되어 표면의 불순물을 제거합니다. 이제 결정의 순도를 분석할 수 있습니다.
용매를 선택할 때 고온과 저온 사이의 용해도 차이가 클수록 용질이 냉각되어 결정을 형성할 때 용액에서 나올 가능성이 높아진다는 점을 명심하십시오. 냉각 속도도 재결정화에서 중요합니다.
급속 냉각은 많은 핵 형성 부위의 형성과 더 작은 결정의 성장을 선호하는 반면, 냉각은 천천히 더 적은 수의 핵 형성 부위의 형성과 더 적고 더 크지만 더 순수한 결정의 성장을 유도합니다. 따라서 느린 냉각이 선호됩니다.
이 실험실에서는 두 가지 불순 유기 화합물인 아세트아닐라이드와 트랜스 신남산을 재결정화한 다음 회수된 화합물의 융점 범위를 문헌의 값과 비교하여 순도를 평가합니다.

재결정화

재결정화
종종 원하는 화학 반응 생성물은 용매, 출발 물질 및 불순물로 구성될 수 있는 더 복잡한 반응 혼합물의 일부입니다. 유기 화합물을 적절하게 정제하는 방법을 배우는 것은 유기 화학에서 중요한 기술입니다. 재결정화는 원하는 화합물의 용해도와 용매의 불순물의 차이를 활용하여 원하는 생성물을 고체로 정제합니다. 정제에는 증류, 추출 및 재결정의 세 가지 표준 방법이 있습니다.
가용성
물질의 용해도는 주어진 온도에서 주어진 용매의 고정된 부피에 용해되는 최대량입니다. 다른 용질은 다른 용해도를 가지며 다른 용매에 용해됩니다. 용질은 재결정화를 위해 이용될 수 있는 정의적인 특성을 가질 수 있습니다. 화합물은 용매에서 다음 중 하나의 거동을 나타냅니다. 첫째, 화합물은 불용성이거나 모든 온도에서 용매에 대한 용해도가 매우 낮을 수 있습니다. 둘째, 화합물은 더 높은 온도에서 용매에 용해 될 수 있습니다. 셋째, 화합물은 모든 온도에서 용매에 용해 될 수 있습니다.
용질이 용매에 용해되어 용액을 형성하는지 여부를 결정하는 주요 요인은 용질과 용매 사이의 분자간 힘의 강도와 유형입니다. 일반적인 경험 법칙은 "like dissolves like"이며, 이는 유사한 유형의 분자간 힘을 가진 물질이 서로 용해된다는 것을 의미합니다. 예를 들어, 식염(NaCl)과 같은 극성 물질은 극성의 물에 잘 용해됩니다.
용해도를 향상시키는 또 다른 핵심 요소는 온도입니다. 많은 물질의 경우 용해도는 더 높은 온도에서 크게 증가합니다. 이는 더 높은 온도에서 증가된 운동 에너지가 분자를 함께 유지하는 용질 간 분자력을 끊기 때문입니다. 이것은 일상 생활에서 볼 수 있습니다. 예를 들어, 우리는 식탁용 소금(NaCl)이 물에 잘 녹는다는 것을 알고 있습니다. 그러나 낮은 온도보다 높은 온도에서 더 많이 용해됩니다.
질적으로, 용해된 용질의 최대량에 아직 도달하지 않은 경우 용액은 불포화 상태로 간주됩니다. 가능한 최대 용질이 용해되면 용액이 포화 상태가 됩니다. 과포화 용액에는 일반적인 조건에서 가능한 최대량보다 더 많은 용해된 용질이 포함되어 있습니다.
<h4>재결정화 </h4>
재결정화는 원하는 생성물과 고온에서 오염 물질 사이의 용해도 차이를 이용합니다. 재결정화의 첫 번째 단계는 제품 혼합물을 최소한의 가열된 용매에 용해시키는 것인데, 이 경우 과포화는 아닌 포화 용액이 생성됩니다. 그런 다음, 용액을 실온으로 냉각시켜 원하는 화합물과 불순물의 용해도를 모두 감소시킵니다.
용액이 냉각됨에 따라 순수 성분의 결정화가 시작되지만 여전히 용해되는 불순물은 시작되지 않습니다. 이것은 관심 성분이 불순물보다 훨씬 더 높은 농도일 때 발생합니다. 첫째, 핵형성 단계에서 용매는 용질 분자의 무작위 응집을 시작하여 시드 또는 핵이라고 하는 첫 번째 결정을 형성합니다. 다음으로, 입자 성장 또는 결정화 단계에서 더 많은 분자가 시드에 추가되어 결정을 형성합니다. 결정에는 순수한 화합물이 포함되어 있고 불순물은 용매에 남아 있습니다.
핵형성은 과포화 용액에서 입자 성장보다 빠르게 진행됩니다. 씨앗이 많을수록 각 결정이 더 작아집니다. 따라서 용액이 과포화가 아닌 포화 상태이면 더 적은 수의 씨앗이 형성되어 더 큰 결정이 생성됩니다. 실온으로 냉각하기 전에 용액을 더 높은 온도로 가열하면 더 높은 농도의 용질이 용해되어 과포화가 감소합니다. 또한 급속 냉각으로 인해 핵 형성이 빨라져 많은 작은 결정이 형성되고 결정 내부에 불순물이 가두게 됩니다. 더 적고 더 큰 결정을 얻기 위해 느린 냉각이 선호됩니다.
용액이 실온으로 냉각되고 결정이 형성되면 진공 여과를 사용하여 용액을 여과합니다. 그런 다음 결정을 건조시킵니다. 회수율은 회수된 생성물의 질량을 조제품의 질량으로 나누어 계산됩니다.
<img src="/CDNSource/lm/labs/58/58_Concepts_1.jpg">
회수율이 100%인 경우는 드물며, 저온에서 화합물의 용해도가 화합물의 결정화되는 양을 결정하기 때문입니다.
<h4>용매 선택</h4>
결정화가 효과적이려면 최적의 용매를 사용해야 합니다. 원하는 제품은 실온에서 선택한 용매에 대한 용해도가 낮아야 하지만 더 높은 온도에서 용매에 대한 용해도가 높아야 합니다. 이상적으로, 불순물은 모든 온도에서 용매에 용해되어야 합니다. 따라서, 혼합물이 고온에서 용매에 첨가되면 원하는 생성물과 불순물이 쉽게 용해됩니다.
용액이 냉각됨에 따라 원하는 생성물의 용해도가 감소하고 결정화가 발생하기 시작하여 정제된 생성물을 형성합니다. 때때로, 불순물은 고온에서도 선택한 용매에 불용성으로 남아 있을 수 있습니다. 용해된 생성물을 포함하는 용액의 고온 중력 여과는 고체 불순물을 제거할 수 있습니다. 그런 다음 샘플을 냉각하여 제품을 재결정화할 수 있습니다.
이상적으로, 사용할 용매는 원하는 제품의 융점보다 훨씬 낮은 온도에서 끓일 수 있어야 합니다. 용매는 또한 불활성이어야 하며 원하는 정제된 생성물과 반응하지 않아야 합니다.
<h4>참조</h4>
<ol>
	<li style="font-size:17px">Kotz, J.C., Treichel Jr, P.M., Townsend, J.R. (2012). 화학 및 화학 반응성. 캘리포니아 벨몬트 : Brooks / Cole, Cengage Learning.</li>
	<li style="font-size:17px">Silberberg, M.S. (2009) 화학: 물질과 변화의 분자적 성질. 매사추세츠주 보스턴: 맥그로 힐.</li>
	<li style="font-size:17px">Harris, D.C. (2015). 정량적 화학 분석. 뉴욕, 뉴욕 : WH 프리먼 앤 컴퍼니.</li>
</ol>

재결정화
종종, 화학 반응의 원하는 생성물은 용매로 구성될 수 있는 더 복잡한 반응 혼합물의 일부입니다.

Education

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JoVE Lab Manual

Chemistry

화학

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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galvanic cells

Battery Power from Redox Reactions

Electrochemistry studies how chemical change and electrical energy are linked. In this branch of chemistry, electrons move from one species to another. That movement can either produce current or be driven by an applied current. The main reaction in electrochemistry is the oxidation-reduction, or redox, reaction. A redox reaction has two half-reactions. Oxidation is the loss of electrons, and reduction is the gain of electrons. These changes also shift the oxidation state of each substance. The...

electrolytic cells

Electroplating with Electrolytic Cells

Electrochemistry links electrical energy with chemical change, and electrolytic cells show how electricity can drive a nonspontaneous reaction. In these reactions, electrons move from one species to another. The key chemistry is a redox reaction, or oxidation-reduction reaction, which happens in two half-reactions. Oxidation is the loss of electrons, and reduction is the gain of electrons. The substance that loses electrons is the reducing agent, and the substance that gains electrons is the...

proper lab notebook keeping

Recording a Reproducible Lab Experiment

Accurate lab notebook records are essential in science and organic chemistry labs. Without clear notes, descriptions, quantities, and observations, an experiment cannot be repeated or checked. Good record keeping supports reproducibility, which means other people can follow the same work and get similar results. A laboratory notebook is useful in school labs, research settings, and industry. It can also become a legal record when an idea, process, substance, or technique is patented and...

basic organic chemistry techniques

Organic Lab Glassware and Safe Handling

Organic chemistry labs use specific glassware, careful weighing steps, and controlled heating to keep reactions safe and accurate. Beakers and Erlenmeyer flasks are often used for mixing or holding solvents, but they are not meant for measuring volume unless only an approximate amount is needed. Volumetric glassware is used when a precise volume matters. This group includes graduated cylinders, volumetric pipettes, and volumetric flasks. A graduated cylinder is commonly used in the organic lab...

melting points

How Impurities Change Melting Points

Melting point helps identify an organic compound by showing the temperature where a solid turns into a liquid at 1 atmosphere of pressure. It is a physical property, like solubility, density, color, and electronegativity. Because the exact start of melting is hard to measure, the result is reported as a range. The lower end is when the first drops of liquid appear. The upper end is when the last solid has melted. Scientists compare this range with literature values to help identify a compound. <br...

boiling points

Boiling Point and Vapor Pressure

Boiling point and vapor pressure are key physical properties of a liquid. For a pure compound, the boiling point can help identify the substance. In experiments, the boiling point is usually recorded as a range because exact values are hard to measure and may differ by a few degrees from the literature value. Vapor pressure explains why a liquid boils. In a sealed container, some molecules at the liquid surface gain enough energy to escape into the gas phase. At the same time, some gas...

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...

extraction

Separating Mixtures with Solvent Extraction

Solvent extraction is a common way to separate and isolate a target compound in organic chemistry. In this process, a solute moves from one phase to another so it can be separated from unreacted starting materials or impurities. Extraction is also useful when a product is dissolved in a reaction solvent that is hard to remove by evaporation, such as a solvent with a high boiling point. There are three main types of extraction. In solid-liquid extraction, the solute moves from a solid phase...

simple distillation

Separating Liquids by Boiling Point

Separating liquids by boiling point is the goal of distillation. A miscible mixture contains liquids that mix evenly, and it can be heated so that the more volatile liquid evaporates first. Volatility is a liquid’s tendency to vaporize, and boiling begins when bubbles form in the liquid phase. In a closed container, liquid and vapor can reach equilibrium, which means the rate of vaporization equals the rate of condensation. The pressure from the vapor above the liquid is called vapor pressure.

steam distillation

Steam Distillation and Low-Boiling Separation

Steam distillation is a separation method for temperature-sensitive organic compounds. It works best when the organic compound is immiscible in water, meaning the two liquids do not mix. The method helps remove the organic molecule from a non-volatile contaminant without using the higher temperatures needed for many other separations. In steam distillation, the immiscible mixture is heated until both water and the volatile organic compound distill together. The vapor rises to a condenser,...

thin-layer chromatography

TLC Separation by Polarity

Thin-layer chromatography (TLC) separates compounds in a mixture by polarity. It uses the same basic parts as other chromatography methods: a stationary phase, a mobile phase, and the solute. In TLC, the stationary phase is a thin coating on a plate instead of material packed into a column. The stationary phase in most TLC systems is polar silica gel, a form of silicon dioxide. Its surface has OH groups that can form hydrogen bonds. The sample is first spotted on a pencil start line with a...

column chromatography

Separating Mixtures by Polarity

Column chromatography separates compounds in an organic chemistry lab by using differences in physical properties. It is especially useful for microscale separations when less than 10 grams of material is available for analysis. The method can separate a mixture based on solubility, polarity, hydrophobicity, size, and charge. Any chromatography system has three main parts: the stationary phase, the mobile phase, and the sample mixture. In column chromatography, the stationary phase is usually...

hydrolysis of an ester

Soap Formation from Triglycerides

Triglycerides are lipids made of a glycerol backbone linked to three fatty acids. They are common in vegetable oils and animal fats. Plant lipids are the basis of vegetable oils, while animal lipids are used as lard or general sources of fats. A fatty acid has a long carbon chain, usually between 12 and 20 carbons, with a weak organic acid at one end. Some fatty acids contain double bonds. These are unsaturated fats. The double bond usually gives the chain a bent, cis shape. As a result,...

synthesis of luminol

Luminol Chemiluminescence in Forensics

Luminol chemiluminescence shows how a chemical reaction can produce light and help with blood detection. Chemical reactions may be endothermic or exothermic, depending on whether they absorb or release heat as reactants become products. The enthalpy of a reaction is the energy difference between reactants and products, or ΔH. A positive ΔH means the reaction is endothermic. A negative ΔH means it is exothermic. Many reactions do not happen in a single step. They can pass through intermediates...

esterification

Ester Formation and Everyday Fragrances

Esters are organic compounds with a carbonyl group attached to an oxygen that connects to another alkyl or aryl group. Their general formula is RCOOR'. They are often made by reacting a carboxylic acid with an alcohol in an esterification reaction. In this process, the hydroxyl group of the acid is replaced by an alkyl or aryl group. Many simple esters have familiar fruit and flower smells because their structures are small and less complex. Formic acid and methanol can combine to form methyl...

identification of unknown aldehydes and ketones

identification-of-unknown-aldehydes-and-ketones

Carbonyl Tests for Aldehydes and Ketones

Aldehydes and ketones are carbonyl compounds, which means they contain a C=O functional group. A ketone has two alkyl or aryl groups attached to the carbonyl carbon. Acetone is the simplest ketone, with two methyl groups. An aldehyde is similar, but it has at least one hydrogen attached to the carbonyl carbon. Formaldehyde is the simplest aldehyde, and acetaldehyde has one hydrogen and one methyl group. The carbonyl carbon in both aldehydes and ketones is electrophilic because the oxygen atom...

uv-vis spectroscopy of dyes

How Dyes Absorb Visible Light

Dyes absorb visible light because their molecular structure lets them interact with specific wavelengths. When light hits a substance, some light is absorbed and the rest is reflected or transmitted. The color we see depends on the wavelength that is reflected. A blue substance reflects blue light, around 430 to 480 nm, and absorbs the complementary orange region, around 590 to 630 nm. Compounds that do not absorb in the visible range appear colorless to the human eye. Light also has energy and...

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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Purifying Crystals by Solubility

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